Gravity is a pull, and a pull is a kind of force. Drop a ball and it falls. Gravity, a pull from Earth, draws the ball in toward Earth's center, and that is why the ball goes down. As the ball falls, it speeds up.

Air gets in the way of things that move through it, and air and water slow such things down. This force is called drag. There is almost no air on the Moon. Dropped together there, a hammer and a feather fell at the same rate, so each fell as fast as the other. With no air, light things and heavy things fall alike.

Weight and mass are not the same. Weight is the pull that gravity puts on a thing. Mass tells how much stuff a thing is made of. On the Moon, gravity has only about one-sixth of Earth's strength. Anyone standing there would weigh much less.

Pushes and pulls are forces, but they do not all work alike. Some forces only work when things touch. A push is one, and friction is another. Two magnets can pull or push each other and never touch. Gravity also works across a gap.

Here is a surprise about pushing: a thing you push pushes back on you, and it pushes with the same strength. Water answers a fish in the same way. In water, a fish pushes backward, and the water pushes forward on the fish.

A bigger push gives a bigger change in how a thing moves. A heavy thing needs more push than a light one to move as much. A lever is a simple machine, and with it a small push or pull can handle a job that would need a big force.

Words to know

• gravity: pull toward Earth
• drag: air slowing things
• mass: amount of stuff
• weight: gravity pull
• lever: simple machine