slide 1 / 159...slide 31 / 159 graphing motion when interpreting a graph, it is important to look...
TRANSCRIPT
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Forces and Motion
· MotionClick on the topic to go to that section
· Graphs of Motion
· Newton's Laws of Motion
· Newton's 3rd Law & Momentum
· Forces
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What does it mean to be in motion?
With a partner, come up with a scientific explanation of what it means if an object is in motion. Don't just say an object is moving!
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What does it mean to be in motion?
An object is in motion if it changes position in relation to a certain place, called a reference point.
Reference points are places or objects used to determine the motion of an object.
It is extremely important to choose reference points carefully.
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Relative Motion
Motion is relative as it is dependent upon the reference point.
Are you in motion right now? Think about it...
Are you in motion compared to your desk? Are you in motion compared to the sun?
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Measuring Motion
Speed is one of the many ways we measure motion. Speed is a measure of the distance traveled per unit of time.
That means you can find the speed of any object that is in motion!
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Finding Speed
Speed = Distance ⁄ Time
and
We can rearrange the letters in the speed equation and solve for any of the other 2 pieces of information.
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Finding SpeedDiscussion time!
· What are some possible units to measure speed? Remember that speed= distance/time!
· How do you think speed is measured when driving in a car?
· How would you measure speed of an engine? For reference, an animation of an internal combustion engine is shown below.
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Units for Speed
The SI unit for distance is meters (m) and for time is seconds (s).
Given these units, what will be the SI unit for speed? (Hint: recall the speed equation speed= distance/time)
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1 A snail travels a distance of 10 m in 6000 seconds. What is the snail's average speed?
A 60000 m/s
B 0.02 m/s
C 600 m/s
D 0.002 m/s
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2 A blimp travels at 3 m/s for 1500 s. What distance does the blimp cover in that time?
A 500 m/s
B 4500 m/s
C 4500 m
D 500 m
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Manipulate the speed of each car, solve for time, and predict which car will win!
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SpeedWhen we talk about speed, it is important to know that there is a difference between instantaneous speed and average speed.
Average speed can be calculated by dividing the total distance by the total time. This is what is usually calculated by runners in a race.
Instantaneous speed is the speed of an object at any moment in time (an instant of time). Can you think of an example where you would use instantaneous time?
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It might take 3 hours to travel 300 km in a car. What would the average speed of the car be? Calculate it now.
Does that mean the car was going that speed for the whole 3 hours of the trip?
When you go on a trip in the car, do you go the same speed the whole time? Talk to a partner about this.
Average vs Instantaneous Speed
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Average vs Instantaneous SpeedHave you ever run a mile? Do you think you ran at the exact same speed the entire mile?
Think about what runners do at the very end of a race.
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Average vs Instantaneous Speed
When you ride on the bus to school, does the bus driver travel at the same speed the entire trip?
School buses and other vehicles have speedometers that measure the speed of the vehicle at a specific moment in time.
Do speedometers measure average speed or instantaneous speed?
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3 Your teacher times your mile run at 7.5 minutes, that means your speed was 0.13 mile/min. Was this your average speed or your instantaneous speed?
AAverage Speed
BInstantaneous Speed
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4 A student has built a model car that is designed to run at a constant speed. What should the student measure to test whether the car runs at a constant speed, average or instantaneous speed?
A Average Speed
B Instantaneous Speed
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5 A swordfish travels for two hours. The first hour he covers 110 kilometers, and the second hour he covers 84 kilometers. What is the average speed of the swordfish?
A 110 km/h
B 84 km/h
C 97 km/h
D 194 km/h
Hint: Remember that Average speed is total distance travelled divided by total time
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6 A dog walks outside to go the bathroom every day. On a given day, she walks 75 meters before she finds a spot to use the bathroom. She walks at an average speed of 2.5 meters per second. How long does it take her to find a place to use the bathroom?
