Friction

Roll a ball along the ground and it slows down and stops, though nobody stopped it. Stop pedalling and your bicycle comes to rest. The force responsible is friction: the force that opposes the motion of one surface over another. It always acts in the direction opposite to the motion, or to the attempted motion.

Why friction arises

Even a surface that looks smooth is covered with tiny hills and valleys when seen under a microscope. When two surfaces are in contact, these irregularities lock into one another. To move one surface over the other, we have to overcome this locking. The rougher the surfaces, the greater the friction.

What friction depends on

• The nature of the surfaces: a rough surface gives more friction than a smooth one. It is easier to slide a box on a polished floor than on a rough road.
• How hard the surfaces press together: a heavy box is harder to push than a light one, because it presses the surfaces together more strongly.

Kinds of friction

• Static friction acts when we try to move an object that is at rest. It must be overcome to start the motion.
• Sliding friction acts when one object slides over another. It is a little less than static friction, which is why a box is easier to keep moving than to start.
• Rolling friction acts when an object rolls. It is much smaller than sliding friction. This is why wheels were one of the greatest inventions, and why heavy luggage has rollers.
• Fluid friction, or drag, is the friction offered by liquids and gases to objects moving through them.

Friction as a friend

• We can walk because friction between our shoes and the ground stops our feet from slipping. On a wet, soapy floor or on ice, there is too little of it.
• We can write: chalk on a board and a pencil on paper work by friction.
• Brakes stop a vehicle by friction, and tyres grip the road with it.
• A matchstick lights because rubbing produces heat.
• Nails and screws stay in walls, and knots stay tied.

Friction as a foe

• It wears things out: the soles of shoes, tyres, the moving parts of machines.
• It produces heat, which wastes energy and can damage machines. Rub your palms together and feel them warm up.
• It makes us use more force, and so more fuel.

Friction is therefore often called a necessary evil.

Increasing friction

When we need more grip, we make surfaces rougher. Tyres and shoe soles have grooves (treads). Sportspersons wear shoes with spikes. Gymnasts and kabaddi players rub coarse powder or soil on their hands.

Reducing friction

• Lubricants such as oil and grease form a thin layer between moving parts.
• Polishing makes surfaces smoother; powder on a carrom board does the same.
• Ball bearings change sliding into rolling, as in the hubs of bicycle wheels and fans.
• Streamlining: giving a body a smooth shape, narrow in front, reduces drag. Fish, birds, boats, cars and aeroplanes are streamlined.