Slide 1
Momentum combines mass with velocity
Momentum combines an object’s mass and velocity. A heavy slow cart can carry the same momentum as a lighter faster cart.
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In one direction, a 2-kilogram cart at 3 metres per second has momentum of 6 kilogram metres per second. A 3-kilogram cart at 2 metres per second has the same value.
Slide 2
Opposite momentum directions can cancel in the total
Momentum has a direction. Equal amounts of momentum in opposite directions can sum to zero for a whole system.
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Two equal carts approaching each other at equal speeds have zero total momentum before collision. They still possess kinetic energy, so zero momentum does not mean that nothing energetic can happen.
Slide 3
Impulse measures a force’s change of momentum over time
Impulse is the change in momentum produced by a force acting over time. A longer stopping time can reduce average stopping force.
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Stopping the same moving object in twice the time halves the required average net force. The peak force need not halve, because its detailed variation during the stopping process also matters.
Slide 4
Conservation depends on the system’s external impulse
Total momentum is conserved when the system receives no net external impulse. Internal collision forces exchange momentum between its parts.
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For two colliding carts, the track may supply small frictional impulses. Treating momentum as conserved is most accurate when the collision is brief enough that those external effects are negligible.
Slide 5
Momentum can be conserved while kinetic energy decreases
Momentum can remain conserved when objects stick together. Their total kinetic energy may decrease as deformation, sound and heating occur.
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For equal masses, one moving cart sticking to an initially stationary cart produces half the initial speed if external impulse is negligible. The combined mass doubles, preserving momentum while reducing kinetic energy.
Sources
- Momentum and Its Conservation
The Physics Classroom
- Newton’s Laws of Motion
NASA