A 30 seconds
B 2 minutes
C 188 seconds
D 90 seconds
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Velocity
Velocity is another way to measure motion. Simply put, velocity is the speed of an object with a direction included.
Runner's speed: 10 km/hrRunner's velocity: 10 km/hr to the East
What do you notice about the units for speed and velocity?
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Why is velocity important?
Have you ever been in an airplane? Would would be the consequence of a pilot only knowing the speed of other nearby planes?
Tornadoes travel at about 170 km/h. Why is knowing the velocity of a tornado important?
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7 Carlos and Gina are riding on their horses to go into town. They travel 70 meters in 7 seconds going west. What is their velocity?
A 490 m/s westB 10 m/s westC 490 m/sD 10 m/s
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8 A car travels 100 km/h west for 2 hours. The car then travels 50 km/h east for one hour. What is the car's final position relative to its starting position?
A 50 km westB 150 km westC 250 km westD 50 km east
Hint: Find the distance and direction traveled for each leg of the trip first. Draw these distances on a number line using 0km as the starting point.
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Graphing Motion
Graphs can be used to show motion and can be used to determine relationships!
When graphing data, position should be on the y-axis and time on the x-axis. Drag and drop the variables onto the correct axis on the graph below.
Position
Time
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Graphing Motion
Use the graph below to calculate speed at 1, 2, and 3 seconds.
Do you notice a pattern?
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Graphing Motion
2 m/ 1s = 2 m/s 4 m/ 2s = 2 m/s
6 m/ 3s = 2 m/s
The speed is 2 m/s at each of the three seconds! In other words, the speed is constant.
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Graphing MotionWhen interpreting a graph, it is important to look for relationships between variables. These relationships can be strong, weak, or not present at all.
The graph below shows a strong relationship between position and time. Can you describe what this relationship is?
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Graphing MotionA weak relationship is when significant changes in one variable cause minimal changes in the other variable. Is there any pattern in the graph below? Circle the patterns.
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Graphing MotionWhen there is no relationship between the variables, there will not be a pattern present. Is there any pattern in the graph below?
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The Importance of SlopePosition versus time graphs can be used to find speed and compare speeds.When we talk about the slope of a line, we are talking about how steep a line is.
Look at the skier to the right. He is on a very difficult trail.
Would an easier trail be steeper or more flat?
How would the slopes of these two trails compare?
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The Importance of SlopeIn a position versus time graph, the y-axis is the position and the x-axis is the time. Recall that the relationship between position and time is speed. So when we are looking at the slope, we are looking at the speed .
The steeper the slope of the line on a position versus time graph, the greater the speed!
The slope of the black line gives us the speed of the bicycler.
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Which walker has a greater speed? How can you tell in terms of the slopes of the graphs?
9
A B
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Relate the shape of the graph (slopes!) to the man's motion.
Graphing Motion Simulation LabClick on the image to the left to launch the simulation. You will need to download it to be able to use it.
Click on the CHARTS tab at the top.
Minimize the velocity graph and the acceleration graph by clicking on the RED dash on each graph. KEEP POSITION!
CLICK PLAY
Drag the man to the RIGHT at a constant speed, STOP, and finally drag the man at a constant speed Left back to the starting point (zero).
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Acceleration
Constant speed is when an object's speed does not change; however, most objects do not travel at a constant speed. Acceleration is a way to measure changing motion.
Do you walk at the exact same speed from class to class? What if you are late for class?
Are you ever accelerating when you go to class? Can you define acceleration?
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speed speedchange in direction
Acceleration
Acceleration occurs when there is a change in velocity. Remember, velocity is speed with direction.
So acceleration occurs any time there is an increase in speed, a decrease in speed, or a change in direction.
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Acceleration
Acceleration is a measure of the change in velocity per unit of time. Here is the equation for acceleration.
vf -vo
ta =
acceleration = (final velocity-initial velocity) time
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10 A school bus driver sees an old man crossing the street at an intersection. The bus driver hits the brake pedal and turns the steering wheel to avoid him. Describe the acceleration of the bus.
