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What weight is

Gravity and orbits · level 2, lesson 1 of 6

A question to start

Hang a bag of apples from a rubber band. The band stretches. Hang a bag with twice as many apples: it stretches more. What is pulling the band down?

The idea

Gravity pulls on every apple. The more apples, the bigger the pull. That pull of gravity on something is its weight.

You cannot see weight, but you can see what it does: it stretches rubber bands and springs, squashes the springs in a bathroom scale, and makes your arms tired. A spring scale is a spring with a pointer and a ruler: hang a thing from the hook and the spring stretches until it pulls up exactly as hard as gravity pulls down.

weighttwice the weight

Scientists measure weight in newtons, named after Isaac Newton. An apple weighs about 1 newton on Earth. A bathroom scale says kilograms, but it really measures how hard you press down on it, and turns that into kilograms assuming you are standing on the Earth.

Worked examples

1. A rubber-band scale. Hang one coin purse: the band stretches 2 cm. Two purses the same: about 4 cm. Three: about 6 cm. Twice the weight, twice the stretch (as long as you do not stretch the band too far).

2. Standing on one foot. Your scale shows the same number whether you stand on two feet or one. Your weight is still pulling down just as hard; it is only pressing on a smaller patch.

Try it

  1. What is weight?
  2. Three apples hang from a spring. Would it stretch more or less than with one apple?
  3. About how many apples make a weight of 5 newtons?
  4. A rover weighs less on Mars. Has any of the rover gone missing?
Answers
  1. The pull of gravity on something.
  2. More, about three times as much.
  3. About 5 apples.
  4. No. It is just as big and made of just as much metal; Mars pulls on it less hard. The next lesson is about this.

Common mistake

"Weight is how big something is." A big balloon weighs much less than a small rock. Weight depends on how much stuff is in a thing and how hard the planet pulls, not on its size.

Where it leads