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e-Techno Text Book VI-Physics (Vol-2) Narayana Group of Schools 1 NEWTON LAW’S OF MOTION AND FRICTION CONTENTS Newton’s First law Inertia, types of inertia Linear momentum Newton’s 2nd Law Units of Force Newton’s Third Law Impulse Free body diagram Friction
e-Techno Text Book VI-Physics (Vol-2) Narayana Group of Schools 2 MEMO GRAPH NEWTON’S LAWS OF MOTION Conservation of linear momentum Newton’s I law Newton’s II law Newton’s III law Inertia F = ma net Inertia of motion Inertia of direction Inertia of rest
e-Techno Text Book VI-Physics (Vol-2) Narayana Group of Schools 3 NEWTON LAW’S OF MOTION NEWTON’S LAWS OF MOTION SYNOPSIS-1 (Newton’s I law of motion, Inertia, Types of Inertia : Inertia at rest, Inertia of motion and Inertia of Direction). Newton has given three laws to describe the motion of bodies. These laws are known as Newton’s laws of motion. Newton’s First law: If no external force acts, a body continues to be in its state of rest or of uniform motion along a straight line. (or) If no external force acts, we can also say that “bodies” go on doing what they are doing. Inertia: In the above law it is clear that a body at rest and a body in uniform motion continue to do what they are doing. Thus, a body cannot change its state of rest or of uniform motion by itself. The inability of a body to change by itself its state of rest or of uniform motion is called inertia of the body or inertia can also be defined as the tendency of a body to remain in the state of rest or uniform motion. Measure of inertia: Mass is the measure of inertia. It is more difficult to move a heavier body than a lighter body from the state of rest. Similarly, it is more difficult to stop a heavier moving body than a lighter body. Thus, the more the mass, the more the inability of the bodies to change their state and hence the more their inertia. Hence we can conclude that mass is the measure of inertia. Force and inertia: We have seen that inertia is defined as the inability of a body to change by itself its state of rest of uniform motion. Then, how can the body overcome this inability (inertia)? The body does it with the help of force. Thus force is an agent which overcomes inertia. Definition: Force may be defined as that physical cause which changes or tends to change the state of rest or the state of motion of a body. Types of Inertia: There are three different types of inertia. They are: (i) Inertia of rest (ii) Inertia of motion (iii) Inertia of direction (i) Inertia of rest: The tendency of a body to continue in its state of rest is called inertia of rest. Example: a) The passengers standing in a bus fall backwards, when the bus suddenly starts because the feet of the passengers start moving along with the bus as it is in contact with the floor of the bus. Whereas, the upper part of the body, due to inertia of rest, tries to remain stationary. Thus the lower portion of the body (foot) moves forward and the upper portion remains at rest due to inertia of rest, and thus the standing passengers fall back. b) When a paper on which a pile of books are placed is suddenly pulled with a jerk, we expect the books also to move, but this does not happen. This is because the books, stay where they were due to inertia of rest when the paper is pulled out.
e-Techno Text Book VI-Physics (Vol-2) Narayana Group of Schools 4 A wooden scale is placed horizontally with its two ends resting on two glass tumblers containing water as shown in figure. Strike suddenly the centre of the ruler with a long rod. Observe that neither the glass tumblers break nor the water spill out. But the scale gets broken into two pieces. Why does this happen? (ii) Inertia of motion: The tendency of a body to continue in its state of motion is called inertia of motion. Example: a) When a fan is switched off, it continues to move due to inertia of motion. b) A running boy falls in the forward direction if he is tripped by a stone because the stone stops his foot, whereas the rest of the body continues moving forward due to inertia of motion and hence the boy falls in the forward direction. c) A rider falls forward when a galloping horse stops suddenly because when a horse stops, the rider due to inertia of motion, continues moving and hence falls in forward direction.