Depends on how well built he is probably for the average American 8 MPH
Answer:
At point A, the cart has high potential energy. At point b, the cart is pulled down by gravity. At point c, the cart gains its highest kinetic energy. At point d, the cart returns back to the same state but with lower potential energy.
The planet closest to the sun; Mercury.
Sorry I had the answer but it wont let me type numbers :(.: