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Chapter 4 & 5 - Newton’s
3rd Law of Motion and
Momentum
Action - Reaction
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Newton’s Third Law
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To every action there is
always an opposed equal
reaction
We call one force the action
force and the other the reaction
force.
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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
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Action-Reaction Examples
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• 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
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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
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Tug 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
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Greatest Force against the
Ground WINS!
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Helicopter Example
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• 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
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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
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Increasing 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
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Decreasing 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
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window.
Decreasing 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
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Law of Conservation of
Momentum
• In the absence of an external force,
the momentum of a system remains
unchanged
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Law of Conservation of
Momentum
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Collisions
• 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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Collisions
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Collisions and the
Conservation of Momentum
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