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Heavy and light

Gravity and orbits · level 1, lesson 2 of 6

A question to start

Find a heavy ball and a light ball of about the same size: a tennis ball and a ball of scrunched-up paper, or an orange and a ping-pong ball. Hold one in each hand at the same height. If you let go of both at the same moment, which one will land first?

Guess before you try.

The idea

Most people guess the heavy one. Try it: drop them together onto a soft carpet and listen. You hear one thump, not two. They land together.

heavylight

Gravity pulls harder on the heavy ball, but the heavy ball is also harder to get moving. The two things balance exactly, so both balls speed up in the same way and stay side by side all the way down.

So why does a feather float down slowly? Because of the air. The feather has to push the air out of its way, and it is so light that the air holds it back a lot. A paper ball pushes through easily.

You can test this. Take two sheets of paper. Scrunch one into a ball and keep the other flat. Drop them together. The ball lands first, even though both sheets weigh the same. Only the shape changed, so it must be the air.

In 1971 the astronaut David Scott stood on the Moon, where there is no air at all, and dropped a hammer and a falcon feather at the same time. They landed together. On Mars there is a little air, very thin, so a feather would fall faster than on Earth but still a bit slower than a hammer.

Worked examples

1. A book and a sheet of paper. Drop them side by side: the book wins. Now lay the paper flat on top of the book and drop them together. They fall together, because the book pushes the air out of the way for the paper.

2. Two balls of clay. A big lump and a small lump of clay, both round, dropped together: they land together.

Try it

  1. You drop a marble and a bowling ball (carefully!) from the same height. Which lands first?
  2. Why does a leaf take longer to fall than an apple?
  3. On the Moon, which lands first: a feather or a hammer?
  4. How could you make a sheet of paper fall faster without adding anything to it?
Answers
  1. They land together (or so close you cannot tell).
  2. The air holds the leaf back much more, because it is light and flat.
  3. They land together. There is no air on the Moon to hold the feather back.
  4. Scrunch it into a tight ball, so it pushes less air.

Common mistake

"Heavy things fall faster." For thousands of years many people believed it. Without air, heavy and light things fall together. When something falls slower, it is the air holding it back, not gravity treating it differently.

Where it leads