Describing Motion

Newton’s Laws

First we need to define the word FORCE:

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

• Two types of forces– Pushes– Pulls

Forces are measured in Newtons

• SI unit of force

• Symbol: N

• Measured by using a spring scale

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

Newton’s Laws

• First Law – Inertia

• Second Law – Acceleration, Force & Mass

• Third Law – Action-Reaction

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]

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

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Second Law

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

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

object

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

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

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.

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 stops the ladder?

Gravity.

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.

Why does the car move [accelerate] when it is hit by the truck?

The heavy and moving truck has more force than the small, at rest car.

Why does the truck stop moving when it hits the car?

The force of the car pushing back on the truck, plus the force of friction between the massive truck and the road slow down, the stop the truck.

The truck hits the car. An action force stops the truck.

What is the equal and opposite reaction force?

The force that pushes the car forward.