CIE AS Physics (9702) exams from 2022

Revision Notes

3.2.2 Elastic & Inelastic Collisions

Elastic Collisions

  • When two objects collide, they may spring apart retaining all of their kinetic energy. This is a perfect elastic collision
  • An elastic collision is one where kinetic energy is conserved

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

Equation for kinetic energy

 

  • Since kinetic energy depends on the speed of an object, in a perfectly elastic collision (head-on approach) the relative speed of approach = the relative speed of separation

 

Worked example - elastic collision, downloadable AS & A Level Physics revision notes

Elastic collision example

 

Exam Tip

Despite velocity being a vector, kinetic energy is a scalar quantity and therefore will never include a minus sign. This is because in the kinetic energy formula, mass is scalar and the v2 will always give a positive value whether its a negative or positive velocity

Inelastic Collisions

  • Whilst the momentum of a system is always conserved in interactions between objects, kinetic energy may not always be
  • An inelastic collision is one where kinetic energy is not conserved

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

Equation for kinetic energy

 

  • The kinetic energy is transferred into other forms of energy such as a heat or sound
  • Inelastic collisions can be when two objects collide and they crumple and deform. Their kinetic energy may also disappear completely as they come to a halt
  • A perfectly inelastic collision is when two objects stick together after collision, as shown in the example below

 

Worked example - inelastic collision, downloadable AS & A Level Physics revision notes

Inelastic collision example

 

Exam Tip

Although kinetic energy may not always being conserved, remember momentum will always be conserved.

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