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Throwing and catching

Gravity and orbits · level 1, lesson 3 of 6

A question to start

Throw a soft ball to a friend. Watch it carefully. Does it fly in a straight line from your hand to theirs? And what happens if you throw it as hard as you can straight up?

The idea

A ball you throw does not go in a straight line. It goes up a little, then curves over and comes down, like a rainbow shape. This curve is called an arc.

top

The ball goes sideways because you threw it sideways. It curves down because gravity pulls it down the whole time it is in the air. Two things happen at once: it keeps moving across, and it is pulled down.

Throw the ball straight up. It slows down, stops for a moment at the top, and comes back down to you. Gravity pulled on it all the way up, slowing it, and all the way down, speeding it up. The harder you throw, the higher it goes before it turns round, but it always comes back.

On Mars the pull is weaker, so a ball goes higher and farther before it comes back. The same throw would go almost three times as far.

Worked examples

1. A ball rolled off a table. It does not drop straight down from the edge. It keeps moving forwards while it falls, so it lands a little way out from the table.

2. Water from a hose. Point a hose a bit upwards. The water makes an arc, just like a thrown ball, because each drop is thrown forwards and pulled down.

Try it

  1. You throw a ball straight up. What happens to it at the very top?
  2. Why does a thrown ball curve down instead of flying straight?
  3. A fountain shoots water up and across. What shape does the water make?
  4. Would a ball thrown on Mars go farther or less far than on Earth?
Answers
  1. It stops for a moment, then starts to fall back down.
  2. Gravity keeps pulling it down while it moves across.
  3. An arc, like a rainbow.
  4. Farther, because Mars pulls less hard.

Common mistake

"At the top, gravity stops." The ball stops for a moment, but gravity never stops pulling. That is why the ball starts falling again straight away.

Where it leads