OT: that reminds me of an interesting physics problem.

If you have a ball sitting on the floor in the aisle of a stationary bus and the bus starts accelerating forward the ball rolls toward the back of the bus. If you have a bus moving at a constant velocity and it start decelerating the ball rolls toward the front of the bus.

Suppose you also have a helium balloon floating in the bus. Does it also move toward the back of the bus when the bus accelerates and toward the front when the bus decelerates? Or does it stay where it is? Or does it move toward the front when the bus accelerates and toward the back when the bus decelerates?