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Momentum

Momentum

Inertia in motion
Linear momentum is the mass times velocity of
an object.
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• A vector quantity
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Bullets and trains
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Momentum

Impulse
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A change in momentum requires a force
• Why?
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How long is the force applied?
• Average Force * Time Interval
Fave Δt = Δ(mv)
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Momentum

Impulse
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Case 1: Increasing Momentum
• Large F and large t
• Cannons and rifles
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Case 2: Decreasing momentum over a Long Time
• Small F and large t
• Airbags, bungee jumping, catching a ball, train
couplings, boxing
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Momentum

Impulse
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Case 3: Decreasing momentum over a Short Time
• Large F and small t
• Boxing, karate
• Bouncing – karate, water wheels
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Momentum

Conservation of Momentum
The total momentum of an isolated system is
constant.
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• If no external net force is present then no change in
momentum is possible.
When studying collisions, we say that the
momentum before the collision must equal the
momentum after the collision.
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Momentum

Collisions
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Elastic – the KE of the system is constant
• Billiards
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Inelastic – the KE of the system is changed
• Car crashes
• Explosions – more KE after. Where does the energy
come from?
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Momentum

Inelastic Collisions
Perfectly inelastic – the objects stick together
after the collision

m1 v1i + m2 v2i = (m1 + m2) vf
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