My homework is to make a rube goldburg machine that demonstrates 3 forms of force in about 3-4 steps and have a ball land in a cup. I can use Gravity as a force during a ramp as 1 step. I can't think of any other forms of force because my teacher doesn't teach for crap. google only pulls up star wars crap!!! HELP! What other forms of force are there that are easy to demonstrate in a sort of roller coaster rube goldburg machine?