Which principle explains the magician's tablecloth trick where the dishes remain undisturbed?

• A. Gravitational force depends directly on mass
• B. Objects at rest tend to remain at rest
• C. Force equals mass times acceleration
• D. An object traveling at constant velocity has no net force on it

Answer: (B)

Inertia is the key idea: objects at rest tend to stay at rest unless a net external force acts on them. When the tablecloth is snapped away, the horizontal force on each dish is brief and not large enough to overcome the dish’s resistance to a change in motion. Because of this inertia, the dishes tend to maintain their initial state, so they stay essentially in place while the cloth moves out from underneath. Friction between the cloth and dishes may help a bit, but the main effect is that the dishes resist the sudden acceleration, making the trick look like the dishes remained undisturbed. The other statements describe forces and motions that don’t directly capture why the dishes don’t follow the cloth.