🛷 Measuring Force and Friction

Hello! If you push a box on ice, it slides a long way. On a carpet, it stops almost at once. Today we will learn how to measure a force, and about friction, the force that stops the box.

A force is a push or a pull on an object. A force can start an object moving, stop it, or change its speed and direction. So how do we measure a force?

We measure force with a force meter. Inside it there is a spring. The bigger the force, the more the spring stretches. The unit of force is the newton, or N for short. Here the spring stretches 2 cm for every 1 N: 1 N, 2 N, 3 N.

When a box slides along the floor, it slows down and stops. The reason is the force of friction. Up close, surfaces are not smooth: they have tiny bumps that catch on each other. The force of friction acts in the opposite direction to the movement.

Let’s let the same car go from the same height. On ice it goes the farthest. On the wood floor it stops sooner. On the carpet it stops the soonest. The rougher the surface, the bigger the force of friction.

We can measure friction with a force meter. When we pull the box at a steady speed, the pulling force is equal to the force of friction. We read 1 N on ice, 3 N on the wood floor and 5 N on the carpet. The more force it takes to pull, the bigger the friction.

Friction is often useful. The soles of our shoes grip the ground, so we walk without slipping. Brake pads rub on the wheel and stop the bicycle. A pencil rubs on paper and leaves a mark.

Sometimes friction is a problem. Shoe soles and tyres wear down over time. The parts of a machine that rub together get hot and wear out. A door hinge squeaks.

We can reduce friction. We put oil where parts rub together. We use wheels instead of sliding. We smooth and polish a surface. This is also why sledges and skis glide so easily over slippery snow and ice.

Summary: we measure force with a force meter, and the unit is the newton. The force of friction slows movement. The rougher the surface, the bigger it is. Friction can be useful and it can be a problem. Now try it in Moon Observatory!