A It increased speed
B It decreased speed
C It changed direction
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11 A dragster launches from rest to 50 mi/hr at the start of a race. How did the car accelerate?
A increased speed
B decreased speed
C changed direction
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The school bus picks you up at the bus stop and takes 60 seconds to accelerate to 120 km/h. What is the acceleration of the school bus?
vf -vo
ta =
Practice Solving for Acceleration
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12 A plane's speed increases from 25 m/s to 60 m/s in 5 seconds. What is the acceleration of the plane?
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13 After traveling for 10 seconds, a runner reaches a speed of 12m/s. If the runner started from rest, what is the runner's acceleration?
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14 A parachute opens and slows a skydiver from 65m/s to 45m/s in a period of 5 seconds. What is the acceleration of the skydiver?
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Acceleration and GraphsIf the speed and direction of an object are constant, then the acceleration is zero. Look at the car's speed at 20s, 60s, and 100s.Is the car accelerating?
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Acceleration and Graphs
Acceleration is positive when speed is increasing.
Negative acceleration (deceleration) is when speed is decreasing.
How could you describe the slopes of each of these lines?
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15 A driver hits the brakes to slow down at an intersection. As the car's speed is decreasing it has:
A positive acceleration
B negative acceleration
C no acceleration
D more information is needed
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16 Which of the following graphs shows a positive acceleration?
A B
C
x
Y
Spe
ed (m
/s)
Time (s)x
YS
peed
(m/s
)
Time (s)
x
Y
Spe
ed (m
/s)
Time (s)
D
x
Y
Spe
ed (m
/s)
Time (s)
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Constant Speed Graphical Analysis Lab
We will us a constant velocity vehicle to collect data and graph our results.
The graph will help us determine the average speed.
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Acceleration and GraphsThe slope of a position versus time graph can also show acceleration.
If the slope curves and gets steeper, then positive
acceleration is occurring.
If the slope curves and becomes less steep, then negative acceleration is occurring.
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Distance vs. Time Graphs of Accelerating Objects
Click on the image to download the simulation.
Click on the CHARTS tab.
Type in 1 m/s2 into the acceleration value and hit play.
Sketch the shape of the distance vs. time graph:
Sketch the shape of the velocity vs. time graph:
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17 Which of the following speed vs time graphs (on the left) correspond to the position vs time graph on the right?
A
B
C
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Accelerated Motion on an Inclined Plane Lab
In this lab you will record the time it takes for a car to go different distances down an inclined plane.
The data will be graphed and then you will analyze the results.
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What Are Forces?
Forces are ________ or
______ in a given direction.
Forces affect how objects move.
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Forces Affect MotionAt your table, make a list of ways forces can affect objects. There are many ways. Write a few of these ways below.
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Forces
Acceleration
cause
The Big Idea...Since forces can cause changes in the speed or direction of an object, we can say that forces cause changes in velocity, so forces cause acceleration!
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Units of Force
Forces are measured in newtons (N).
You probably measure yourself on a scale in pounds. One pound is equal to 4.448 newtons.
Just like velocity, force has direction. When forces are demonstrated both magnitude and direction should be shown.
10 N
What is the magnitude of the force shown above? How about the direction?
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Balanced Forces
If you and a friend both push with the same strength on each side of a table, will it move?
Two or more opposite forces acting on an object are considered balanced if their effects cancel each other out. Balanced forces do not cause a change in an object's motion.
8 N 8 N
The box to the right is at rest and will remain at rest since the opposite forces acting on it are balanced.
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Unbalanced ForcesIf the effects of the forces don't cancel each other out (one force is stronger than others), the forces are unbalanced forces. Unbalanced forces do cause a change in motion; speed and/or direction.
Think about Tug-Of-War. How does one side win?
