Note: You'll have to have an understanding (and background knowledge) on the conservation of momentum and energy in order to solve this riddle. Try not to use the internet!
One fateful morning, a cart, which was parked at the top of a hill the night before, is found in a mangled mess along with a cart that was parked below it in the bottom of a lake. When the carts are found, the supervisor says that the engineer must have forgotten to put on the brake on the top cart. However, the engineer still has a counter-argument! He said that it was impossible (due to physics) that the cart could have pushed both carts (which latched on contact) over the hill and into the lake and that someone must have given it a push in order for it to have made it into the lake.
Which person is correct?
Here's a diagram with all of the necessary information:
Oh, and if you find the answer, don't make it obvious and encode it in ROT-13 or something so others can have fun figuring this out. I really had fun solving this problem.