Posted In: Physics

Newton’s laws of motion are the basic building blocks of physics and mechanics. They describe the relationship between an object and the forces acting upon it and the corresponding motion of the object in response to these forces.

There are 3 laws of motion.

First law of motion
Newton’s first law of motion states that:
“Every object will remain in its state of rest or of uniform motion in a straight line unless it is forced to change that state by external forces acting upon it.”

The equation can be mathematically expressed as follows:

Thus as per this law, an object which is at rest will continue to remain at rest unless a force acts upon it and an object in motion will not change its velocity unless a force acts upon it. The moving object will continue to move in a straight line path indefinitely in the absence of net forces.

This law is also known as the law of uniform motion. Sometimes it is also referred to as the law of inertia.

For example, When you apply the brakes while driving a car, the unbalanced force of the road on the wheels provides the force to stop the car but there is no unbalanced force to stop you. Thus you continue in motion and slide along the seat in a forward motion. This is why seat belts are needed when a vehicle is moving at high speed. The seat belts provide the unbalanced force to bring you to a state of rest.

Second law of motion

Newton’s second law of motion states that:
“The rate of change of momentum of a body, is directly proportional to the force applied. This  change in momentum occurs in the direction of the applied force.”

Force is equal to the change in momentum of an object (mass times velocity) per the change in time. The equation can be expressed as:

For a constant mass, force equals mass times acceleration.

It can be stated in terms of acceleration as

Where  F is the force applied, m is the mass and a is acceleration.

Thus when force is applied to a body it produces acceleration. The acceleration of an object is directly proportional to the net force on the object and inversely proportional to the mass of the object. If the force on an object is increased the acceleration also increases and if the mass of the object is increased the acceleration of the object is decreased.  

Third law of motion
Newton’s third law of motion states that:
“For every action on an object there is equal and opposite reaction.”

If two object a and b are exerting forces on each other then the two forces are equal in magnitude and opposite in direction. It can be mathematically expressed as

Fa = - Fb
where Fa is the force exerted by object A and Fb is the force exerted by object B. The force exerted by object A is called ‘action’ and the force exerted by object B is called ‘reaction’.

The third law means that all forces are due to interaction between different bodies. There is no such force which is not accompanied by equal and opposite force.


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