A parked shopping cart won't budge until you push it, and once rolling it won't stop until something (a wall, friction) pushes back. Forces are those pushes and pulls, and they're the only thing that can change how something moves.
A force is a push or a pull
Nothing changes its motion by itself. A book stays put until you push it; a ball keeps rolling until something slows it. That something is a force — a push or a pull. Forces have both a size and a direction.
When forces cancel out (balanced), motion does not change. When one side wins (unbalanced), the object speeds up, slows down, or changes direction. In a tug-of-war, nobody moves until one team pulls harder.
Friction is a force that resists sliding. It is why a pushed box eventually stops and why shoes grip the floor. Rougher surfaces make more friction; smooth, slippery ones make less.
a = F / m = 5.0 m/s²
Push a block with more force and it speeds up faster; make it heavier and the same force barely budges it.
- Prop a book up to make a gentle ramp. Let a coin slide down and watch how it moves.
- Now tape a strip of paper towel to the ramp and slide the coin again.
- Which surface let the coin slide farther and faster?
- Explain the difference using the word friction.
What you should see: You saw friction slow motion down and connected surface roughness to how hard it is to slide.