Newton’s 1st and 3rd Laws

How do you start and stop the motion of an object?

Newton’s First Law

• If you know how to start and stop the motion of objects you know something about Newton’s First Law

• The Law states:• An object at rest will remain at rest and an object in motion will

remain in motion unless acted on by an unbalanced force

• Without an unbalanced force, a moving object will keep moving and an object at rest will remain motionless

What happens to your body when…..

Inertia

• The forward movement of your body is caused by inertia• The brakes stop the car but not your body, so your body keeps

moving forward

• Inertia is the tendency of an object to resist a change in motion

Inertia and Mass

• The inertia of an object depends on its mass

• Objects with greater mass have greater inertia

• To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object

Newton’s 3rd Law

Newton’s 3rd Law

• Yesterday we talked about how to start and stop the motion of an object

• What did we say about how to start a skateboard?

• Why does the skateboard move in the opposite direction of the push from your foot?

Newton’s 3rd Law

• The law states:• Every action has an equal and opposite reaction

• Forces always act in pairs• First an action occurs

• Pushing against the ground with your foot on a skateboard• Then a reaction occurs

• The skateboard moves forward

Action and Reaction Forces

• The reaction force is always equal in strength to the action force but in the opposite direction• Blue arrow = action force• Green arrow = reaction force

• These forces can be represented with arrows• Way arrow points = direction of force• Size of arrow = strength of force