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The newton

Forces and motion · level 3, lesson 4 of 6

A question to start

We measure length in metres and mass in kilograms. What do we measure force in? How big is one?

The idea

Force is measured in newtons, written N, after Isaac Newton, who worked out the laws of motion in the 1680s.

A newton is not very big. Hold a small apple, about 100 g, in your hand: the push your hand gives to hold it up on Earth is about 1 newton.

One kilogram is pulled down by gravity with a force of about 9.8 N on Earth, and 3.7 N on Mars. That pull is the weight of the kilogram.

Weight of Mars Earth
A small apple (100 g) 0.37 N 0.98 N
A bag of sugar (1 kg) 3.7 N 9.8 N
A child (30 kg) 111 N 294 N
Perseverance (1,025 kg) 3,800 N 10,000 N
1 kg1 kg 9.8 N on Earth3.7 N on Mars

A newton has an exact meaning: it is the force that makes a 1 kg mass speed up by 1 metre per second, every second, with nothing else pushing or pulling. You meet that in level 5.

Worked examples

1. A 5 kg bag on Earth. Each kilogram weighs about 9.8 N, so \(5 \times 9.8 = 49\) N.

2. The same bag on Mars. \(5 \times 3.7 = 18.5\) N. A bit more than a third.

3. Lifting a suit pack. A 40 kg backpack weighs \(40 \times 9.8 = 392\) N on Earth, but \(40 \times 3.7 = 148\) N on Mars, about what 15 kg feels like on Earth.

Try it

  1. What is the unit of force?
  2. About how much does a 100 g apple weigh on Earth?
  3. Find the weight of a 2 kg rock on Mars.
  4. Find the weight of the same rock on Earth.
  5. An astronaut and suit have a mass of 150 kg. What is their weight on Mars?
Answers
  1. The newton, N.
  2. About 1 N.
  3. \(2 \times 3.7 = 7.4\) N.
  4. \(2 \times 9.8 = 19.6\) N.
  5. \(150 \times 3.7 = 555\) N.

Common mistake

"Weight is in kilograms." In everyday talk we say so, but in physics, kilograms measure mass. Weight is a force, measured in newtons, and it changes from planet to planet while the mass stays the same.

Where it leads