Motion &

Forces

1. Force (F)- push or pull (cause of acceleration, or change in object’s velocity)

A. Examples: pushing a box across the floor, hitting a baseball, throwing a basketball, etc.

2. Forces influence motion

1. Changes the velocity- speeds up, slow down or changes the direction

2. Forces do not cause motion; they cause acceleration

3. Net force = the combination of all forces acting on an object.

4. Objects accelerate in direction of the net force (if it is zero, then there is no acceleration)

1. Balanced forces – Net Force = 0 (They cancel each other out)= no acceleration but could be at constant velocity.

2. Unbalanced forces – Net force ≠ 0 (they do not cancel) =objects will accelerate

3. Forces in same direction add, opposite directions subtract

1. Measured in Newtons (N)- amount of force it takes to cause1kg of mass to accelerate at 1m/s2.

(1 N= 1 kg•m/s2).

2. Representing Force-use an arrow

The arrow points in the direction of the force.

The length of the arrow represents the strength of the force.

Examples: Give direction and which is strongest:

A B

C

1. Arrows pointing in same direction add together.

2. Arrows pointing in opposite directions subtract from each other.

Example:

= 4N to right

= 2N to right 1N

1N

3N

3N

Look at the Force diagrams below: Figure up the net Force and in which direction the object will accelerate.

DO NOT COPY - CHECKING YOUR UNDERSTANDING

If the forces acting upon an object are balanced, then

the object

a. must not be moving.

b. must be moving with a constant velocity.

c. must not be accelerating.

d. none of these The answer could be A and it could be B .

An object having balanced forces definitely cannot be accelerating.

This means that it could be at rest and staying at rest (one option) or could be

in motion at constant velocity (a second option).

Either way, it definitely is not accelerating

1. Friction- force that acts in a direction opposite the moving object

2. Causes the object to slow down and eventually stop

3. To move an object, your force must be larger than the force of friction

1. Static Friction- force that surface acts on an object to keep it from moving. Ex: box sitting on ground

2. Sliding friction- force that opposes the direction of motion of an object as it slides over a surface: Ex: slide

3. Rolling friction- force that opposes motion of a rolling object. Ex: tires

4. Fluid friction- friction opposing object moving through a fluid such as gas or water. Ex: airplane, submarine

1. Gravity – attraction between two objects due to their mass

2. All falling objects accelerate at the same rate (9.8 m/s2)

3. It depends on the mass of the two objects as well as the distance between them

4. Gravity depends on: the mass of the two objects

the distance between the objects

5. Greater mass = greater the attraction of gravity

6. Greater distance b/t objects =

gravity decreases

1. Free fall – motion of an object when gravity is the only force acting on it

2. All objects near earth accelerate at 9.8 m/s2 in the absence of air resistance

Weight – measure of the force of gravity on an object ‘s mass (changes as gravity changes)

Weight = mass x gravity(9.8 m/s2)

w = mg

Unit: Newton

g = 9.8 m/s2 near earth

1 N = 0.225 lb; 1 lb = 4.448 N

Mass = does not change EVER

Weight = changes as gravity changes= Will change when you go to another planet.

1. A 10 kg mass would weigh ____

1. A 50 kg mass would weigh ____

1. A 100 kg mass would weigh ___

1. An astronaut that weighs 600 N on Earth is standing on an asteroid with a gravitational force one hundredth that of Earth. What is his weight on the asteroid?

98 Newtons

490 N

980 N

6 Newtons

Air resistance is the friction caused by air.

Terminal Velocity – when air resistance balances weight and the object stops accelerating and reaches its maximum constant velocity

320 km/h (200 mi/h)

Fair

Fgrav

no net force

no acceleration

constant velocity

DO NOT COPY - CHECKING YOUR UNDERSTANDING

Which of the following statements are true of the quantity mass?

