Guide to Mechanics pp 93-118 | Cite as

# Collisions

Chapter

## Abstract

In Chapter 3, the term momentum was defined as the product of mass and velocity. It has particular significance when we study collisions. Let us demonstrate this with a simple example. A heavy lorry and a car are travelling side by side at the same speed and need to stop at a red light. Common experience leads us to expect that the braking force required to bring the lorry to a stop must be greater than that required to stop the lighter car. Newton’s second law states that:
or, for constant mass:
The right-hand side of this equation represents the rate of change of momentum. Hence, the lorry, which has the larger momentum, needs a greater force to make it stop than does the car.

$$force = mass\,\; \times \,\frac{{d(velocity)}}{{dt}}$$

$$force = \,\frac{{d(mass\,\; \times velocity)}}{{dt}}$$

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## Copyright information

© Philip Dyke & Roger Whitworth 1992