physical science chapter 2 motion and speed 1 note: you will not be able to view the videos from the...
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Physical Science Chapter 2
Motion and Speed
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Note: You will not be able to view the videos from the internet version of this presentation. Copyright laws prevent that option.
Intro Video AA.24.00
Motion occurs when an object changes its position.
Section 1—An object’s speed depends on how far an object travels in a unit of time.
Section 2– Acceleration describes how the velocity of an object is changing.
Section 3—An objects motion changes only if the forces acting on the object are unbalanced.
Distance and time are important. In order to win a race, you must cover the distance in the shortest amount of time.
How would you describe the motion of the runners in the race?
If you are sitting in a chair reading this sentence, you are moving.
You are not moving relative to your desk or your school building, but you are moving relative to the other planets in the solar system and the Sun.
2:1 Motion and Speed
A. Motion—when an object changes its position relative to a reference point.
1. Distance—how far an object has moved.2. Displacement—distance and
direction of an object’s change of position from a starting point.
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2:1 Motion and Speed
• Suppose a runner jogs to the 50-m mark and then turns around and runs back to the 20-m mark.
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•The runner travels 50 m in the original direction (north) plus 30 m in the opposite direction (south), so the total distance she ran is 80 m.
2:1 Motion and Speed
• Sometimes you may want to know not only your distance but also your direction from a reference point, such as from the starting point.
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•Displacement is the distance and direction of an object's change in position from the starting point.
2:1 Motion and Speed
• The length of the runner's displacement and the distance traveled would be the same if the runner's motion was in a single direction.
• The displacement would be less if the runner’s motion was reversed.
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Consider the motion of a Hot Wheels car down an incline, across a level and straight section of track, around a 180-degree curve, and finally along a final straight section of track. Such a motion is depicted in the animation following. The car gains speed while moving down the incline - that is, it accelerates. Along the straight sections of track, the car slows down slightly (due to air resistance forces).
Again the car could be described as having an acceleration. Finally, along the 180-degree curve, the car is changing its direction; once more the car is said to have an acceleration due to the change in the direction. Accelerating objects have a changing velocity - either due to a speed change (speeding up or slowing down) or a direction change.
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2:1 Motion and SpeedB. Speed—distance an object travels per unit of time.
1. Rate—any change over time.
2. Calculation for speed:
speed = distance/time.
3. Speed that doesn’t change over time—constant speed
4. Speed is usually not constant; usually an object has changing speed.
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2:1 Motion and Speed
5. The SI unit of length or distance is the meter (m). Longer distances are measured in kilometers (km). (copy into your notebook.)
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The distance Earth travels around the sun in one year is about 942 billion meters. One year has approximately 31,558,000 seconds. Calculate Earth’s speed.
Extra Credit
TAE 39
Assignment
Page 40. Speed Equation.
Your assignment is to answer the 4 Practice Problems at the bottom of the
page.
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2:1 Motion and Speed
5. Average speed—speed of motion when speed is changing;
speed= total distance/total travel time
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2:1 Motion and Speed
If the total distance traveled was 5 km and the total time was 1/4 h, or 0.25 h. The average speed was:
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2:1 Motion and Speed
6. Instantaneous speed—speed at any given point in time. A speedometer shows how fast a car is going at one point in time or at one instant. The speed shown on a speedometer is the instantaneous speed.
18Measuring Motion 2.55
•When something is speeding up or slowing down, its instantaneous speed is changing.
•If an object is moving with constant speed, the instantaneous speed doesn't change.
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Practice Problems 1
• The motion of an object over a period of time can be shown on a distance-time graph.
Graphing Motion 2.12.1
Describing MotionDescribing Motion
• Time is plotted along the horizontal axis of the graph and the distance traveled is plotted along the vertical axis of the graph.
Click image to play movie
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2:1 Motion and Speed
C. Distance-time graph displays motion of an object over time.
1.Plot distance on a(n) vertical axis
2. Plot time on a(n) horizontal axis
D. Velocity—speed and direction of an object’s motion
E. Motion of Earth’s crust—so slow we don’t notice it.
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• Because velocity depends on direction as well as speed, the velocity of an object can change even if the speed of the object remains constant.
