What are Newton’s Laws of motion

The following are the Newton’s laws of motion :

Newton’s I Law: A body continues to in its state of rest or of uniform motion along a straight line unless an external force is applied on it.

Show that Newton’s 1st Law of Motion defines force?

According to the law of motion, a body continues to be in its state of rest or of uniform along a straight line unless it is acted upon by an external force.

So, we define force in the form of a push or pull which may change or tends to change the state of rest or of uniform motion of a body.

Newton’s II Law: The rate of change of linear momentum of the body is directly proportional to the external force applied on the body and this change takes place in the direction of the applied force.

Discuss the consequences of Newton’s 2nd Law of Motion.

Ans: The consequences are-:

(i) No force is required to move a body uniformly along a straight line.

In case of uniform motion the velocity remains the same through out the motion. Hence the acc. is zero

As                                                F = ma,     If a = o       then   F = o

(ii) Acceleration motion is always due to external force.

If no external force is applied then the body will continue to be in its state of rest or of uniform motion.

i.e. If a motion has to be accelerated or to bring a change in velocity we have to apply an external force for this purpose.

(iii) The Newton’s 2nd Law of motion helps us to measure the force by using the equation.

F = ma   or      F = m dv/dt

(iv) Concept of  Inertial Mass:

From Newton’s 2nd Law                           F = ma

a = F/m

So larger the mass smaller will be the acceleration produced in it by the force and hence greater will be the inertial.

i.e. The mass of a body given us the measure of  inertial of the body in linear motion and such a mass is known as InertialMass.


Newton’s III Law: To every action there is an equal and opposite reaction.

Note: The force of action and reaction do not cancel each other because they act an different bodies.

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