1Mass and weight
Mass is the amount of matter in something, in kilograms. Weight is the force of gravity on it, in newtons. On the Moon a bag keeps its mass but weighs less.
massweight
Lesson 30.1 notes
Gravity and weight
Weight = mass × g, gravity on other worlds, and how gravity holds the Moon and the planets in orbit.
2Weight = mass × g
Weight (N) = mass (kg) × g (N/kg). g is the gravitational field strength, about 10 N/kg on Earth. So a 2 kg bag weighs 2 × 10 = 20 N.
gravitational field strength
3Other worlds
g is different on other worlds. A 60 kg person would weigh 96 N on the Moon, 600 N on Earth, about 1500 N on Jupiter and over 16 000 N on the Sun.
4More mass, more gravity
The more mass a planet or star has, the stronger its pull at the same distance. Gravity also gets weaker the further away you go, but it never quite reaches zero.
5Gravity holds the Moon
The Earth's gravity pulls the Moon towards it, bending its path into a circle: an orbit. Without that pull, the Moon would fly off in a straight line.
orbit
6Pulls both ways
Gravity pulls both ways. The Moon pulls the Earth just as hard as the Earth pulls the Moon. The Moon's pull on the oceans helps to raise the tides.
tides
7The Sun holds the planets
The Sun has over 99% of the Solar System's mass. Its gravity keeps all eight planets in orbit, including the Earth, which goes round once a year.
Explore: Weight lab
Hang a mass on a newton meter on the Moon, Mars, Earth or Jupiter. The mass stays the same, but the weight changes. Then try jumping on each world.
Common mistake: Your mass doesn't change in space
A 60 kg astronaut is still 60 kg on the Moon: she's made of the same matter. Only her weight changes, from 600 N on Earth to 96 N on the Moon, because the Moon's gravity is weaker.
Remember
- Mass is the amount of matter in something, in kg. Weight is the force of gravity on it, in newtons (N).
- weight = mass × g. g is 10 N/kg on Earth, 1.6 N/kg on the Moon and about 25 N/kg on Jupiter.
- Every mass pulls on every other. The Earth's pull keeps the Moon in orbit, and the Sun's pull keeps the Earth in orbit.
Going further at GCSE