Newton’s Laws of Motion

Published on Jul 15, 2016

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Isaac Newton is famous for his work on gravity but his contribution to the mechanics of physical systems is of paramount importance. He compiled his three laws of motion in Mathematical Principles of Natural Philosophy and helped further the study of forces and the reaction produced by bodies that were subjected to these forces.

In classical physics, these laws are applied to every object in order to investigate their motion. An important assumption when applying these laws is to treat the object under study as a point object or an object with negligible size and point-shaped. Also, these laws are applicable only in an inertial frame of reference. The frame of reference is the frame in which we, the observers, are present. Earth is an inertial frame of reference. Unless we are in an accelerating frame of reference like an accelerating car, we are in the inertial frame of reference and we can apply Newton’s laws of motion.

Law of inertia

First law is also referred to as the law of inertia. When you are riding in a bus and the bus makes a sharp turn, have you ever noticed that you tend to fall as if you wanted to continue moving in the direction that the bus was earlier moving in? This ability of the physical system to resist the change in their motion is a property known as inertia. The same thing happens when you are riding in a car and brakes have to be applied suddenly. You are shaken off your seat and thrown forwards as if your body wished to continue moving. The first law states that an object at rest will not start moving on its own and an object in motion will not come to rest until and unless complied by an external force to do so. Thus, if the net force, that is if the effect of all the forces that are being applied on an object is zero, the object will be at rest.

Second law of motion

According to the second law, if a force is applied on an object which causes its momentum to change, then the rate at which the momentum of the object changes is directly proportional to the amount of force applied. Momentum is the product of the mass and velocity of the object. It is important to note that the change in momentum is in the same direction as the force applied. In all other directions, the component of momentum will remain unaffected.

Action-reaction law

Suppose you and one of your friends are wearing roller blades and stand facing each other. Now, if you try to push your friend away from you, you will observe that you also move away from the spot over which you were standing. Newton’s third law of motion states that every action has an equal and opposite reaction. The force which you exert on your friend is the action. This sets up a reaction force which acts on you and in the opposite direction so that you move away from the spot over which you were standing.

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