Newton’s Laws of Motion

Newton’s First Law of Motion

Newton’s First Law of Motion

“Objects tend to keep on doing whatever it is that they are doing unless another force acts on them”

The Law of Inertia!!Objects will stay in motion/at rest

until acted upon by a force

Mass is a measure of Inertia• The mass of an object corresponds to it’s

inertia.– Ex. Which has more inertia:

– An elephant or a feather?– A car or a semi truck?

The more mass an object has, the more inertia it has!!

Mass is not the same as Volume

Mass: a measure of the amount of matter in an object

Volume: a measure of the amount of space occupied by an object

Mass is not the same as weight!!Mass: the amount of matter in an objectWeight: A measure of gravity’s pull on an object

Newton’s Second Law of Motion

Newton’s 2nd Law of Motion

• The force acting on an object is equal to the mass of the object multiplied by its acceleration.

Force = mass x acceleration

F = m • a

Acceleration

Definition: the rate of change of velocity

An object is said to accelerate when it does any one of the following:

• Slows down

• Speeds up

• Changes direction

Which of the following shows an example of acceleration? Why or why not?

• A car going from a dead stop to 60 mph.

• A car going around a turn while maintaining a constant speed.

• A car slowing down as it rounds a turn.

• A car coming to a stop.

• A car going down a straight stretch of road at 35 mph.

Which car or cars (red, green, and/or blue) are undergoing an acceleration? Study each

car individually to determine the answer.

Forces and Acceleration

• In order for an object to accelerate, a force has to act on it (either a push or a pull)

• Usually, more than one force acts

• Acceleration depends on the net force

Newton’s Third Law of

Motion

Newton’s Third Law of Motion

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Or more simply…

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

Forces

• Forces are not simply single actions (a push or a pull)

• They are really interactions between two objects– Ex. A car crashing into a tree:

• The car exerts force on the tree

• The tree exerts force on the car

Forces always occur Forces always occur in pairs!!!in pairs!!!

What is the force pair acting on you as you sit in your chair at your desk?

What is the force pair acting as you walk across the floor?

Action and Reaction Forces

• Neither can exist without the other

• They are equal in strength and opposite in direction

Action and Reaction Forces

• Action: Object A exerts a force on

object B.

• Reaction: Object B exerts a force

on object A.

Which exerts more force, the Earth pulling the moon, or the

moon pulling the earth?

Give the action/reaction forces that would occur when a football player

tackled another football player.

What is the action reaction force pair when a

cannonball is shot from a cannon?

• The force the cannon exerts on the cannonball is exactly the same as the force the cannonball exerts back on the cannon.

• Why then, if the forces are the same, does the cannon not accelerate back as fast as the cannonball accelerates forward?