Friction

"Why doesn't a heavy box move when I push it?" Because friction between the box and the floor pushes back, against your push. The box moves only when your push is bigger than the friction, giving a net force forwards.

Other forces are acting on the box too:

• its weight, pulling it down;
• the normal force from the floor, pushing it up (straight out from the surface).

These two balance each other. Air friction is usually small enough to ignore.

Stop pushing and the box slows down and stops, because friction keeps acting against its motion. So a force is needed to keep it moving only to cancel out friction.

Activity 6.1: the coin stack. Use the same stretched rubber band to flick a stack of four coins across different surfaces:

• on polished marble, it goes farthest;
• on a laminated table top, less far;
• on wood, least far.

Friction depends on the surfaces in contact: the smoother they are, the less friction.

Activity 6.2: measuring friction. Pull a block along with a spring balance so that it moves at a steady speed. Now the net force is zero, so your pull exactly equals the friction, and the spring balance reads the friction. The reading is smallest on the surface where the coins went farthest.

"What if there were no friction at all?" Then an object, once moving, would never slow down. It would keep going forever at the same velocity. (Think of how you slip on a wet or oily floor, where there is very little friction.)