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Unbalanced Forces
15 N 8 N
Two ways to interpret this diagram are to say:
there is a 15N force to the right and an 8N force to the left
OR
we can say there is a +15N force and a -8N force.
What does the negative on the -8N tell us?
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Unbalanced Forces
15 N 8 N
The box was initially at rest. Since the forces acting on the box are unbalanced, the box will start moving.
Does the box accelerate? Which way do you think the box will move and why?
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18 Forces are all around us. Which of the following do you think are examples of forces? (choose all that apply)
A Gravity
B Friction
C Muscles
D Wind
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19 What is the SI unit for force?
A Pounds
B Kilograms
C Newtons
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20 A +10 N force acts on a car and at the same time, a -20 N force acts on the car. What is the net force acting on the car and is it balanced?
A -30 N unbalanced
B -10 N unbalanced
C -10 N balanced
D + 30 balanced
Hint: Net force is the total (or sum) of all the forces acting on an object.
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Friction
Forces are present all around us, but can not usually be seen. Friction is a force that resists motion and we experience daily.
When you run, walk, sit on the couch, brush your hair, and write you experience friction.
You are experiencing friction right now!
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Friction The force of friction is caused by microscopic particles touching each other. These microscopic pieces on both surfaces cause friction.
Friction is affected by how hard the surfaces push together and the types of surfaces involved.
Click here to see the force of friction clip
There are many types of friction.
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Static Friction
Static friction acts on objects that are not moving. Have you ever wondered why it is so hard to start moving a heavy object like a dresser or couch, but then once it starts moving it is easier?
That is because you have to overcome the force of static friction! Static friction holds the couch in place and keeps it from moving.
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Sliding Friction Sliding friction occurs when objects slide over each other. Sliding friction is easier to overcome than static friction. That is why the couch is easier to move once it starts sliding!
Click here to see sliding friction in action.
Sliding friction is also known as kinetic friction.
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Fluid Friction Fluid friction occurs when objects move through a fluid.
Remember, air is a fluid, so you continuously experience fluid friction!
Click here to see the effects of fluid friction.
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Rolling Friction
Rolling friction exists when objects roll across surfaces.
With a partner, make a list of other examples of rolling friction. Hint: There are many examples in sports.
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Friction
The force of friction is measured in Newtons like all other forces. When you are trying to determine where to put the friction force, just remember that friction acts opposite to motion!
stationary boxforce applied by person pushing box
static friction force
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21 Friction acts more on objects in motion than it does on objects at rest.
True
False
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22 Friction always opposes the motion of an object.
True
False
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Gravity
Forces are present all around us and always act in pairs, so we usually experience more than one force. Gravity is a force that pulls objects towards each other.
How does gravity act on objects here on Earth? Use the picture to the right to help you with your answer.
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Law of Gravitation
The Law of Universal Gravitation tells us that gravity acts between all objects in the universe. This means that without exception, any two objects in the universe attract each other!
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Factors Affecting Gravity
Gravity is affected by both mass and distance. The greater the distance between two objects, the less the gravitational force between them. The greater the mass of the object, the greater the object's gravitational force.
You have learned about mass in the past. Can you define mass?
Which of these examples has more gravity?Why?
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23 The force of gravitation between an object and a planet is increased as they move away from each other.
True
False
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24 The force of gravitation between an object and a planet is decreased if the object increases mass.
True
False
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Mass and WeightWhat is weight?
You have also learned the difference between mass and weight in the past. What can you recall about mass and weight?
Think about astronauts on the
International Space Station. What do you
know about their masses and
weights?
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Wait, so mass affects gravity?
Weight is a measure of the gravitational force exerted on an object. Weight varies depending on gravitational force, but mass does not.
Weight = mass x gravity
Our moon has a gravity that is one sixth of that on Earth.
What would that do to your weight on the moon?
Would your mass change on the moon?
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What is "g"?
"g" is the acceleration due to gravity. On Earth, our "g" is approximately 9.8 m/s2.
