 ### Change in Momentum (Δp) ↔ Impulse

So, what is this "change in momentum" also called IMPULSE all about?.

As an example, and objects velocity can change from one value to another. Then obviously the momentum would have changed. This "change in the momentum" is called IMPULSE.

In this diagram, the trolley has a MOMENTUM of 2 at first (initially) and then it got faster. The new final MOMENTUM is 7.
The difference in the two is called "change in momentum" or impulse. In this case the change in the momentum is 5. More formally, we write this as :

Δp = 5kg.m.s-1

The word "change" is represented by the triangle symbol Δ and is pronounced DELTA.
Hence we can say "delta p = 5 kg.m.s-1

Obviously we have a formula to make calculations easier! You are required to copy the formulae as is on the data sheet, and then you can make changes. By factorising, you get a very useful layout. Some practice questions!

Question 1
The velocity of a 4kg trolley increased from 3m.s-1 to 9m.s-1 all rightwards. 1.1 What does the term Δp mean?
change in momentum
1.2 Calculate the change in momentum of the trolley. Question 2
The velocity of a 5kg trolley changed from 4m.s-1 right to 7m.s-1 left. • Draw carefully
• The velocities are in TWO directions, so there will be TWO SIGNS
• It is wise to call the FINAL VELOCITY POSITIVE always!
• This will make the initial velocity negative
• And the formula will have TWO negatives, one from the formula itself, and the other from the initial velocity.
2.1 Why must the velocities have opposite signs?
The velocities are in opposite directions.
2.2 Calculate the change in momentum. Where did the "left" come from?
Notice that the answer has a positive sign. And according to our diagram, positive meant left!
So Δp is leftwards!