4. Loaded Grocery Cart

Big brother Thaiya cannot lift up a filled grocery bag, yet little Maiya can easily push a full cart forward. Why?

Pushing and lifting involve different forces. When Thaiya lifts upward, the opposing force is gravity acting downward. It takes a lot of strength to lift the weight of that heavy bag. Pushing a cart on wheels is much easier. Maiya just needs to push forward hard enough to overcome the opposing force of friction. Friction is the force between the cart's wheels and the floor as they slide past one another. If the wheels turn easily, and the floor is smooth, there is not much force of friction for Maiya to overcome. A cart that is too heavy to lift or drag can be rolled easily! Aren't wheels a great invention?