Newton's principles

While trying to explain how gravity determines the planets' movement, Newton has discovered 3 simple laws which describe how forces make bodies move. These "laws of movement" are the basics of the entire Physics and apply for any kind of body, from mice to elephants and from atoms to planets.

1. A body upon which there is no force acting is, either not moving, or keeps moving in a straight line, at a constant speed.
...in other words
when you push a push-cart in your local supermarket, it will keep moving in a straight line until it hits something. This is the law of inertia. If the first part of the law is easy to understand (a body holds still if not disturbed by a force), what can we say about the second part? We have all noticed that moving objects do not keep moving forever. They eventually stop. This breaking is caused by friction and other forces. But, if we ignore friction, it will be easy to understand.

2. Forces cause acceleration. The bigger the force and the lighter the body, the greater the acceleration is.
...in other words
it's much easier to accelerate a bike rather than a truck. This law explains what happens when you push an object. In fact, acceleration means increasing the speed and that happens when we push an object. The harder you push the pedals on your bike, the faster you go. The little you and your bike weigh, the easier it is for you to accelerate. In physics, acceleration doesn't only mean increasing the speed, but any modification to the state of repose or linear movement at a constant speed. That means that when you push the breaks in order to slow down, friction causes a negative acceleration, called deceleration.

3. For any action there is a reaction, equal and on the opposed way.
...in other words
when a rocket gives off hot gases, these are the ones that push it further into space. Newton understood that forces (which he named "actions") always go into pairs. If a body pushes an other with a force, that one also pushes the first on the opposed way. The forces are equal, but their effect can be different. If you throw a ball with your hand, it also pushes your hand, but only the ball flies. If you press your feet hard and fast on the ground, it pushes back and you jump! Your feet push the Earth too, but it's movement is imperceptible!