Skip to content

Air gets in the way

Gravity and orbits · level 3, lesson 2 of 6

A question to start

Hold your hand out of the window of a moving car (only if a grown-up says it is safe). At slow speeds you hardly feel the air. Faster, and it pushes your hand back hard. What happens to a falling thing as it gets faster and faster?

The idea

Air is made of tiny pieces, far too small to see, and a falling thing has to push through them. The air pushes back. This push is called air resistance, or drag.

The faster you go, the harder the air pushes back. At first a falling thing is slow, the drag is small, and gravity wins easily: it speeds up. But as it speeds up, the drag grows. In the end the drag pushing up is as strong as gravity pulling down. The two forces balance, and the thing stops speeding up. It keeps falling at a steady speed, called its terminal speed.

just let gofasterterminal speed

How big the terminal speed is depends on the thing. A skydiver falling flat reaches about 200 km/h. A raindrop, only about 30 km/h. A feather, a walking pace. Light things with a big flat shape have low terminal speeds; heavy, narrow things have high ones.

On Mars, the air is very thin, about sixty times thinner than ours. Gravity is weaker too, but the thin air matters more. Something falling through the air of Mars builds up a much bigger speed before the drag can balance it. A skydiver there would fall several times faster than on Earth. Dust is so fine, though, that it can still float in the thin air for weeks.

Worked examples

1. Two coffee filters. Drop one paper coffee filter, then two stacked together. The pair falls faster: twice the weight, but almost the same shape, so it needs more drag, and so more speed, to balance.

2. A skydiver changing speed. A skydiver who curls into a ball goes faster; one who spreads out flat goes slower. Their weight has not changed, only the shape the air has to push on.

Try it

  1. What is air resistance?
  2. Why does a falling thing stop speeding up in the end?
  3. Which has the higher terminal speed: a pebble or a leaf? Why?
  4. On Mars, would a skydiver fall faster or slower than on Earth?
Answers
  1. The push of the air on something moving through it.
  2. Drag grows with speed, until it is as strong as the weight. Then the forces balance and the speed stays the same.
  3. The pebble: it is heavy for its size, so it needs a lot of drag to balance its weight.
  4. Faster: the air is so thin that drag is small, even though gravity is weaker.

Common mistake

"At terminal speed, the thing stops falling." It keeps falling, just at a steady speed. Only the speeding up stops.

Where it leads