Monday, 14 May 2012

Shooting for momentum

Momentum is a tricky concept to grasp.  It’s not entirely clear, at least to students, what the advantage of using momentum is compared with say, velocity or force.

It doesn’t help that it’s quite tricky to clearly show differences in momentum.  The typical example that I used to use was to compare a truck and a car, or perhaps a motorhome and sit-on toy.
The question is then, which would you prefer to be hit by?  Why? 

A large motorhome ... would hurt if it hit you!
Toy Tractor - would still hurt, but not as much

However, the difficulty here is that it’s difficult to separate out the ideas of size, mass, and speed from momentum.

I wanted to think of a way that would allow a more direct comparison of objects for a discussion of momentum.  One of my colleagues uses a Nerf gun as a demo of momentum, and I wondered if it would be possible to adapt it to make some more quantitative measurements of momentum.  She has stuck a ballbearing onto one of the darts for the gun. She then asks a volunteer to stand against a wall and asks them which they would prefer to be shot with. 

Which would you want to be shot at you?  Why? 
Most people choose the dart without the ballbearing.  Are they right?  Let's see.

Adapting the demo:
Step 1: Get hold of a Nerf gun (or in my case, cheap replica).
Bought from a cheap shop!

Step 2: Use blu-tac and small washers to add mass to two of the darts.  I stuck one washer and two washers on the darts to see if there was a relationshop between the distance they flew and the added mass.

The prepared experiment!

Step 3: Find a large room and fire the three darts, keeping the gun straight and as close to the same position as possible.

In that classic POE process – predict what you would expect to happen?  Why?
Step 4: Observe. 
As I expected the darts flew different distances – with the unweighted dart going the furthest.  I tried this a number of times and each time the lightest dart flew the furthest.

The advantage of using the gun is that the force applied in each case is the same, and the shape of the darts are (pretty much) the same.   

This makes it easier to discuss the idea of momentum as a product of mass x velocity and not get hung up on being run over by a motor home.

I’m planning on repeating my experiment, but actually taking measurements, and possibly videoing and then analysing the results to calculate momentum.  Details to follow…