Physics
Inertial Frame of Reference
Quick fact
Galileo famously argued that you cannot tell whether a ship is moving at constant speed or is stationary by observing the behavior of objects below decks – the first statement of the principle of relativity.
Why this is interesting
When you're sitting in a smoothly moving train, a ball on the floor stays still. But if the train brakes hard, the ball rolls forward – why does it suddenly seem to move without any apparent push?