States that a body continues in its state of rest or of uniform motion in a straight line unless compelled by an external force to change that state. This law can be put in simple words : “Departure from a straight line motion path indicates the presence of an external force.”
Illustration (i) A man jumping of the moving train falls forward because his feet suddenly come to rest, while his body is in motion with the train.
(ii) A running person falls down when is foot stumbles against a stone. This is because the foot is brought to rest while the upper part remains moving.
NEWTON'S SECOND LAW OF MOTION
The rate of change of momentum is in proportion to the impressed fore and takes place in the direction of force. The harder a football is kicked, the faster will it move, and it will move in the same direction in which it is kicked.
NEWTON'S THIRD LAW OF MOTION
Says that to every action, there is an equal and opposite reaction.
Illustrations. (i) If we step out of a boat, we go in one direction and the boat goes in the other direction.
(ii) When a bullet is fired from a gun, the bullet goes forward and the gun kicks backward. This is called the recoil of the gun. Unless there is action we cannot have reaction.
(iii) We cannot drive a nail into a wooden block unless it is supported against something to offer reaction.
(iv) We cannot cut a piece of paper with one blade of a pair of scissors, since there will be no reaction.
(v) When a man wants to walk or run, he presses the ground backward with his feet. The ground, therefore, exerts an equal and opposite force on the man. This force acting on the man enables him to move forward.
(vi) In order to fly an airplane the engine is first started and the propellers or blades are made to revolve rapidly. These blades are curved in such a way that, on revolving, they throw the air back with a great force, and consequently the air pushes them forward with an equal and opposite force. Hence, if the obstacles placed before the wheels are removed, the airplane runs forward at a great speed.
Is the pull of the earth with which it attracts bodies towards itself.
LAW OF GRAVITATION
States that “every particle in this universe attracts every other particle with a force which is directly proportional to the product of other masses and inversely proportional to the square of the distance between them.” We believe this law to be true because the motion of heavenly bodies can be easily explained by this law. Astronomical calculations based on this law predicted the existence of new stars.
When a small piece of stone tied to a string is whirled, one has to pull the string inwards. This pull on the stone is called the centripetal force. An equal and opposite force is exerted by the stone on the hand. This force on the hand is called centrifugal force. Thus centripetal force is directed towards the center while centrifugal force is directed away from the centre.
Illustration. (i) Mud sticking to a bicycle tyre is pulled in radially by the force of adhesion; when the latter force is less than the centripetal force required to move the mud in a circle, the mud files off tangentially.
(ii) The sparks, which fly off from the grinding stone of a blacksmith, are also due to the same phenomenon.
(iii) Curved roads or rail tracks are banked or raised on one side so that a fast-moving train or a motor car leans inwards, thus providing the required centripetal force to enable it to move round in curve.
(iv) A cyclist while rounding a curve leans inwards so as to provide him with the necessary centripetal force, which would enable him to take a turn on the circular path.
(v) In a laundry, wet clothes are whirled in a large vessel the sides of which are perforated with hundreds of small holes. The water is thrown off at a tangent through these holes, and clothes thereby get dried soon.
(vi) In a dairy, cream is separated from the rest of the milk, (skimmed milk) by means of a cream separator. The principle is that heavier the body the greater is the centripetal force required by it to move in circle. Skimmed milk is heavier than cream, its particles require a greater force to move than the cream particles, and, therefore, the former remains nearer the walls of the spinning vessel, and the latter remains near the axis.