On other planets, acceleration due to gravity will vary depending on the mass of the planet. In general, the more massive the object, the larger the value of g!
Jupiterg = 26.1 m/s2
Earth's Moong = 1.67 m/s2
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25 A 50 kg kid is on planet Earth, where g = 9.8 m/s2. What is the boy's weight?
A 5.1 m/s
B 490 N
C 490 kg
D 5.1 N
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26 A 50 kg kid is on planet Earth, where g = 9.8 m/s2. What is the boy's mass?
A 50 kg
B 490 N
C 490 kg
D 5.1 N
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27 A 50 kg kid is on the moon, where g = 1.67 m/s2. What is the boy's weight?
A 83.5 kg
B 29.9 kg
C 83.5 N
D 29.9 N
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28 A 50 kg kid is on the moon, where g = 1.67 m/s2. What is the boy's mass?
A 83.5 kg
B 50 kg
C 83.5 N
D 50 N
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29 The larger the planet, the larger the __________.
A mass
B weight
C gravity
D all could be correct
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Gravity and MotionGalileo showed that falling objects accelerate equally no matter what their mass is. This is strictly true when gravity is the only force acting on a falling object and is known as free fall.
When an object is in free fall, it accelerates at 9.8 m/s2
10kg
If these two stones are dropped from the same height at the same time, which hits the ground first?
1kg
Click here to see a bowling ball and golf ball being
dropped
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Gravity and Motion
The following video shows a feather and a ball bearing being dropped from a small height.
Which simulation showed the objects in free fall?
Critical thinking: What was happening in the simulation that did not illustrate free fall?
Click here to see a feather and a ball bearing being dropped.
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Air Resistance
Try this: Crumple up two pieces of paper individually and drop them both from the same height at the same time. Which hits the ground first? Now drop a crumpled piece of a paper and a non-crumpled piece of paper in the same way. What's the difference?
Objects are not always truly in free fall because they experience air resistance. Air resistance is a fluid friction experienced by falling objects. When objects experience air resistance, they don't fall at a rate of 9.8 m/s2.
Click here to see a feather and a hammer being dropped on the moon. What happens
with no air resistance?
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Air ResistanceThe flat piece of paper fell at a slower rate because it had more surface area. This greater surface area resulted in the paper experiencing air resistance. For the crumpled paper, air resistance was probably very tiny and thus could be ignored. The crumpled paper was essentially in free fall.
People use parachutes when they jump out of planes. Why?
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Net Force
As you know, many forces are acting on us and other objects. To determine the total force acting on an object, the forces are added and subtracted as appropriate to find the net force.
When several forces are acting on the same object, the net force might be zero...
5 N -5 NNet Force = 0
If the net force on an object is zero, then it is in equilibrium. When an object is at rest, the net force is zero.
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If the net force is not equal to zero, then there is a change in the motion of the object. The object is not in equilibrium. What is the net force in the
example below?
5 N -12 NNet Force =
Unbalanced Forces
What direction is this box going to move?
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5 N -12 N Net Force = -7 N
In this case, the object will accelerate towards the left because the NET FORCE is toward the left.
Unbalanced Forces
On Earth, gravity and friction are two of the unbalanced forces that
frequently change an object's motion.
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30 What is the net force acting on the object below? Is the object in equilibrium?
7 N
-8 N
-10 N
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31 What is the net force acting on the object below? Is the object in equilibrium?
25 N
-15N
-10 N
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32 Acceleration due to gravity on Jupiter is 26.1 m/s2. How much would a 60 kg person weigh on Jupiter?
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33 Acceleration due to gravity on Earth's Moon is 1.67 m/s2. How much would a 60 kg person weigh on Earth's Moon?
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Sticky Sneakers Lab
How does the amount of friction between a shoe and a surface compare for different shoes?
Which shoes would be best for playing basketball?
Which would be best for bowling?
These questions will be answered in this lab.
