Momentum

The greater the force you apply to an object (say a shot) and the longer you apply this force for the greater will be the change of velocity of the object. However something else affects this change in velocity. Think about what happens if you try and throw a heavier shot?

Even if you use the same force for the same time it is obvious that you wont be able to throw a heavy javelin as fast as a light one and so it won't go so far.

To find out the change in velocity we must use the idea of MOMENTUM.




If we apply a certain impulse to an object we will get a certain change of momentum. The velocity change depends on the mass of the object:


where u is the initial velocity and v the final velocity. The units of momentum are the same as those for impulse, that is Newton seconds (Ns).

If we think of an object at rest either before or after an impulse is applied to it, then the impulse becomes either
(a) something that gives a stationary object a momentum, or
(b) something that takes the momentum away from a moving object.
The formula can be rewritten as:

where v is the velocity gained or lost. From this formula you can see that for a given
momentum you either need a large force acting for a short time or a small force acting for a long time.

We can use the idea of momentum and impulse to work out the change of speed of an object when a certain impulse acts on it.





You can feel this if you try to stop an object moving. If you catch a cricket ball with your hands still it will hurt, you are stopping it in a very short time and so the force will be large. However, if you let your hands give it makes the time longer and so the force will be smaller.

A fun demonstration of the effect of the time of stopping on the force used to stop is to use eggs! First throw them into a hard wall and then into a sheet that is being held vertically by two people. The sheet will give – the eggs will slow down slowly and won't break!

When you jump off a wall onto the ground you should always bend your knees on landing. If you did not you would be trying to stop yourself in a very short time and the force on you would be very large.

Don't by this as you may seriously damage your back.


(Note: A very good video clip of people jumping off a stool onto the ground is shown in the Multimedia Motion II CD produced by Cambridge Science Media)

Think about how momentum and impulse apply to the following cases:
Iron cricket bats or hockey sticks, the design of rigid or squashy car bumpers.



Momentum and direction change



Which has the biggest change of momentum? Actually the rubber ball because it has had its DIRECTION of motion chanqed. Momentum is a vector quantity like velocity.