WEBVTT

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Here is the problem: A 60 kg object moving at 5 m/s collides and sticks to a 40 kg object moving at 0 m/s. What is their common velocity?

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One-dimensional collision; positive velocities point one way, negative the other.

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Total momentum before.

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Perfectly inelastic: the objects move together.

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Kinetic energy before and after.

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So the answer is v equals 3 m/s (perfectly inelastic; 300 J of kinetic energy lost, 40 percent).

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Momentum is conserved in every collision when no external force acts. Kinetic energy is conserved only in elastic collisions; in a perfectly inelastic collision the objects stick together and the maximum possible kinetic energy is converted to heat, sound and deformation.

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Momentum before 300 equals after 300 kg times m/s, which checks out.