Mass is the measure of how much matter an object contains -- the total number of subatomic particles (electrons, protons, and neutrons) in the object. It is harder to push a car than a bicycle because the car is made of more particles, i.e. the car has more mass.

Acceleration is the measure of how the speed and direction, or velocity, of something changes. When you speed up in a car, you are accelerating. When something falls to the Earth, it moves faster and faster as it falls; it is accelerating.

A force is a push or pull. If one object exerts a force on another, the second object exerts an equal and opposite force on the first. The Earth accelerates the fruit more than the fruit accelerates the Earth because the Earth has more mass and is harder to move.

Gravity is a force that all objects with mass exert on all other objects. The force of gravity we are familiar with is the force exerted on us by the Earth. We measure this force as weight.

The force of gravity from an object gets weaker as you get farther away from the object. Astronauts in space feel very little gravity because they are far from the Earth. (When in a space station the primary reason is free fall, explained below.)

Weight and mass are not the same! The acceleration of an elevator can simulate higher and lower gravity, as it pushes you up and down. When the force of gravity changes, your weight changes, but your mass is always the same.

When there is no air resistance, objects of different masses fall at the same rate. The weight of the more massive object is greater, but since it has more mass it is also harder to move.

If an object is moving sideways as it falls, it still falls at the same rate as an object that falls straight down. The object's horizontal velocity does not effect its vertical velocity.

Suppose you throw something so fast that the Earth curves off under it before the object can fall back down.

The object is in orbit.

A man in free fall feels the same as a man in orbit because the man in orbit is just falling around the Earth instead of falling to the Earth.

Scientists have used these rules to launch rockets off of the Earth, send probes to other planets, put men on the Moon, and set space stations in orbit around our planet!

Above is a computer model of the International Space Station, currently being constructed in orbit around the Earth.