Friction: the grabby force
Why things slow down and stop, why shoes grip, and why rubbing your hands together makes them warm.
Slide a book across a table. It slows down and stops, even though nothing is blocking it. Why? A force is working against it. That force is called friction.
What is friction?
Friction is a force that tries to stop things from sliding over each other. It acts where two surfaces touch, and it always pushes against the movement.
Why does it happen? Look very closely at any surface, even a smooth one. It is full of tiny bumps and dips.
When one surface slides over another, the bumps catch on each other. Rough surfaces have bigger bumps, so more friction. Smooth surfaces have smaller bumps, so less friction.
Friction that helps
Without friction, life would be very strange. You could not walk, because your shoes would slide. You could not hold a pencil. A car could not brake or turn. Nails would fall out of wood.
We even make friction on purpose:
- Shoe soles and tyres have patterns that grip the ground.
- Brakes press rough pads against a wheel to slow it down.
- Matches light when friction makes heat on the matchhead.
- Sandpaper is rough on purpose, so it can wear wood away.
Friction that gets in the way
Sometimes friction is a nuisance. It wastes energy and wears parts out. So we reduce it:
- Oil coats surfaces so the bumps slip past each other.
- Wheels roll instead of sliding, which is much easier than dragging.
- Smooth, shiny surfaces like ice have very little friction.
That is why a marble rolls so far in Marble Machine. A rolling ball has little friction compared with a sliding block.
Friction makes heat
Rub your hands together hard. They get warm! The bumps catching and tearing at each other turn some of the movement energy into heat. This is exactly what the grey part of the energy bar shows in Marble Machine. Try the three friction settings in Free Build, and watch how much energy becomes heat.
What changes how much friction there is?
- The surfaces. Rough surfaces give more friction than smooth ones.
- How hard they are pressed together. A heavier box has more friction, because it is pressed harder onto the floor.
Other kinds of friction: drag
Air and water make friction too. When you push through them, they push back. We call this air resistance or water resistance, or just drag. That is why a cyclist crouches down low, and why a cheetah's slim shape helps it run. A smooth, pointed shape cuts through air with less drag.
Make a slope with a board and a pile of books. Put a small block on it. Slowly raise the slope until the block starts to slide. Now put a cloth on the board and try again. On which surface can you make the slope steeper before it slides? That surface has more friction.
Words to know
- Friction
- A force that resists sliding between two surfaces.
- Rough
- A surface with bigger bumps, which gives more friction.
- Lubricant
- Something slippery, like oil, that reduces friction.
- Drag
- Friction from air or water pushing back on something moving through it.
Check yourself
Which direction does friction push?
Show the answer
Against the movement. It tries to stop things from sliding.
Why do rubbing hands get warm?
Show the answer
Friction turns some of the movement energy into heat.
Name one place where we want more friction and one where we want less.
Show the answer
More: shoe soles, tyres, brakes. Less: oiled machine parts, wheels, ice for skating.