Slide a book across a table and it slows down. The force that slows it is friction, which the table exerts where the two surfaces touch. Like any force, it adds into the net force.
Key idea
Friction acts where two surfaces touch and pushes against their sliding (or against the sliding they would do). It is bigger when the surfaces press together harder and when they are rougher.
Why it works
Even smooth-looking surfaces have tiny bumps that catch on each other. Press harder and more bumps lock together, so friction grows. While an object is still, static friction matches your push, up to a limit; push harder than that limit and the object slides, and the smaller kinetic friction takes over.
Worked example: Not moving yet
A heavy box needs 60 N to start sliding. You push it with 45 N and it doesn't move. How big is the friction force?
- The box stays still, so the net force is zero.
- Friction must cancel your push exactly: 45 N, pointing against your push.
- It is not 60 N: that is only the most static friction can give.
Answer: 45 N
Worked example: Sliding
You push a box with 70 N and it slides; kinetic friction is 50 N. Find the net force.
- Friction points against the sliding: $70 - 50 = 20$ N in the direction of the push.
Answer: 20 N forward
Common mistake
Thinking friction only ever slows things down. When you walk, your shoe pushes backward on the ground, and friction from the ground pushes you forward. That forward friction is what moves you; on smooth ice it is missing, and your feet slip. A car's tires work the same way.
Tilt a ramp until a block slips in the Science Lab: Forces at Work.