• Philip Dyke
  • Roger Whitworth
Part of the Macmillan Mathematical Guides book series (MG)


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:
$$force = mass\,\; \times \,\frac{{d(velocity)}}{{dt}}$$
or, for constant mass:
$$force = \,\frac{{d(mass\,\; \times velocity)}}{{dt}}$$
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.


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Copyright information

© Philip Dyke & Roger Whitworth 1992

Authors and Affiliations

  • Philip Dyke
    • 1
  • Roger Whitworth
    • 2
  1. 1.Polytechnic South WestUK
  2. 2.Droitwich High SchoolUK

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