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The History of the Laws of Motion
Aristotle, a Greek philosopher, and Galileo Galilei, an Italian astronomer, may have been two of the first scientists to try to explain
gravity and motion.
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In the late 1600s, Sir Isaac Newton used Galileo's ideas to create three basic laws of motion.
The History of the Laws of Motion
Sir Isaac Newton contributed to advances in physics, mathematics, and astronomy.
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Laws of Motion
Newton may be one of the greatest scientists in history. The three laws of motion he created are three of the most used natural laws in science.
These laws help us to make sense of the world around us.
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Newton was inspired by the apple falling from the tree and asked himself if gravity might also be the force holding the
moon in orbit.
Newton found that gravity plays a role in other orbital motions as well!
Laws of Motion
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Newton's First Law of MotionThe first law of motion tells us that an object at rest stays at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless acted on by an unbalanced force.
On Earth, gravity and friction are two of the unbalanced forces that frequently change an object's motion.
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The first law of motion is sometimes referred to as the law of inertia.
Newton's First Law of Motion
Inertia is the tendency of an object to resist a change in motion.
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The First Law of MotionThe first law basically tells us that motion will not change without a net force. So, if an object stops moving or starts moving, you
know there is a net force.
If there is a net force, then the forces are unbalanced. As you know, unbalanced forces cause changes in motion!
Apply Newton's First Law of Motion to the baseball player sliding into second base. What are some forces acting on him?
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Inertia Simulation
Click on the image to the left to launch simulation.
Click on the motion tab, check speed to add speedometer, and place a person on the skateboard.
Apply a force to the object and look at the speedometer. What happened?
Stop applying the force. What happens?
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Application of The First Law of Motion
Have you ever wished that you could just tell your clothing to move itself to the closet?
Unfortunately, we know that objects don't move on their own. An unbalanced force is required to make an object change its state of motion.
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Imagine you need to move a few pieces of furniture in your room. Would you rather move your dresser with everything in it or your dresser when its empty?
Application of The First Law of Motion
Be sure to use the term inertia in your answer.
Based on your answer, how are inertia and mass of an object related?
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34 Inertia is the resistance of an object to change in its state of motion.
True
False
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35 The law of inertia applies to___________.
A moving objects
B nonmoving objects
C both moving and nonmoving objects
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Which has more Inertia?
A Tennis Ball or a Bowling Ball?
Why?
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36 Which object has the greatest inertia?
A Car
B Tennis Ball
C Moving Freight Train
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37 Which object has the greatest inertia?
A Car
B Tennis Ball
C Freight Train at rest
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38 Which object has the greatest inertia?
A Moving Tennis Ball
B Tennis Ball at Rest
C Both have the same Inertia
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39 A ball will accelerate when it is acted on by 2 equal forces pointing in opposite directions.
True
False
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Newton's Second Law
Newton's second law states that acceleration depends on both force and mass. It supports the idea that unbalanced forces cause acceleration.
Remember, acceleration is an increase in speed, a decrease in speed, or a change in direction.
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Newton's Second Law
Unbalanced forces cause acceleration.
There is a direct relationship between force and acceleration.
Mass and acceleration are inversely proportional.
Forces that cause a net force on an object are unbalanced.
As force increases, acceleration increases.
As mass increases, acceleration decreases.
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Newton's Second Law
Unbalanced forces cause acceleration, so unbalanced forces cause an increase in speed, a decrease in speed, and/or a change in direction.
The second law relates force, mass, and acceleration.
Force = mass x acceleration
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We can rearrange this to solve for the other variables. You do not need to know how to rearrange it, but you do have to be able to select the right formula for solving your problem.
F = ma
a = Fm m = F
a
Newton's Second Law
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Application of Newton's Second Law
The second law states that if force is increased, acceleration will also increase.
If you want to shoot a very fast penalty kick into the soccer net, how do you kick it?
