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Mechanics

Elastic and inelastic collisions in one dimension

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Overview

Recall questions on what separates an elastic from an inelastic collision, where the lost kinetic energy goes, and how momentum is still conserved when a ball rebounds.

What you need to know

Answer each one before you reveal it.

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Process

How do the momentum changes of two colliding vehicles compare, and what distinguishes an elastic from an inelastic collision?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is the difference between an elastic and an inelastic collision?

    Model answer

    Kinetic energy is conserved in an elastic collision but not in an inelastic one, while momentum is conserved in both.

  2. 2

    What is a perfectly inelastic collision?

    Model answer

    It is a collision in which the bodies stick together and move off with a common velocity, losing the greatest possible amount of kinetic energy.

  3. 3

    Into what is the kinetic energy lost in an inelastic collision transferred?

    Model answer

    Mainly into the internal energy of the colliding bodies as they deform, along with a small amount as sound.

  4. 4

    A ball bounces back off a wall, so how can momentum still be conserved?

    Model answer

    The wall and the Earth attached to it are part of the system and take up an equal and opposite change of momentum, which goes unnoticed because their mass is so enormous.

  5. 5

    Why is the momentum of a stone falling freely towards the ground not constant?

    Model answer

    Its weight is a resultant external force acting on it, so the stone considered alone is not an isolated system.

  6. 6

    In terms of momentum, how does a rocket accelerate in empty space?

    Model answer

    It expels exhaust gases backwards at high speed, and since the total momentum must stay constant the rocket gains an equal magnitude of momentum forwards.

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