Rocket stands on the bathroom scale with the playground ball. "Look, the ball has weight. The number went up."
Raven holds a page from a physics text. "What do you notice? This table says the same ball would weigh much less on the Moon."
"Less stuff on the Moon?" Rocket asks.
"Same stuff," Nova says. "Same mass. What changes is how hard the Moon pulls."
Rocket steps off the scale. "So weight is a pull. Mass is the stuff being pulled."
Raven writes it down. "One kilogram on Earth, one kilogram on the Moon. But the weight is not the same."
"Then a scale does not measure what I thought," Rocket says. Nova hums. "Read the table."
OpenStax explains that mass and weight are very different, although they are closely related.
Mass is the quantity of matter in an object and does not vary. Weight is the gravitational force on an object.
The SI unit of force is the newton. OpenStax gives the weight of a one kilogram object on Earth and on the Moon.
| Object | Mass on Earth | Weight on Earth | Mass on the Moon | Weight on the Moon |
|---|---|---|---|---|
| A 1.0 kilogram object | 1.0 kilogram | 9.8 newtons | 1.0 kilogram | about 1.7 newtons |
NASA Glenn describes weight as the force generated by the gravitational attraction of the Earth on any object.
On Earth's surface, gravity speeds a falling object up by 9.8 metres per second every second. NASA also gives it as 32.2 feet per second per second.
NASA says the gravity on the surface of the Moon is one sixth of Earth's. That is why astronauts seem to bounce.
OpenStax agrees: the mass of an object is the same on Earth, in orbit, or on the surface of the Moon.
A bathroom scale actually detects weight, OpenStax notes, but it is set up to display mass for Earth.
| Statement | True or false? |
|---|---|
| Mass is the amount of matter and does not change from Earth to the Moon. | ? |
| Weight is a force measured in newtons. | ? |
| A 1.0 kilogram object weighs more on the Moon than on Earth. | ? |
| A bathroom scale detects weight but shows a mass. | ? |
Excellent reading. Tomorrow you will find force and mass at work in sport, and review the week.