List all that apply.

a) The mass of an object is dependent upon the value of the

acceleration of gravity.

b) The standard metric unit of mass is the kilogram.

c) Mass depends on how much stuff is present in an object.

d) The mass of an object is variable and dependent upon its location.

e) An object would have more mass on Mount Everest than the same

object in the middle of Lake Michigan.

f) People in Weight Watcher's are really concerned about their mass

(they're mass watchers).

g) The mass of an object can be measured in pounds.

h) If all other variables are equal, then an object with a greater mass

would have a more difficult time accelerating.

i) The mass of an object is mathematically related to the weight of

the object.

ANSWER: B, C, F, H, I

Newton’s First Law- states an object

in rest will remain at rest, or an

object in motion will remain in

motion unless a force(unbalanced

force) acts upon it.

Also known as the Law of Inertia

What will happen

when the car

hits the wall?

1. While riding a skateboard you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.

2. Headrests are placed in cars to prevent whiplash injuries during rear-end collisions

Do Not Copy Other Real life examples:

1. Blood rushes from your head to your feet while quickly stopping when

riding on a descending elevator.

2. The head of a hammer can be tightened onto the wooden handle by

banging the bottom of the handle against a hard surface.

3. A brick is painlessly broken over the hand of a physics teacher by

slamming it with a hammer. (CAUTION: do not attempt this at home!)

4. To dislodge ketchup from the bottom of a ketchup bottle, it is often turned

upside down and thrusted downward at high speeds and then abruptly

halted.

5. Headrests are placed in cars to prevent whiplash injuries during rear-end

collisions.

1. Inertia - the tendency of an object to resist a change in its motion

2. Inertia depends Only on the mass of an object

More mass = more inertia Examples:

In car crashes, you tend to remain in motion until you are acted on by a force (until you hit something that resists you)

Seatbelts

Prevents people from being thrown from the car- provides unbalanced force to stop inertia

Increases time over which patient slows

DO NOT COPY- CHECKING YOUR UNDERSTANDING

1. A 2-kg object is moving horizontally with a speed of 4 m/s. How

much net force is required to keep the object moving at this speed

and in this direction?

Answer: 0 N

An object in motion will maintain its state of motion. The presence of

an unbalanced force changes the velocity of the object.

2. Mac and Tosh are arguing in the cafeteria. Mac says that if he

flings the Jell-O with a greater speed it will have a greater inertia.

Tosh argues that inertia does not depend upon speed, but rather

upon mass. Who do you agree with? Explain why.

Tosh is correct. Inertia is that quantity which depends solely upon

mass. The more mass, the more inertia.

3. Supposing you were in space in a weightless environment, would it

require a force to set an object in motion?

Absolutely yes!

Even in space objects have mass. And if they have mass, they have

inertia. That is, an object in space resists changes in its state of

motion. A force must be applied to set a stationary object in motion.

Newton's laws rule - everywhere!

1. The unbalanced force acting on an object equals the object’s mass times its acceleration

m F

a F = ma

a=acceleration (m/s2)

m-mass (kg)

F= force (N) newtons

1N = 1kg x m/s2

m

F

a

1. Determine the accelerations which result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object

A 3-kg object experiences an acceleration of 4 m/s/s. A 6-kg object experiences an acceleration of 2 m/s/s.

2. An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

4000N/1000kg=4m/s2

1. For every action force, there is an equal and opposite reaction force.

2. Forces occur in pairs

4. The action force and the reaction force occur to different objects so the force is not balanced. (movement occurs)

1. You push on wall & wall pushes on you

2. Swimming = you push on water and

water pushes on you

3. The hammer exerts a force on the nail

to the right. The nail exerts an equal but

opposite force on the hammer to the

left.

4. The rocket exerts a downward force on

the exhaust gases.

5. The gases exert an equal but opposite

upward force on the rocket.

DO NOT

COPY

If you dropped a 1.5 kg book and a 15 kg rock from the same height, which would hit the ground first?

ANSWER:

They would

hit at about

the same

time

The extra mass of the heavy object exactly compensates for the additional gravitational force.

A heavier object experiences a greater gravitational force than a lighter object

But –

A heavier object is also harder to accelerate because it has more mass (greater inertia)