CIE A Level Physics復習筆記3.2.2 Elastic & Inelastic Collisions

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月16日上午11:40

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

3.2.1.3-Kinetic-energy-equation 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? WE-Elastic-collision-question-image

WE-Elastic-collision-answer-image

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

3.2.1.3-Kinetic-energy-equation 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.

WE-Inelastic-collision-question-image