Slide a book across a carpet and across a smooth table. On the carpet it stops quickly. A force is pushing against it: friction.
Key idea
Friction is a force between two surfaces that rub together. It acts against the motion. Rough surfaces make more friction, smooth ones less. Gravity is a pull between things; Earth's gravity pulls everything toward the center of the Earth.
Friction is often helpful. It lets your shoes grip, your bike brakes stop and a match light. Sand on an icy path adds friction. Oil and wheels make friction smaller.
Why it works
Every surface has tiny bumps, even ones that feel smooth. When two surfaces slide, the bumps catch on each other.
"Down" is toward the center of the Earth everywhere, which is why people in Australia do not fall off. Gravity also holds the Moon near the Earth.
Worked example: Which falls first?
You drop a heavy book and a light book flat from the same height at the same time. Which lands first?
- Gravity makes both fall faster and faster at the same rate.
- Air pushes on both, but these are both heavy and flat-shaped enough that it hardly matters.
Answer: They land at almost the same time.
Common mistake
Thinking heavy things always fall much faster. A feather falls slowly because air pushes it, not because it is light. On the Moon, which has no air, astronaut David Scott dropped a hammer and a feather in 1971, and they landed together.
Try it at home
- Make a ramp with a board or a big book. Let a toy car roll down onto a bare floor, then onto a towel.
- Measure how far it goes each time. Which surface has more friction?
Going further. In the Book Cart lab, push a cart and change the friction.