Mass, Weight and Gravity

Remember from Grades 3, 4 and 5

In Grade 3 you learned that gravity is a non-contact force that pulls things toward Earth. In Grade 4 you met the newton (N), the unit for force. In Grade 5 you learned that mass is the amount of matter in something. Now you will connect these ideas and see why mass and weight are different, especially in space.

Would you weigh less on the Moon?

Astronauts on the Moon bounce along in giant hops. Their bodies and spacesuits did not shrink. They have the same amount of matter as on Earth. So why are they so bouncy?

Because the Moon's gravity pulls on them much less strongly than Earth's gravity does. Their mass stayed the same, but their weight changed.

Mass is "how much stuff"

Mass is the amount of matter in an object. It is measured in grams (g) and kilograms (kg).

Your mass is the same on Earth, on the Moon, on Mars or floating in a space station. You are made of the same amount of stuff wherever you go.

Weight is "how hard gravity pulls"

Weight is a force: the pull of gravity on an object's mass. Since it is a force, scientists measure it in newtons (N).

On Earth, gravity pulls on each kilogram with about 9.8 N (almost 10 N). So:

weight (N) = mass (kg) × strength of gravity (N per kg)

A 1 kg bag of flour weighs about 9.8 N on Earth, but only about 1.6 N on the Moon.

What makes gravity stronger or weaker?

Every object with mass pulls on every other object. This pull is usually too tiny to notice, but a whole planet or star has so much mass that its pull is strong.

• More mass → stronger gravity. Jupiter has much more mass than Earth, so it pulls harder. The Moon has much less mass than Earth, so it pulls less.
• Farther away → weaker gravity. The pull gets weaker as you move farther from a planet's centre.

So the same mass has different weights in different places, because the force of gravity is different.

Bathroom scales and floating astronauts

A bathroom scale shows a number in kilograms, but it really measures how hard you push down on it, which is your weight. It is set up for Earth's gravity, then shows the matching mass. On the Moon, the same scale would show a much smaller number, even though your mass had not changed.

Astronauts on the International Space Station float. That is not because there is no gravity up there. Earth's gravity is still strong at the station's height. They float because the station and everyone in it are falling around Earth together as they orbit. Since nothing pushes on a scale, they feel weightless. This is called microgravity. Their mass has not changed at all.

Code it: a weight calculator (A2.1)

Computers can get input in different ways: a person typing, pressing a button, choosing from a menu, or a sensor taking a measurement. Here is a plan for a program that asks for a mass, lets the user choose a world, and calculates the weight. You can build it in Scratch with an ask block and if blocks, or on a micro:bit using buttons to choose the world.