5.3 Contact Forces
Forces come in two big families, told apart by one simple question: do the two objects have to touch? When they do, the force is a contact force.
Contact Forces A contact force acts only when there is physical contact between the objects. The contact can be direct (your hand on a box) or indirect (through a stick or rope). The two most important contact forces are muscular force and friction.
Muscular force
Every time you walk, run, lift, jump or stretch, the force comes from your muscles as they contract and relax. This is muscular force.
Muscles Working — Inside and Out Animals, birds, fish and insects all use muscular force to move and survive, and humans have long used the muscular force of animals (bullocks, horses) for heavy tasks. Muscular force even works inside your body: muscles help you chew and push food through the digestive canal, and the rhythmic squeezing of your heart muscles circulates blood — quite literally keeping you alive.
Friction
Roll a ball on flat ground and it slows and stops on its own. Stop pedalling and your bicycle gradually halts — sooner on a rough road than a smooth one. Something is slowing these objects, even when nothing seems to be pushing them.
Activity 5.3 — The hidden stopper Give a lunch box (or any flat-based object) a gentle push across a table. It slides a little way and stops. Push it the other way — it stops again. A force must be acting against its motion to bring it to rest. That force is friction.
What Is Friction? Friction is the force that comes into play when an object moves, or tries to move, over another surface. It is a contact force, and it always acts in the direction opposite to the motion (or attempted motion) — which is why it slows things down. Friction arises from the tiny irregularities on the two touching surfaces: even surfaces that look perfectly smooth are covered in microscopic bumps that catch on one another. Rougher surfaces → more friction, which is why the bicycle stops faster on a rough road.
Worked Example
Why do we slip on wet ice or a polished floor?
Walking works because friction between your feet and the ground grips you and lets you push off. On ice or a polished floor, the surfaces are very smooth, so there are far fewer irregularities to catch on — friction is very low. With too little friction to grip, your foot slides instead of pushing off, and you slip. Friction, often blamed for “wasting” motion, is exactly what lets us walk at all.