Topic One (1.1): Energy in Collisions

Introduction

This topic extends the idea of conservation of momentum to elastic collisions, in which, because energy stored kinetically is conserved, useful information can also be found by calculating the changes in energy stored kinetically in the colliding objects. It is a continuation of the theme of potential energy and kinetic energy and momentum in Module 2 (Topic 2.1 and Topic 2.4) of the AS Level course. Many sports and games, such as baseball and ping-pong, illustrate the ideas of momentum and collisions. Usually, elastic is taken to imply that energy stored kinetically is conserved. In some texts, this is written as perfectly elastic. Inelastic describes a collision in which some energy stored kinetically is dissipated or stored less usefully.

Reading

Edexcel International AS/A Level Physics Student Book 2 pages 2 to 5

Edexcel International A Level Physics Lab Book pages 36 to 40

Tasks

Complete the following:

  • Read pages 4 to 5 of the textbook and carefully work through the examples.
  • Read the Practical Skills box on page 5. It contains an experiment on which you may be tested in the final examination.

WATCH – Elastic and Inelastic Collisions (YouTube) Links to an external site.

WATCH – Elastic/Inelastic Collision Demonstration (YouTube) Links to an external site.

WATCH – Energy Lost in an Inelastic Collision: Sample Physics Problem (YouTube) Links to an external site.

WATCH – Momentum (YouTube) Links to an external site.

SIMULATION – Newton’s Cradle (Walter Fendt) Links to an external site. Is this an example of elastic or inelastic collision?

SIMULATION – Collision Lab (PHET) Links to an external site. Assume elastic 1D collisions for now and practice using various masses and velocities (this is a HTML 5 prototype)

SIMULATION – Collision Cart Interactive (Physics Classroom) Links to an external site. Complete the activity ‘Exercise 2’

Extend

  • Stretch and Challenge – Discuss how a rocket ship works, as a controlled explosion in which reaction mass travels backwards. The rocket needs nothing to lift off except the expended fuel.

Practical Knowledge

  • Investigate the relationship between the force exerted on an object and its change of momentum. (Core Practical 9)

WATCH – A-Level Core Practical 9 (Vimeo: Abingdon Physics Videos) Links to an external site.

  • Work through pages 36 to 40 in the Lab Book.

Download REVIEW – Core Practical 9 Answers

Top Tips

Make sure that you understand the following key points:

  • The differences between the conservation laws can be confusing (momentum is always conserved and total energy is always conserved, but kinetic energy is not always conserved).
  • When you are deciding whether a collision is elastic or inelastic, you must only consider the conservation of kinetic energy. Total energy in all forms must always be conserved.
  • Remember that the letter delta LaTeX: \Delta Δ is a shorthand for change in.
  • Newton’s 3rd law of motion relies on the principles of conservation. The pair of interacting systems must conserve momentum.
  • Momentum is a vector so has direction.

Key Terms

Add the following key terms with definitions to your word list:

  • Elastic collision
  • Inelastic collision