by Ernest Washburn

Created on: April 15, 2009

Newton's first law (law of inertia) states that an object in motion continues to stay in motion and an object at rest continues to stay at rest as long as no outside force acts upon that object. Although the law is named after Newton it was originally discovered by Galileo. Galileo used a thought experiment when he discovered the law of inertia. His experiment was that if a ball was rolled down a symmetrical U-shaped ramp without friction then the ball would accelerate as it moved toward the earth and decelerate at an equal rate when moving away from the earth. When the ball stopped it would be at exactly the same height that it was at originally. He then imagined rolling a ball down an inclined plane without friction. Galileo imagined that the ball would accelerate as it rolled down the ramp and then would move at a constant velocity forever unless another force acted upon it.

The law of force and acceleration, aka Newton's second law, says the when an unbalanced force is exerted on an object then it will move in the direction of the force with an acceleration directly proportional to the force. Another way of stating it is with Newton's famed equation

F = m * a

where F is the force, m is the mass, and a is the acceleration of the body. Since it was Newton who discovered this relationship the unit for force is the Newton which is the force required to accelerate one kilogram of mass at one meter per second squared. There is, however, a flaw with this formula. Einstein's theory of relativity states that the faster an object moves the more massive it becomes. This means that as an object moves faster the more mass it gains and the more force is required to increase the objects speed giving it even more mass therefore requiring more force etc. Einstein therefore proved that no object can travel faster than the speed of light and as a result showed that Newton's equation was flawed. The equation for determining the change in mass is

m = m[o] * {1/(1 - v^2/c^2)^(1/2)}

where m is the mass of the object in motion, m[o] is the mass of the object at rest, v is the velocity of the object in motion, and c is the speed of light (3 * 10^8 m/s). Even though Newton's equation is flawed the mass gained by an object with a velocity below 90% of the speed of light is insignificant therefore it is mostly correct when dealing with normal speeds in every day situations.

Newton's third and final law, the law of action and reaction, simply says that for every action there is a equal and opposite reaction. An example of this is someone standing. Gravity is pulling down on them with a force equal to their mass multiplied by 9.8 m/s^2 (the constant acceleration of an object near the earth's surface), but the electromagnetic force between the atoms of that persons feet and the ground exerts an equal force on that person in the opposite direction thus keeping them stationary.

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