Chapter 4 & 5 - Newton’s 3rd Law of Motion and Momentum

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Chapter 4 & 5 - Newton’s 3rd Law of

Motion and Momentum

Chapter 4 & 5 - Newton’s 3rd Law of

Motion and MomentumAction - ReactionAction - Reaction

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Newton’s Third LawNewton’s Third Law

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To every action there is always an opposed

equal reaction

To every action there is always an opposed

equal reaction

We call one force the action force and the other the

reaction force.

We call one force the action force and the other the

reaction force.

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Simple Rule to Identify Action Reaction

Simple Rule to Identify Action Reaction

• Action: Object A exerts a force on object B

• Reaction: Object B exerts a force on object A

• Example: Man pushes on wall, wall pushes back at man

• Action: Object A exerts a force on object B

• Reaction: Object B exerts a force on object A

• Example: Man pushes on wall, wall pushes back at man

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Action-Reaction Examples

Action-Reaction Examples

• Cannon is fired - Cannon exerts force on ball (accelerates ball - ball exerts force on cannon (it recoils)

• Rocket pushes on ground ground - ground pushes back

• Cannon is fired - Cannon exerts force on ball (accelerates ball - ball exerts force on cannon (it recoils)

• Rocket pushes on ground ground - ground pushes back

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• Balloon exhaust (air) pushes on outside air molecules - air molecules push back

• Horse pushes on ground - ground pushes back

• Balloon exhaust (air) pushes on outside air molecules - air molecules push back

• Horse pushes on ground - ground pushes back









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Action-Reaction act on Different objects

Action-Reaction act on Different objects

• Action-Reaction forces do not cancel out because they act on different objects

• Two forces acting on the same object can cancel out each other producing a net force of zero

• ∑F=0

• Action-Reaction forces do not cancel out because they act on different objects

• Two forces acting on the same object can cancel out each other producing a net force of zero

• ∑F=0

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Tug of War ExampleTug of War Example

• Each team pulls and produces enough rope tension to keep the rope from accelerating (Equal and opposite forces)

• The winning team?• The team that produces more force

against the ground, not the team that exerts the greatest force on the rope, wins the Tug O’ War

• Each team pulls and produces enough rope tension to keep the rope from accelerating (Equal and opposite forces)

• The winning team?• The team that produces more force

against the ground, not the team that exerts the greatest force on the rope, wins the Tug O’ War

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Greatest Force against the Ground WINS!

Greatest Force against the Ground WINS!





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Helicopter ExampleHelicopter Example

• The whirling blades are shaped to push air particles down (action)

• The air forces the blades up (reaction)

• This upward force is called lift

• The whirling blades are shaped to push air particles down (action)

• The air forces the blades up (reaction)

• This upward force is called lift





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Summary of Newton’s 3 Laws

Summary of Newton’s 3 Laws


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Momentum and Collisions

Momentum and Collisions

• Momentum = mass x velocity• P = m x v• Impulse = change in momentum• Or, change in m x v in a time

interval

• Momentum = mass x velocity• P = m x v• Impulse = change in momentum• Or, change in m x v in a time

interval

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Increasing MomentumIncreasing Momentum

• 1) Apply the greatest force you can• 2) Extend the time of application of the

force• Example: Long barreled cannons• Long barrels produce higher velocities than

short ones as long as the force remains relatively large!

• Why? The force of the exploding gun powder acts on the cannonball for a longer time

• 1) Apply the greatest force you can• 2) Extend the time of application of the

force• Example: Long barreled cannons• Long barrels produce higher velocities than

short ones as long as the force remains relatively large!

• Why? The force of the exploding gun powder acts on the cannonball for a longer time

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Decreasing MomentumDecreasing Momentum

• Long time means less force, if force is able to decrease

• Example: you are in a fast-moving car and the brakes fail. Would you want to stop by hitting a brick wall or hit a haystack and move until you eventually come to rest?

• By hitting the haystack you extend the contact time (time in which your momentum is brought to zero by opposing forces)

• Force is reduced and you survive • If force is constant or is able to increase over time

than - Long time less force is thrown out the window.

• Long time means less force, if force is able to decrease

• Example: you are in a fast-moving car and the brakes fail. Would you want to stop by hitting a brick wall or hit a haystack and move until you eventually come to rest?

• By hitting the haystack you extend the contact time (time in which your momentum is brought to zero by opposing forces)

• Force is reduced and you survive • If force is constant or is able to increase over time

than - Long time less force is thrown out the window.

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Decreasing MomentumDecreasing Momentum

• Short time means more force (inversely related)

• Example: When boxing, if you move into the punch you are in trouble. Move away and the force is decreased, force is given chance to decrease.

• Example: Breaking a stack of bricks. Would you want a short quick blow or a long extended blow? Remember: force increases with more momentum, quicker blow, more momentum, thus more force

• Short time means more force (inversely related)

• Example: When boxing, if you move into the punch you are in trouble. Move away and the force is decreased, force is given chance to decrease.

• Example: Breaking a stack of bricks. Would you want a short quick blow or a long extended blow? Remember: force increases with more momentum, quicker blow, more momentum, thus more force

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Law of Conservation of Momentum


• In the absence of an external force, the momentum of a system remains unchanged

• In the absence of an external force, the momentum of a system remains unchanged

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CollisionsCollisions

• Elastic - When objects collide without being permanently deformed and without generating heat

• Inelastic - Whenever colliding objects become tangled or coupled together• In a perfect inelastic collision both

objects stick together.

• Elastic - When objects collide without being permanently deformed and without generating heat

• Inelastic - Whenever colliding objects become tangled or coupled together• In a perfect inelastic collision both

objects stick together.

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CollisionsCollisions

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Collisions and the Conservation of

Momentum

Collisions and the Conservation of

Momentum


Documents

Chapter 4 & 5 - Newton’s 3rd Law of Motion and Momentum