Velocity 2.12.1
• The speed of this car might be constant, but its velocity is not constant because the direction of motion is always changing.
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• As you look around the surface of the Earth from year to year, the basic structure of the planet seems the same.
Motion of Earth's Crust 2.12.1
• Yet if you examined geological evidence of what Earth's surface looked like over the past 250 million years, you would see that large changes have occurred.
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Motion of Earth's Crust 2.12.1Describing MotionDescribing Motion
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Pangea 2.55
Moving Continents 2.12.1
• Together the crust and the top part of the upper mantle are called the lithosphere.
(add to your notes)
• How can continents move around on the surface of the Earth? Earth is made of layers.
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Moving Continents 2.12.1
• The lithosphere is broken into huge sections called plates that slide slowly on the puttylike layers just below.
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• These moving plates cause geological changes such as the formation of mountain ranges, earthquakes and volcanic eruptions.
Moving Continents 2.12.1
• The movement of the plates also is changing the size of the oceans and the shapes of the continents.
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2:2 Acceleration
A. Acceleration—change in velocity’s rate1. Positive acceleration—speed is increasing
2. Negative acceleration—speed is decreasing
3. When an object changes speed or direction, it is accelerating.
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Speeding Up and Slowing Down
• When you think of acceleration, you probably think of something speeding up. However, an object that is slowing down also is accelerating.
• Acceleration also has direction, just as velocity does.
2.22.2AccelerationAcceleration
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Speeding Up and Slowing Down 2.22.2
AccelerationAcceleration
• If the acceleration is in the same direction as the velocity, the speed increases and the acceleration is positive.
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Speeding Up and Slowing Down 2.22.2
AccelerationAcceleration
• If the speed decreases, the acceleration is in the opposite direction from the velocity, and the acceleration is negative.
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Changing Direction
• A change in velocity can be either a change in how fast something is moving or a change in the direction of movement.
• Any time a moving object changes direction, its velocity changes and it is accelerating.
2.22.2AccelerationAcceleration
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Changing Direction
• The speed of the horses in this carousel is constant, but the horses are accelerating because their direction is changing constantly.
2.22.2AccelerationAcceleration
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Calculating Acceleration
• To calculate the acceleration of an object, the change in velocity is divided by the length of time interval over which the change occurred. Acceleration = change in velocity/time.
2.22.2AccelerationAcceleration
• To calculate the change in velocity, subtract the initial velocity—the velocity at the beginning of the time interval—from the final velocity—the velocity at the end of the time interval.
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Calculating Acceleration
• Then the change in velocity is:
(copy this formula into your notes)
2.22.2AccelerationAcceleration
35Initial means first.
Calculating Acceleration • Using this expression for the change in
velocity, the acceleration can be calculated from the following equation:
Copy this formula:
2.22.2
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Calculating Acceleration
• If the direction of motion doesn't change and the object moves in a straight line, the change in velocity is the same as the change in speed.
2.22.2AccelerationAcceleration
• The change in velocity then is the final speed minus the initial speed.
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Calculating Positive Acceleration
• The airliner is traveling in a straight line down the runway, so its speed and velocity are the same.
2.22.2AccelerationAcceleration
• Because it started from rest, its initial speed was zero.
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Calculating Positive Acceleration
• Its acceleration can be calculated as follows:
2.22.2AccelerationAcceleration
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The unit for acceleration – meters per second squared.
Calculating Negative Acceleration 2.22.2
AccelerationAcceleration
• The final speed is zero and the initial speed was 3 m/s.
• Now imagine that a skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a
stop in 2 s.
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Calculating Negative Acceleration
• The skateboarder's acceleration is calculated as follows:
2.22.2AccelerationAcceleration
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Calculating Negative Acceleration 2.22.2
• The acceleration always will be positive if an object is speeding up and negative if the object is slowing down.
• The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative.
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Amusement Park Acceleration
• Engineers use the laws of physics to design amusement park rides that are thrilling, but harmless.
2.22.2
• The highest speeds and accelerations usually are produced on steel roller coasters.
