THE THREE LAWS

1. First Law: Tis law is often simplified into the sentence "A particle will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force"

2. Second law: This law is often stated as "F = ma: the net force on an object is equal to the mass of the object multiplied by its acceleration."

3.Third law: This law is often simplified into the sentence "Every action has an equal and opposite reaction."

Newton's first Law: Law of Inertia

Essentially, it makes the following two points:

• An object that is not moving will not move until a net force acts upon it.


• An object that is in motion will not change its velocity (accelerate) until a net force acts upon it.

Newton's second Law: Law of resultant Force

Using modern symbolic notation, Newton's second law can be written as a vector differential equation:

where:

is mass

is the velocity vector

is time.

The product of the mass and velocity is the momentum of the object (which Newton himself called "quantity of motion"). Therefore, this equation expresses the physical relationship between force and momentum for systems of constant mass. The equation implies that, under zero net force, the momentum of a system is constant; however, any mass that enters or leaves the system will cause a change in system momentum that is not the result of an external force.

It should be noted that, as is consistent with the law of inertia, the time derivative of the momentum is non-zero when the momentum changes direction, even if there is no change in its magnitude. See time derivative.

Since the mass of the system is treated as constant this differential equation can be rewritten in its simpler and more familiar form:

where:

is the acceleration.

A verbal equivalent of this is "the acceleration of an object is proportional to the force applied, and inversely proportional to the mass of the object". If momentum varies nonlinearly with velocity (as it does for high velocities—see special relativity), then this last version is not accurate.

Newton's third Law: Law of recIprocal actions

In other words "For every action there is an equal, but opposite, reaction".

The Third Law means that all forces are interactions - that there is no such thing as a unidirectional force. If body A exerts a force on body B, simultaneously, body B exerts a force of the same magnitude body A, both forces acting along the same line. As shown in the diagram opposite, the skaters' forces on each other are equal in magnitude, and opposite in direction. Although the forces are equal, the accelerations are not: the less massive skater will have a greater acceleration due to Newton's second law. It is important to note that the action/reaction pair act on different objects and do not cancel each other out. The two forces in Newton's third law are of the same type, e.g., if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward.