Friction, Drag and Terminal Velocity

Explained

Friction, drag, and why falling objects stop speeding up

Friction acts between two surfaces in contact and opposes their relative motion, or the attempted sliding between them.

Note the careful wording. Friction opposes sliding, which is not the same as saying it always slows things down. Without friction between your shoe and the ground you could not walk, and without friction between a tyre and the road a car could neither accelerate nor steer. In both cases friction is what makes the motion possible.

That distinction is what the specification is after. Friction opposes the relative motion of the surfaces in contact, and the surface being pushed backwards is what drives the object forwards.

Contact friction and drag

Contact friction happens between solid surfaces. It depends on how rough the surfaces are and how hard they are pressed together, and it does not depend on speed.

Drag happens when an object moves through a fluid, meaning a liquid or a gas. Air resistance is drag in air. Unlike contact friction, drag increases as the object moves faster, and it also depends on the object's shape and cross sectional area.

That difference, one depending on speed and one not, is what makes terminal velocity possible.

Reaching terminal velocity

A falling object starts with its weight acting downwards and almost no drag, so the resultant force is large and downwards, and it accelerates.

As it speeds up, drag increases. The resultant force is therefore smaller, so the acceleration is smaller. The object is still speeding up, but less rapidly than before.

Eventually drag equals weight. The resultant force is zero, so the acceleration is zero, and the speed stops changing. That constant speed is the terminal velocity.

Weight has not changed at any point in this. It is drag that has grown to match it, and answers that say the weight decreases have the mechanism backwards.

Opening a parachute

The parachute increases the cross sectional area, so drag increases sharply and becomes larger than weight.

The resultant force is now upwards, so the skydiver decelerates. As the speed falls, drag falls too, until it again equals weight and a new, much lower terminal velocity is reached.

Following the graph: a steep rise, curving to a horizontal line at the first terminal velocity, a sharp drop when the parachute opens, then levelling off again at a lower constant speed.

Reducing friction and drag

Friction is reduced by lubricating the surfaces with oil, by using ball bearings, or by smoothing the surfaces. Drag is reduced by streamlining, which is why cars, aircraft and cyclists' helmets are shaped as they are.

Both transfer energy to the thermal store of the surroundings, which is why brakes get hot and why a spacecraft re-entering the atmosphere needs a heat shield. That energy has not been destroyed; it has been dissipated, spread out too thinly to be recovered.

Answering the standard question

Name the two forces, compare their sizes, state the resultant, and say what that means for the motion.

At the start, weight is greater than drag, so there is a resultant force downwards and the object accelerates. At terminal velocity, drag equals weight, so the resultant is zero and the speed is constant.