1G20.71 - Liquid Accelerometer on Inclined Track
Elegantly shows the inertial frame of the liquid. When the cart is freely accelerating down the track, the liquid level is parallel to the incline, and unintuitive result.
A cart is released from a height on a track or launched up the track by the instructor. Initially the liquid is parallel to the earth. When the cart is launched, students must predict how the liquid will be aligned.
Initially, the liquid is level with the earth. When the cart is released or launched it experiences constant acceleration by gravity. The unique property of gravity (General Relativity) is that when you can't tell when you are being accelerated by it compared with if you are at rest (unless everything around you isn't in your frame, i.e. the ground below you should you choose to bungee-jump). For the enclosed liquid in the cart, the only force it is aware of is the normal force of the inclined track and so we see the liquid align parallel to the track, not to the earth.