Momentum (WJEC GCSE Physics)

• A moving object has momentum which is defined by the equation:

• Where:
  • p = momentum in kilogram metre per second (kg m/s)
  • m = mass in kilograms (kg)
  • v = velocity in metres per second (m/s)

• This means that an object at rest (i.e. v = 0) has no momentum

• Momentum keeps an object moving in the same direction
  • It is difficult to change the direction of an object with a large momentum

• Since velocity is a vector the momentum of an object also depends on its direction of travel
  • This means momentum can be either positive or negative

• If an object travelling to the right has positive momentum, then an object travelling in the opposite direction (to the left) will have negative momentum
  • This isn't a solid rule, but in questions, usually the positive direction is to the right and negative to the left

How does the momentum of a ball change after a collision?

The momentum of the tennis ball is positive as it approaches the wall and negative after the collision as it moves in the opposite direction