Physics notes for class 11 – Laws of Motion | NCERT 11th Science

Physics notes for 1 - Laws of Motion - NCERT 11th Science

Laws of Motion Physics notes for class 11 | NCERT 11th Science

Laws of Motion Physics notes for class 11 – What is Inertia?

The tendency of a body in motion continues to remain in motion, moving with uniform velocity and a body at rest continues to remain at rest unless acted upon by a force. This tendency of the body is known as ‘inertia’ (i.e. resistance to change). In Newton’s First Law of Motion, this concept is quantified.

Inertia is a quality, all the objects/bodies in nature made of matter that possess. Mass is a measure of inertia. Objects with higher mass resist changes in motion more than objects with lower mass.

Laws of Motion

Sir Isaac Newton presented the three laws of motion.  Newton’s laws of motion give us the relations between the forces acting on a body and the motion of the body.

These laws are as follows:

Newton’s First Law of Motion also known as Law of Inertia states that every object persists to stay in uniform motion in a straight line or in the state of rest unless an external force acts upon it.

Newton’s Second Law of Motion states that force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration, i.e. F = m*a.

Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction.

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Derivation of Newton’s Second Law of Motion

According to Newton’s second law: F-> =dP-> /dt,

where P->  = momentum and  P-> =mv->

If the time interval for the applied force is increased, then the value of the applied force will decrease. In cricket players use this while catching the ball. They pull their hands backward so that time of contact with ball increases and they would experience less jerk due to the motion of the ball.

From Newton’s second law of motion,

F-> ∝ dP->/dt
F-> =k×dP-> /dt=kma->
For simplicity, the constant of proportionality (k) is chosen to be 1, therefore
F-> =ma->


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Applications of Laws of Motion

Newton’s First Law of Motion

A car traveling on a highway at a fixed speed tends to maintain uniformity in its motion and everything else inside the car. When a force from outside is applied to the car in motion, like a sudden change in direction, the car will respond to this sudden change on its own, although the passengers in the car or the objects inside it are still responding to inertia, wherein their motion will still be in a straight line. When in fact the direction has already changed causing the passengers or the objects to be thrown off. This event is explained by the first law of motion.

Newton’s Second Law of Motion

The application of the second law of motion can be seen in determining the amount of force needed to make an object move or to make it stop. For example, stopping a moving ball or pushing a ball.

Newton’s Third Law of Motion

The application third law of motion can be seen via an illustration wherein let’s say a glass is on a table, even though the glass is at rest it is actually exerting a force on the table and the table, on the other hand, is exerting equal opposite force thus making the glass stay.

Study Law of Motion with interactive animated learning videos and get solved NCERT solutions, sample papers, and helping guide to solving any numerical problem related to Laws of motion. For details click on Class 11 Science.

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Tag: Laws of Motion, Applications of Laws of Motion, Newton’s laws of Motion, 11th Science, Physics notes for class 11, What is Inertia?



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August 31, 2019

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