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Describing Motion

Newton’s Laws

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First we need to define the word FORCE:

• The cause of motion (what causes objects to move)

• Two types of forces– Pushes– Pulls

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Forces are measured in newtons

• SI unit of force

• Symbol: N

• Measured by using a spring scale

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Forces may be balanced or unbalanced

• Balanced forces – all forces acting on an object are equal– There is NO MOTION

• Unbalanced forces – one or more forces acting on an object are

stronger than others – There is MOTION

• A NET FORCE

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Newton’s 3 Lawsdescriptive names

• First Law – Inertia

• Second Law – Acceleration, Force & Mass

• Third Law – Action-Reaction

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First Law

• Inertia– An object at rest

[not moving] remains at rest unless acted on by a force [push or pull]

– An object in motion remains in motion unless acted on by a force [push or pull]

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more First Law• Inertia & Mass

– Mass is the amount of matter in an object

– The more MASS an object has, the more INERTIA the object has.

– Bigger objects are harder to start & stop

http://toons.artie.com

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Second Lawforce = mass x acceleration

• Acceleration & Mass Definitions– Acceleration is a change in velocity

[speed or direction]– Mass is the amount of matter in an

object

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more Second Law

• Acceleration & Force– The more force placed on an object,

the more it will accelerate [change its motion]

• Acceleration & Mass– The more mass [or inertia] an object

has, the more force it takes to accelerate the object

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Third Law

• Action – Reaction– Forces are always produced in pairs

with opposite directions & equal strengths

– For every force there is an equal and opposite force

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The truck is in motion. What is the force that causes it to stop?

The push of the stopped car.

The car is at rest. What is the force that causes it to move?

The push of the truck.

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What about the ladder on top of the truck?

The ladder is in motion because the truck is in motion.

When the truck stops, the ladder stays in motion.

The truck is stopped by the force of the car, but the ladder is not.

What force makes the ladder fall while it is moving horizontally?

Gravity.

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The truck is in motion, the car is at rest. How do each of these vehicles accelerate?

The truck stops moving. The car starts moving.

Which one will be the hardest to accelerate?

The truck because it has the most mass.

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In which direction does the truck push the car?

To the right.

In which direction does the car push the truck?

To the left.

left right

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The truck hits the car. Let’s call this an action force (to the right).

What is the equal and opposite reaction force?

The car’s force on the truck (to the left).

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