Friction: The Force That Slows Things Down

What is friction?

Friction is a force that resists, or works against, motion whenever two surfaces are in contact. Whenever one surface tries to slide across another, friction acts in the opposite direction, trying to stop it.

Friction is closely linked to the other pushes and pulls you may already know. The difference is that friction is a force you usually want to either increase or reduce, depending on the job.

Slide a book across a table and it slows to a stop. Roll a ball across grass and it stops far sooner than on a smooth floor. In both cases, friction is the reason.

What causes friction?

Surfaces that look perfectly smooth are actually covered in tiny bumps and ridges, far too small to see. When two surfaces touch, these microscopic bumps catch and grip on each other.

To move one surface across the other, you have to push hard enough to break past all those tiny grips. The catching and dragging of the bumps is what we feel as friction.

Two main things affect how much friction there is:

• How rough the surfaces are. Rough surfaces like sandpaper or rubber grip strongly and have high friction. Smooth surfaces like ice or polished glass grip weakly and have low friction.
• How hard the surfaces are pressed together. A heavier object presses down harder, so there is more friction. Pushing a full box is harder than pushing an empty one.

Static and moving friction

Friction comes in two stages:

• Static friction holds a still object in place. It is why a heavy box does not slide on its own, and why you have to push quite hard before it suddenly starts to move.
• Kinetic (sliding) friction acts on an object that is already moving. It is usually a little weaker than static friction, which is why the box is easier to keep moving once it is going than it was to start.

When friction is useful

Friction often gets a bad name, but life would be impossible without it.

• Walking: the grip between your shoes and the ground stops your feet from sliding out behind you.
• Holding things: friction between your fingers and a cup keeps it from slipping.
• Brakes: a bicycle or car stops because brake pads press against the wheel, using friction to slow it down.
• Tyres: the tread pattern on a tyre increases grip with the road, especially in the rain. The friction inside the braking system and the grip of the tyres do useful work to keep us safe.

When friction is unwanted

Sometimes friction works against us:

• It wastes energy. Friction turns useful movement energy into heat, which is why machine parts get warm.
• It wears parts out. Constant rubbing slowly grinds down surfaces, like the soles of well-worn shoes.
• It slows machines down, making them less efficient.

This is one reason engineers design simple machines carefully — reducing friction lets the same effort move a bigger load.

Reducing and increasing friction

Because friction can help or hurt, we often want to control it.

To reduce friction:

• Use lubricants such as oil or grease, which fill the gaps with a slippery layer that keeps the rough surfaces apart.
• Use wheels, rollers or ball bearings so surfaces roll instead of drag.
• Polish surfaces to make them smoother.

To increase friction:

• Add a rougher surface, like rubber grips or treads.
• Press the surfaces together more firmly.

Friction in air and water: drag

Friction does not only happen between solids. Moving through a fluid (a liquid or a gas) also causes friction, called drag. Friction with air is known as air resistance.

This is why a feather flutters down slowly while a coin drops fast, and why fast cars, planes and cyclists are given smooth, streamlined shapes — to slip through the air with as little drag as possible.

Try it yourself! 🧪

Test how surfaces change friction using a simple ramp.

You will need: a stiff board or large book, a small toy car or a coin, and different materials such as a towel, kitchen foil, sandpaper, and a smooth table.

1. Prop one end of the board up to make a gentle ramp.
2. Place the toy car at the top and let go. Note how far it travels along the floor before stopping.
3. Now lay each different material over the ramp and floor, one at a time. Run the car down again from the same height.
4. Record how far the car travels on each surface.

You will find the car goes furthest on smooth surfaces (low friction) and stops soonest on rough ones like the towel or sandpaper (high friction). For a fair test, always release the car from the same height so it starts with the same energy.

Stay safe: Keep the ramp low and the floor clear, and use a light toy rather than anything heavy that could roll into someone.