Video narration
Watch these two shopping carts receive the same push side by side. The empty cart jumps forward, while the full cart barely gathers speed. That difference feels familiar: loading a cart makes it harder to get moving quickly. But notice what changed here. The push is the same; only the carts' mass is different. The empty cart has less mass, and the full cart has more. Acceleration means how quickly an object's speed changes. So the full cart does move, but its speed builds more slowly. This is Newton's second law: F equals m a-force equals mass times acceleration. In this controlled comparison, the effective net force stays the same. If mass increases while force stays fixed, acceleration must decrease. Push both carts again, and the rule is visible. Same push, more mass, less acceleration-that's why the full cart moves off more slowly.