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Up into the sky

Rockets and landing · level 1, lesson 4 of 6

A question to start

A rocket stands on the launch pad, as tall as a building. The engines light, fire and smoke pour out, and for a moment it just sits there. Then, slowly, it starts to rise. Why does it go up slowly at first?

The idea

Two things pull and push on the rocket.

  • Gravity pulls it down. A big rocket full of fuel is very heavy.
  • The engines push it up. Hot gas rushes down out of the engines, and that pushes the rocket up.
engine pushgravity

To go up, the engines must push harder than gravity pulls. If they pushed exactly as hard, the rocket would just hover. A big rocket's engines push only a bit harder than its weight at first, so it rises slowly, and then faster and faster.

As the rocket burns its fuel, it gets lighter. The engines keep pushing just as hard, so the rocket speeds up more and more. By the time it reaches space, it is going faster than anything on Earth.

On Mars it is easier. Mars pulls only a bit more than a third as hard, so a rocket there needs much less push to lift off.

Worked examples

1. Lifting a box. To lift a box, you must pull up harder than it weighs. If you pull exactly as hard, it just stays where it is.

2. A rocket getting lighter. At lift-off, most of a big rocket's weight is fuel. As the fuel is used up, the same push moves less weight, and the rocket speeds up.

Try it

  1. Which way does gravity pull a rocket? Which way do the engines push it?
  2. What happens if the engines push exactly as hard as gravity pulls?
  3. Why does a rocket go faster and faster as it climbs?
  4. Why would it be easier to launch a rocket from Mars?
Answers
  1. Gravity pulls down; the engines push up.
  2. It hovers: it stays at the same height.
  3. It burns fuel and gets lighter, but the engines push just as hard.
  4. Mars's gravity is weaker, so less push is needed to lift the rocket.

Common mistake

"The fire pushes on the ground." The rocket keeps going faster after it is far above the ground, and even in space. The push comes from the gas the engine throws out, not from the ground.

Where it leads