What is friction?
Roll a ball along the ground and it slows down and stops, even though nobody stops it. The force responsible is friction: the force that opposes the relative motion between two surfaces in contact.
Friction always acts opposite to the direction of motion, or to the direction in which an object is trying to move.
What causes friction?
Even surfaces that look smooth have tiny bumps and hollows when seen under a microscope. When two surfaces touch, these irregularities interlock. Moving one surface over the other means overcoming this interlocking, and that resistance is friction.
Friction depends on:
the nature of the surfaces: rougher surfaces have more friction. A ball rolls farther on glass than on a carpet.
how hard the surfaces are pressed together: a heavier box is harder to slide, because its surfaces are pressed together more firmly.
Types of friction
Type | When it acts | Size |
|---|---|---|
Static friction | when an object is at rest and you try to move it | largest |
Sliding friction | when an object is sliding | less than static |
Rolling friction | when an object rolls over a surface | much less than sliding |
This explains a familiar experience: getting a heavy box started is harder than keeping it moving, because static friction is greater than sliding friction.
Rolling reduces friction a great deal. This is why we put wheels on suitcases, and why machines use ball bearings between the moving parts of fans, bicycle hubs and motors.
Friction: a necessary evil
Friction is useful
It lets us walk without slipping. Walking on a wet, polished floor or on ice is hard because friction is low.
It lets us hold a pen, a cup or a rope.
Brakes use friction to slow and stop vehicles.
Striking a matchstick produces enough heat from friction to light it.
Nails and screws stay in walls because of friction.
Friction is harmful
It wears out shoe soles, tyres, and machine parts.
It produces heat that wastes energy in machines.
It makes us use more fuel and effort to keep things moving.
Increasing friction
When we need more grip, we increase friction:
shoe soles and tyres have treads (grooves),
athletes wear shoes with spikes,
gymnasts rub a powder on their hands,
brake pads are made of special high-friction materials.
Reducing friction
When we want things to move easily, we reduce friction:
Lubricants such as oil, grease and graphite form a thin layer between surfaces so that they do not rub directly,
ball bearings and wheels replace sliding with rolling,
surfaces are made smooth and polished,
a little powder on a carrom board helps the pieces glide.
Fluid friction (drag)
Liquids and gases are called fluids. Objects moving through them also experience friction, called drag or fluid friction. It depends on the object's speed, its shape and the fluid.
To reduce drag, objects are given a streamlined shape: pointed at the front and tapered at the back. Aeroplanes, fast cars, boats, birds and fish all have streamlined bodies.