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Newton’s 1st and 3rd Laws
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How do you start and stop the motion of an object?
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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
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What happens to your body when…..
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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
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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
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Newton’s 3rd Law
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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?
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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
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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