Impulse

Momentum is always conserved, so how can it change?

You have two objects X & Y.They collide. X will experience a change in momentum. So will Y. If you add up the total momentum (of X & Y) before the collision it will equal the total momentum (of X & Y) after the collision (the conservation of momentum principle).

Units

Impulse/Newton seconds/Ns

Force/ Newtons/ N

Time/ Seconds/ s

Momentum/ kgms-1

Impulse- the basic ideas

Whenever two objects collide, there is an impulse. The impulse causes a change of momentum. In fact, the impulse is equal to the change of momentum. So we can write

Impulse= change in momentum (1)

If an object changes its momentum, there must have been force acting for a period of time. This is an important point, so we will say it again. A force causes a change in momentum. Some students get this the wrong way around and think the change in momentum creates the force, but this is not right. The force comes first and this causes a change in momentum. The longer the force acts, the greater the change in momentum. We can write

Force x time=change in momentum (2)

Combine (1) and (2) and you have

Impulse=Force x Time= Change in Momentum Ft= mv-mu

How to find the impulse in an exam

If you are asked to calculate an impulse you will find the answer from one of the 4 ways listed below, so learn them all. Easy marks available here.

  • If you can spot a constant force (F) and the time (t), then it's easy (F x t)
  • If you find that the force is changing (they'll probably give you a graph) what value of force do you use. You use the average force and multiply this by the time taken
  • If you are given a graph showing force and time, then you are in luck, you can find the impulse by working out the area under the graph.
  • If you are able to calculate the momentum before and after the collision then the impulse can be found by working out the change of momentum.

    Exam tip-How to calculate momentum and velocities from Impulse

    If you have calculated the Impulse, you have also calculated the change in momentum. As you know, they are equal to each other.

  • If the object starts from rest, then the change in momentum is the same as the final momentum
  • If the objects stops moving, then the change in momentum is the same as the intial momentum.
  • If you know the mass of the object, then its easy to work out its intial or final speed.

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    • (c) The Undercover Physicist 2007.