Edexcel A Level Physics

Revision Notes

6.5 Elastic & Inelastic Collisions

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Elastic & Inelastic Collisions

  • In both collisions and explosions, momentum is always conserved
  • However, kinetic energy might not always be
  • A collision (or explosion) is either:
    • Elastic – if the kinetic energy is conserved
    • Inelastic – if the kinetic energy is not conserved

  • Collisions happen when objects strike against each other
    • Elastic collisions are commonly those where objects colliding do not stick together; instead, they strike each other then move away in opposite directions
    • Inelastic collisions are commonly those where objects collide and stick together after the collision

Elastic & Inelastic Collisions, downloadable AS & A Level Physics revision notes

Elastic collisions are those following which objects move away in opposite directions. Inelastic collisions are where two objects stick together

  • An explosion is commonly to do with recoil
    • For example, a gun recoiling after shooting a bullet or an unstable nucleus emitting an alpha particle and a daughter nucleus

  • To find out whether a collision is elastic or inelastic, compare the kinetic energy before and after the collision
  • The equation for kinetic energy is:

Kinetic energy equation, downloadable AS & A Level Physics revision notes

Worked example

Trolley A of mass 0.80 kg collides head-on with stationary trolley B at speed 3.0 m s–1. Trolley B has twice the mass of trolley A.

The trolleys stick together and travel at a velocity of 1.0 m s–1. Determine whether this is an elastic or inelastic collision.

Collisions Worked Example Answer (1), downloadable AS & A Level Physics revision notesCollisions Worked Example Answer (2), downloadable AS & A Level Physics revision notes

Worked example

Discuss whether a head-on collision between two cars is likely to be an elastic or inelastic collision. 

Step 1: Define an elastic and inelastic collision

    • An elastic collision is one in which kinetic energy is conserved 
    • An inelastic collision is one in which kinetic energy is not conserved, but is transferred to other forms, e.g. heat and sound

Step 2: Describe the effects of head-on car collisions

    • When cars collide, a large amount of kinetic energy is transferred due to work by internal forces
    • This is mainly due to crumpling where the collision of the car causes plastic defamation of the car's bodywork
    • Other energy transfers will include kinetic energy into heat and sound

Step 3: Link the effects to energy transfers

    • Since the cars are brought to rest by the collision, the total KE before the collision does not equal the total KE after
    • Therefore, the collision is inelastic

Exam Tip

If an object is stationary or at rest, its velocity equals 0, therefore, the momentum and kinetic energy are also equal to 0.

When a collision occurs in which two objects stick together, treat the final object as a single object with a mass equal to the sum of the two individual objects.

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