# 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

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

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

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

Inelastic collision example

#### Exam Tip

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

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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