What momentum is
- Momentum is a property carried by every moving object, telling you how hard it would be to stop or to turn
- A heavier object moving at the same speed has more momentum than a lighter one; the same object moving faster has more momentum than when it was moving slowly
- A stationary object has zero momentum, so at v = 0, p = 0
The equation
p = m × v
- where:
- p = momentum (kg m/s)
- m = mass (kg)
- v = velocity (m/s)
- The SI unit follows from the equation: kilograms × metres per second → kg m/s
Momentum is a vector
- Velocity carries a direction, and momentum is mass × velocity, so momentum is a vector quantity that points the same way the object is moving
- When working with momentum on a straight line:
- Pick one direction as positive (rightwards and upwards are the usual conventions)
- The opposite direction is then negative
- A momentum that comes out negative simply means the object is moving the opposite way to the direction you chose as positive
When momentum changes
- An object's momentum changes whenever any of the following happen:
- its velocity increases or decreases (any acceleration along the line of motion)
- its direction of travel changes (which is also an acceleration, even at constant speed)
- its mass changes (such as a rocket burning off fuel)
Example — work out which has the greater momentum: a 0.045 kg golf ball travelling at 80 m/s, or a 5.0 kg medicine ball travelling at 0.72 m/s.
- Golf ball: p = m × v = 0.045 × 80 = 3.6 kg m/s
- Medicine ball: p = m × v = 5.0 × 0.72 = 3.6 kg m/s
- The two have exactly the same momentum, because the medicine ball's much larger mass is offset by the golf ball's much higher speed. On impact (with the same contact time) they would feel a similar stopping force