Forces and Friction

Friction is a force that opposes motion between two surfaces in contact. It acts along the surfaces, opposite to the direction the object moves or tries to move. It is caused by the tiny bumps on both surfaces catching on each other.

Static friction acts when an object is at rest and stops it from starting to move. Sliding (kinetic) friction acts while it slides. Rolling friction acts when something rolls, like a wheel, and is the smallest. Fluid friction (drag) acts on objects moving through air or water.

When an object is at rest, the bumps on the two surfaces settle into each other and lock more firmly. Once it slides, they don't have time to lock, so less force is needed to keep it moving than to start it.

Forces are balanced when they cancel out, so the net force is zero: the object stays at rest or keeps moving at a steady speed. They are unbalanced when the net force is not zero: the object speeds up, slows down or changes direction.

By making surfaces smoother or polishing them, using lubricants like oil or grease, using wheels and ball bearings so things roll instead of slide, and giving vehicles a streamlined shape to reduce air drag.

Yes. Without friction we couldn't walk, write with a pencil, hold things, or stop a bicycle. Tyres and shoe soles have treads to increase friction and grip. But friction also wears out parts and wastes energy as heat.