Friction

Roll a ball on flat ground and it stops by itself. Stop pedalling a bicycle on a flat road and it slows down and stops, and sooner if the road is rough. But changing speed always needs a force. So which force is acting here?

Try it (Activity 5.3). Put a flat-bottomed object on a table and give it a gentle push. It slides a little way and stops. Push it the other way: it stops again.

So a force must be acting between the two surfaces that touch, the bottom of the object and the table, and it must act against the motion. That force is friction. It always acts opposite to the way an object is moving, or trying to move. Because it needs two surfaces in contact, it is a contact force.

Where does it come from? Every surface, even one that looks perfectly smooth like glass, is covered in tiny bumps you can't see. When two surfaces touch, their bumps lock into each other and resist one sliding over the other.

Compare surfaces (Activity 5.4). Push the same object on glass, cloth, wood, tile and sand. It stops after different distances. Friction depends on the kind of surfaces in contact, and it is greater on rough surfaces. That's why the rough road was harder for Sonali and Ragini to pedal on.

Friction is useful too: without it your shoes couldn't grip the ground and you'd slip at every step, and a pen would skid without writing.

Friction isn't only between solids. Air, water and other liquids also push against things moving through them. That's why aeroplanes, ships, boats and high-speed trains are given special streamlined shapes, to cut down the friction of the air or water around them.