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## Issac Newton's Three Laws

Issac Newton, a 17th-18th century English physicist and mathematician (also the guy who had an apple hit his head supposedly), stated three laws that basically describe the statics and dynamics of objects. Statics is the study of forces on an object at rest. Dynamics is the study of how forces affect the motion of a body.

Newton's First Law of Motion (Law of Inertia): Every body continues in its state of rest or of uniform speed in a straight line unless it is compelled to change that state by forces acting on it.
What does that mean? It means that if I throw a ball it will keep on going at the same velocity (speed and direction) unless a force of some kind changes it. Usually there is air friction that pushes against the ball. Also, gravity changes its direction by making it fall down. Finally, the ball stops when it hits the ground. Therefore, air friction, gravity, and the resistance of the ground are forces.

Newton's Second Law of Motion: The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the applied net force.
It is a simple idea actually. If you push a cart, it will accelerate in the direction you are pushing. Also, it will accelerate faster if you push it harder (greater force). So if you push it towards the school, you are accelerating it towards the school. This is pretty much common sense. You don't expect that if you push a cart east that the acceleration will be to the north. It just would not make much sense.

Newton's Third Law of Motion (Law of Action-Reaction): Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first.
This is also a pretty simple idea. Let's imagine that one ball is moving in a straight line toward another ball and collides with it. The moving ball exerts a force on the ball at rest, which accelerates. However, the ball at rest exerts the same magnitude of force in the opposite direction on the moving ball, decelerating it (perhaps even making it reverse direction).

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