Forces and Motion

Steve CaseUniversity of Mississippi

NSF North Mississippi GK-8September 2005

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What words can you think of to describe the motion of an

object?

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VELOCITY

The word scientists use to describe how fast an object is moving

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Velocity is measured as:

change in distance

change in time

a change in distance per change in time

What are some things you can think of that are measured like this?

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What are some examples of objects that move at high

velocities?

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What are some examples of objects that move at low

velocities?

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What units would you use to measure the velocity of an object moving in this classroom?

meters per second

m

s

Meters per second tells us how many meters something can move in one second.

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What else can we know about an object’s motion besides velocity?

• What if the object slows down?

What if the object’s velocity changes?

•What if the object speeds up?

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Scientists have a special way to measure a change in

velocity:

•Acceleration

Acceleration is a change in velocity.

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• If you’re speeding up, you have positive acceleration.

• If you’re slowing down, you have negative acceleration.

• If you’re changing speed quickly, you have high acceleration.

• If you’re changing speed slowly, you have low acceleration.

The Way Acceleration Works:

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Example:

You’re driving along in your car and suddenly have to slam on your brakes for a red light. Negative or positive acceleration? High or low acceleration?

High negative acceleration because you’re slowing down quickly.

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You’re an astronaut waiting in your shuttle for takeoff. Suddenly the rockets fire and you’re on your way to Mars. High or low positive or negative acceleration?

High positive acceleration because you’re speeding up quickly.

Example:

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You’re driving down the highway when your car runs out of gas and you slowly roll to a stop on the side of the road.

Low negative acceleration because you’re slowing down slowly.

Example:

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You’re walking to class when you realize you’re running just a little bit late. You start walking a bit fast, but you still hope to make it on time. Finally the bell rings so you break into a jog and then a run, trying to get to class before the teacher notices you’re late.

Low positive acceleration because you’re speeding up slowly.

Example:

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Challenge Question:

You throw a baseball straight up in the air. It rises to a certain height and then falls back to earth, where you catch it again in your glove.

First, high positive acceleration because it speeds up quickly as it leaves your hand.

Then, as it rises, low negative acceleration, because it begins to slow down slowly because of gravity.

As it begins to fall again, low positive acceleration, because it speeds up slowly.

Finally, when you catch it, high negative acceleration because it slows down very quickly in your hand.

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How Do I Graph the Motion of a Particle?

Distance vs. Time

02468

101214

0 1 2 3 4 5

Time (seconds)

Dis

tan

ce (

met

ers)

Car 1

Car 2

Car 3

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Quick Review

• Velocity: speed of motion• Acceleration: change in velocity

What causes an object’s velocity to change?

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• If you’re riding your bike and want to speed up, what do you do?

• If you’re driving a car and want to slow down,

what do you do?

• If you’re playing soccer and want to move the ball,

what do you do?

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Pushing pedals, pressing brakes, and kicking soccer balls are all examples of

forces.A force is anything that changes the velocity

of an object.

But what’s a change in velocity?

A change in velocity is acceleration, so . . .

A force is anything that gives an object acceleration.

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Nothing will change speed unless acted upon by a

force. If it’s sitting still, it will keep sitting still. If it’s

moving fast, it will keep moving fast.

Only a force can change the velocity of an object.

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Then why do things slow down?

• Flying baseballs slow down and eventually stop because of the force of gravity.

• Cars and bikes eventually slow down and stop because of the force of friction.

• Spacecraft in outer space with no friction and no gravity will keep going at the same speed forever without a force to slow them down or speed them up.

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Final Review

• What is an object’s speed of motion?• What is a change of an object’s

velocity?• What is the only thing that can

change an object’s velocity and give it acceleration?