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Application of Newton's Second Law
The second law also tells us that the greater the mass, the less the acceleration (if the force is constant).
How hard is it to pull one person in the wagon?
What happens when you add another person to the wagon?
Have you ever pulled your friends in a wagon?
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40 When the force acting on an object increases, the resulting acceleration will:
A remain constant
B increase
C decrease
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41 When an object's mass increases but the applied force stays the same, the resulting acceleration will:
A remain constant
B increase
C decrease
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42 You are riding your bicycle to the park to meet a few friends. As you ride, you apply a force of 30 N and accelerate at a rate of 0.4 m/s2. What is the total mass of the bicycle and you?
A 75 kg
B 12 kg
C 120 kg
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43 You are pushing a shopping cart so that it accelerates at 0.2 m/s2 . You start filling the shopping cart with food. Do you need to change how you push on the cart for it to maintain the same acceleration? If so, in what way?
A No
B Yes, push harder
C Yes, push softer
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Newton's 2nd Law Simulation
Click on the image to the left to download the Simulation.
Click the Acceleration Lab Tab
Check show Forces, masses, acceleration, and turn friction to none.
Place 1 crate onto the surface and apply a 500 N force, note the acceleration.
Stack 2 crates onto the surface and apply a 500 N force,note the acceleration.
What was the effect of adding mass to the simulation on the resulting acceleration produced?
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Newton's Third Law of Motion
Newton's third law of motion, unlike the first and second, pertains to forces between two objects.
When you kick a soccer ball, do you feel the force of the ball against your foot?
Does the ball "feel" the force of your foot?
Newton's third law explains this occurrence.
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Have you ever jumped off a skateboard? What happens to the board when you jump off? Draw an arrow below to show what direction the board will go.
Newton's Third Law of Motion
This is Newton's third law of motion at work! You applied a force to the board and the board applied an equal (in magnitude) and opposite (in direction) force on you.
We call these Action-Reaction Forces.
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44 When you sit on a chair, the seat of the chair pushes up on you with more force than your weight.
True
False
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45 Action-Reaction forces are always found in pairs that are equal and opposite.
True
False
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Newton's Third Law of MotionForces always exist in pairs! Newton's third law defines these action and reaction forces.
Click here to see how Newton's Third Law applies to the physics of a rocket.
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As you kick the soccer ball, you apply an action force to the ball, and the ball applies a reaction force on your foot.
These forces are equal in strength and opposite in direction. Why does the ball accelerate more quickly than you and your foot?
Newton's Third Law of Motion
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Newton's Third Law of MotionThe third law states that for every action force, there is an equal, but opposite reaction force.
actio
n
reac
tion
This means that if one object applies a force to another object, then the other object exerts an equal and opposite force on the first object.
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46 When you jump off a skateboard, which of the following is true?
A You accelerate more than the skateboard.
BYou and the skateboard accelerate equal but opposite amounts.
C The skateboard accelerates more than you.
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Newton's Third Law of MotionAccording to Newton's 3rd law, the cart pulls on the man just as hard as the man pulls on the cart. Do these forces cancel each other out preventing the cart and man from moving?
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Newton's Third Law of Motion
Action: The man applies force to the cart that moves the cart forward.
Reaction: The cart applies an equal and opposite force on the man.
Each force in the pair acts on a different object!
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The Truth about Action Reaction Forces
If a force occurs, there are action reaction forces!
Action reaction forces:
can cause changes in motion
are equal in strength but opposite in direction
act on different objects
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47 Which of the following statements pertains to the third law of motion?
AAction and reaction pairs always act on the same object.
BMass is indirectly proportional to acceleration.
CAn object at rest will remain at rest unless acted on by an unbalanced force.
DIf a force occurs, action reaction forces are present.
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48 A textbook is resting on a table. Tommy pushes the book rightwards. What is an action reaction pair in this scenario?
AThe book exerts a downward force on the table. The table exerts an upward on Tommy.
