1 January

Notes

1 January 2001 Number 09 Linear Momentum 1. What is linear momentum? The linear momentum of a body is defined as the product of its mass and its velocity p = mv p = momentum (kgms-1) m = mass (kg) v = velocity (ms-1) Tip: The “linear” in linear momentum is there to distinguish it from angular momentum. A body has linear momentum if it is moving along; it has angular momentum if it is turning. Angular momentum will be covered in a later Factsheet. Since velocity is a vector, linear momentum is a vector quantity – it has both magnitude and direction. Its units are kgms-1 or, equivalently, Ns (exercise: show these units are equivalent). Example 1. What is the momentum of a body of mass 50 grammes moving at 20 kmh -1 due North Tip: If you would be tempted to put 50
20 = 1000 as your answer to this example – go back to the definition of momentum and look at the units! First we must change everything to the appropriate units – kg for mass and ms-1 for velocity 50 grammes = 0.05 kg 20 kmh -1 is 20
1000 = 20 000 metres per hour which is 20 000  3600 = 9 50 ms-1 Note: it is sensible to keep this answer as a fraction to avoid rounding errors So momentum = 0.05
9 50 = 0.278 kgms-1 due ...