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The Physics of Roller Coasters 24
The Stop Height Principle 1.39
2:2 Acceleration
B. Calculating acceleration1. Acceleration = change in velocity /time.
2. Change in velocity = final velocity -initial velocity.
3. Unit for acceleration – meters per second squared.
4. Positive acceleration – positive number with a positive slope on a velocity-time graph
5. Negative acceleration –negative number with a negative slope on a velocity-time graph.
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Fairground and 'thrill' rides rely on rapid changes of velocity - speed and direction - to generate large forces on the body and to give the thrill sensations. The 'Nemesis' ride at Alton Towers lasts for just 40 seconds but exerts forces of 4g (4 times normal gravity) through rapid acceleration in tight turns together with several seconds of weightlessness! Not for the faint hearted.
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Video A. 6.28
2:2 Acceleration
C. Amusement park acceleration— roller coasters
1. Changes in speed cause acceleration.
2. Changes in direction cause acceleration.
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Practice Problems 2b.
What is force?
• A force is a push or pull.
• Sometimes it is obvious that a force has been applied.
2.32.3
• But other forces aren't as noticeable.
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Changing Motion
• A force can cause the motion of an object to change.
2.32.3
• If you have played billiards, you know that you can force a ball at rest to roll into a pocket by striking it with another ball.
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Changing Motion 2.32.3
• The force of the moving ball causes the ball at rest to move in the direction of the force.
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Balanced Forces
• Force does not always change velocity.
• When two or more forces act on an object at the same time, the forces combine to form the net force.
2.32.3
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Balanced Forces
• The net force on the box is zero because the two forces cancel each other.
• Balanced forces are equal in size and opposite in direction.
2.32.3
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Unbalanced Forces
• When two students are pushing with unequal forces in opposite directions, a net force occurs in the direction of the larger force.
2.32.3
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Unbalanced Forces 2.32.3
• They are considered to be unbalanced forces.
• The net force that moves the box will be the difference between the two forces because they are in opposite directions.
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Unbalanced Forces
• The students are pushing on the box in the same direction.
2.32.3
• These forces are combined, or added together, because they are exerted on the box in the same direction.
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Unbalanced Forces 2.32.3
• The net force that acts on this box is found by adding the two forces together.
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Unbalanced forces are unequal in size and / or are not in the same direction.
2:3 Motion and Forces
B. Inertia and Mass1. Inertia—an object’s resistance to any
change in motion.
2. Objects with greater mass have greater inertia.
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Newton's Laws of Motion
• The British scientist Sir Isaac Newton (1642–1727) was able to state rules that describe the effects of forces on the motion of objects.
2.32.3
• These rules are known as Newton's law's of motion.
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Newton's First Law of Motion
• Newton's first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.
2.32.3
• If an object is at rest, it stays at rest unless an unbalanced net force acts on it.
• This law is sometimes called the law of inertia.
58Newton’s First Law 2.13
What happens in a crash? 2.32.3
• When a car traveling about 50 km/h collides head-on with something solid, the car crumples, slows down, and stops within approximately 0.1 s.
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What happens in a crash? 2.32.3
• Within about 0.02 s (1/50 of a second) after the car stops, unbelted passengers slam into the dashboard, steering wheel, windshield, or the backs of the front seats.
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Safety Belts
• The force needed to slow a person from 50 km/h to zero in 0.1 s is equal to 14 times the force that gravity exerts on the person.
2.32.3
• The belt loosens a little as it restrains the person, increasing the time it takes to slow the person down.
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Safety Belts
• This reduces the force exerted on the person.
2.32.3
• The safety belt also prevents the person from being thrown out of the car.
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• Air bags also reduce injuries in car crashes by providing a cushion that reduces the force on the car's occupants.
2.32.3
• When impact occurs, a chemical reaction occurs in the air bag that produces nitrogen gas.
• The air bag expands rapidly and then deflates just as quickly as the nitrogen gas escapes out of tiny holes in the bag.
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2:3 Motion and Forces
C. Auto crashes—the law of inertia at work.
1.A passenger not wearing a seat belt keeps moving forward at the car’s speed even after the car stops.
2. A passenger wearing a seat belt slows down as the car slows down and stops.
64Video 1 Video 2
The End of Chapter 2
Test Coming Up.
Homework is due.