In 1686, Newton published his book Philosophia Naturalis Principia Mathematica which people now call the Principia for short.

In it he set out three laws:

One of the laws is :

Everything stays still, or keeps moving in a straight line at the same speed, unless a force comes along and makes it change.

So if something speeds up, slows down or changes direction then there must be a force acting on it. Galileo knew this but Newton went further. His law explained that if you throw a ball into the air it would go on forever if there was no air resistance, gravity or friction...

Or if you throw it fast enough you could escape Earth's gravity!

Another of Newton's laws is:

When a force is applied to an object it changes its momentum. The force moves the object in the same direction as the force is moving, at a speed proportional to the strength of this force.

This means that things will keep moving at a constant speed if there is no force acting on them.

But when you apply a force and the total forward force is greater than zero, the object accelerates or gets faster.

How much faster depends on two things - its mass and the size of the force.

Mass is how much stuff or "matter" a thing contains.

Mass is NOT weight. - Weight is the force of the Earth's gravity pulling you towards it. If you were to go to different planets the gravity would be different on those planets and so would your corresponding weight.

Newton's last law states:

For every action there is an equal and opposite reaction.

People find it hard to believe that if they hit a wall - the wall will hit back with the same force.

Or a book "resting" on a table is pushing down on the table and the table is pushing back against the book with the same force.

It just doesn't sound like common sense at first, does it?

This page uses Flash 5 - please download the current player.