Drag forces and motion

Factors affecting drag

When trying to reduce drag, shape and speed are the two major factors.

After you have managed to do your best with these then move on to experimenting with the surface of the object to further reduce the drag. The aim is to disturb the fluid as little as possible as the object moves through it – so achieving smooth streamlines. You don't want any abrupt changes in the motion of the fluid past the surface of the object.

Swimming, cycling and motorcar racing are all activities where (frictional) drag forces are a hindrance. Modern science has helped us go faster by reducing the effect of drag forces, often studying the movement of animals for inspiration. Even low-tech strategies like shaving hair off the head and legs help swimmers to streamline their shape and so go faster.

Relating drag to speed

Here is an account of how the varying force of drag comes into equilibrium with the force of gravity to speed falling objects up to their terminal speed.

As a general rule, the faster you move the greater is the drag force acting against you. Objects falling through air experience a drag force that prevents them from speeding up too quickly. Eventually this drag force will balance the downward pull due to gravity resulting in no more speeding up – the object just continues to fall at a steady speed.

Skydivers experience this effect. As they fall they speed up, but the faster they go the greater is the drag caused by the air. Eventually they will reach a speed at which the size of the drag force equals the size of the pull of gravity. This is called the terminal speed (we'd recommend avoiding terminal velocity, unless you really do want to introduce the distinction between speed and velocity here).

It is interesting to appreciate that when this happens the falling skydiver is in fact in equilibrium. Can you make an argument to explain why?