Friction

Push a book across the table and let go: it slides a bit and stops. On ice it would glide much further, on a carpet hardly at all. Something slows the motion down, sometimes more, sometimes less. That something is called friction, and without it you could neither walk nor brake nor hold a pen.

Friction is an opposing force

Wherever two surfaces rub against each other, a force appears that acts against the motion. Under a microscope even smooth material looks rough: tiny bumps hook into each other. If you pull an already sliding crate with 30 N and the friction force is also 30 N, the two forces cancel and the crate keeps sliding at constant speed. Pull harder and a leftover force remains, so the crate speeds up.

Getting it moving is harder than keeping it moving

You may know this from moving furniture: the hardest moment is the very first jolt. As long as an object sits still, static friction holds it in place. Once it slides, only sliding friction acts, and that is smaller. That is why you need more force to get something moving than to keep it moving. If something rolls on wheels, only rolling friction is left, and that is much smaller still. That is why shopping trolleys have wheels.

Useful and annoying at the same time

Often friction is exactly what you want: bicycle brakes press pads against the rim, and shoe soles have tread so you do not slip on wet ground. In bearings and gears, however, friction is a nuisance because it eats energy and wears the parts down. Lubrication helps: oil forms a thin film between the surfaces. Air slows you down too: air resistance grows the faster you go. That is why racing cyclists crouch low and cars are streamlined.