BTommy exerts a rightward force on the book. The book exerts a downward force on the table.
CThe book exerts a leftward force on Tommy. Tommy exerts a rightward force on the book.
DTommy exerts a downward force on the table. The table exerts a rightward force on Tommy.
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49 Action reaction pairs cancel each other out since they are equal and opposite to each other.
True
False
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MomentumNewton's third law tells us that action reaction forces are equal and opposite, but that does not mean that the effects of those forces are equal.
After watching the video, discuss the following:
· what was the action reaction pair?· did both carts accelerate the same? Why or why not?· how is this related to Newton's 1st and 2nd law of motion?
Click here to see how equal action reaction pairs cause different motions.
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Momentum
Remember Newton's law of inertia. Objects with more mass (more inertia) have more reluctance to change their motion and vice versa.
Also recall Newton's 2nd law. Objects with more mass accelerate less than objects with less mass under the same force (and vice versa).
Both carts experience the same strength in force (Newton's 3rd law). But the cart on the right experiences a greater change in motion because it has less mass. Why?
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Momentum
If we understand Newton's third law and momentum, we can predict how the motion of colliding objects will change.
Momentum is the result of the mass of the object times the object's velocity.
momentum (kg-m/s) = mass (kg) x velocity (m/s)
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Momentum
Find the momentum of a skateboarder with a mass of 50 kg traveling at a velocity of 4 m/s west.
p = mv
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50 How can a small insect have the same momentum as a large car?
A insect has large speed
B both car and insect are at rest
C insect has no mass
D A & B
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51 If a 2 kg toy truck is moving at 4 m/s, what is the toy's momentum?
A 3 m/s
B 2 kg m/s
C 8 kg m/s
D 8 m/s
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Law of Conservation of Momentum
Momentum is conserved (remains the same) during an interaction as long as the objects are not affected by outside
forces.
This means that the total momentum of objects prior to hitting each other will equal the total momentum of the objects after
the interaction.
Any momentum lost by one object is gained by the other!
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Momentum Simulation
Click on the image to the left to launch and play the simulation.
Make each object the same mass by moving sliders (1 kg each works best).
Click show values.
Compare the total momentum of the balls added before and after the collision.
What happens to the total amount of momentum before and after the collision?
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Law of Conservation of MomentumFill in the missing values. Remember that the law of conservation tells us that the total momentum before a collision is equal to the total momentum after a collision.
Car Mass (kg)
Velocity (m/s)
Momentum (kg m/s)
1 1000 6
2 2000 0
Car Mass (kg)
Velocity (m/s)
Momentum (kg m/s)
1 1000 -12
2 2000
Before Collision Data After Collision Data
Total Momentum: ___________ Total Momentum: ___________
HINT: p=mv can be rearranged to v=p/m and a negative momentum means the object moves left!
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52 What two variables does momentum depend on?
A mass and volume
B mass and acceleration
C mass and velocity
D mass and force
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53 The total amount of momentum before and after a collision may vary.
True
False
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Newton's Third Law and Momentum LabClick on the picture to go to the website for the lab.
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Newton's Laws of Motion Lab
Rotate from station to station in the classroom. Follow the directions that correspond with your station. Make observations, record your results, and answer the questions for each station.
Station 1: Inertia is Nuts!Station 2: Balloon Blow OutStation 3: Spinning PennyStation 4: Water WhirlStation 5: DominoesStation 6: Yay for Seatbelts and AirbagsStation 7: Free Fallin'Station 8: Rolling Chair
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Images CitedA7N8X 2012, 4-Stroke Engine, gif, viewed 29 June 2015,
<https://commons.wikimedia.org/wiki/File:4-Stroke-Engine.gif>.
Luke Ma 2012, Pull Carts Kyoto, Japan, jpg, viewed 7 July 2015,
<https://commons.wikimedia.org/wiki/File:Pull_carts,_Kyoto,_Japan_(8587844087).jpg>