Release a cart on a live ramp, mark how far it gets, then use your own data to work out how steep a ramp you cannot even see is.
A cart sits at the top of a low-friction ramp. Let go, it speeds up the whole way down, so the farther it runs the faster it is moving. Below you will get a working ramp with three knobs: the run time t you allow before a strobe fires, the cart's mass m, and the local gravitational field strength g. The ramp's tilt is fixed at 30 degrees for now. The strobe flashes at your chosen time and marks exactly where the cart is, in centimeters from the release point.
First, commit to a prediction. Then the apparatus unlocks.