CIE A Level Physics复习笔记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
Worked Example
Two similar spheres, each of mass m and velocity v are travelling towards each other. The spheres have a head-on collision. What is the total kinetic energy after the impact?
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
Worked Example
Two trolleys X and Y are of equal mass. Trolley X moves towards trolley Y which is initially stationary. After the collision, the trolleys join and move off together. Prove that this collision is inelastic.
Exam Tip
Although kinetic energy may not always being conserved, remember momentum will always be conserved.
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