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Parachutes

Gravity and orbits · level 3, lesson 3 of 6

A question to start

Make a parachute from a plastic bag, four strings and a toy figure. Drop the figure without it, then with it. Why does the parachute make it fall so much more slowly? And what if you made the parachute bigger?

The idea

A parachute is a way of making a lot of drag. When it opens, it catches a huge amount of air. All that air pushes up on it, so the drag grows big while the speed is still small. The drag balances the weight at a gentle speed, and the parachutist lands softly.

The bigger the parachute, the more air it catches, and the slower the landing. A heavier load needs a bigger parachute to land just as gently.

small: fastbig: slow

Parachutes on Mars. Mars's air is so thin that a parachute catches very little of it. To land the Perseverance rover in 2021, NASA used a parachute 21.5 m across, about as wide as a tennis court is long. Even so, it only slowed the lander to about 320 km/h, far too fast to land. The last part of the landing used rocket engines.

On Earth, the same parachute with the same load would come down about five times more slowly. Same parachute, same load: the air makes all the difference.

Worked examples

1. Testing your parachute. Drop your toy from the same height with a small bag and then a big bag. Time each fall. The big one should take longer.

2. A heavy load. Add a second toy to the same parachute. It falls faster: more weight needs more drag to balance, and the parachute only makes more drag by going faster.

Try it

  1. How does a parachute slow a fall?
  2. Which lands more softly: a small parachute or a big one, with the same load?
  3. Why do parachutes on Mars have to be so big?
  4. Why did Perseverance also need rockets to land?
Answers
  1. It catches a lot of air, so the drag balances the weight at a low speed.
  2. The big one.
  3. The air is very thin, so a parachute needs a huge area to catch enough of it.
  4. Even its huge parachute left it going about 320 km/h, too fast to touch down safely.

Common mistake

"A parachute stops you falling." It does not stop the fall; it makes it slow and steady. You still come down, just at a speed that is safe to land at.

Where it leads