index >> cosmology >> Entropy Page 6 Mass and Gravity
MASS is how much stuff things are made of. Xandra's mass weighs 24 Kg. on Earth and 4 Kg. on the Moon. That is because the Moon has less mass than the Earth.

When Spaceboy gives Xandra a ride in his spaceship to the Moon she can jump much higher, kick a soccer ball right across the Moon, and throw a rock into orbit. And Spaceboy can do almost as well as she does.

GRAVITY is the force that pulls things with mass towards each other. Xandra can't feel her Mum's gravity because her Mum does not weigh much, not much mass. Not as much as the Earth. The Earth has a lot of mass. When Xandra jumps she is pulled back down to Earth's gravity.

Gravity is weird: it is strong enough to pull Xandra back to Earth when she jumps. But when Xandra rubs a ballon on her hair, it will stick to a wall. A little bit of static electricity is strong enough to overcome the pull of gravity which wants to pull the balloon back to the floor. Gravity is a weak force but some things like the Sun have so much mass that the gravity pull can keep the Earth in orbit.

The Sun has a lot more mass than the Earth, and is much bigger. The Sun pulls the earth round like a sinker on the end of a fishing line just as the Earth pulls the moon round in its orbit.

How much does the Earth weigh? Who knows! Who is going to put the Earth on a scale? You would need a very big doctor's office to have a scale big  enough to weigh the Earth! But you can measure how much mass the Earth has. Scientists have measured how much stuff there is on Earth, counting all the rocks and iron and water and all the other stuff that the Earth is made of. And they know how much gravity the Earth has, because they can measure how the Moon swings round the Earth and how the Earth swings round the sun, like a sinkers on a fishing line, or a YoYo on its string.