It would be wool. (: Like rubbing your feet against carpet ? same affect. (:
Good luck, rockstar, and I hope you pass!
Gravity largely depends on the comparison of two objects; it's why you have the equation F= (GMm)/r^2. On Earth, you have different altitudes that, with the formula, will give different results for gravity because the radius is different everywhere. This difference on calculations, however, are seen to be miniscule. We know gravity as 9.81 m/s^2 but it might be different by thousandths or hundreds of thousandths of a decimal.
The impulse J is equal to the magnitude of the force applied to the cannonball times the time it is applied:

But the impulse is also equal to the change in momentum of the cannonball:

If we put the two equations together, we find

And since we know the magnitude of the average force and the time, we can calculate the change in momentum: