Mechanics & Properties of Matter - Part 2

MECHANICS & PROPERTIES OF MATTER - PART 2

CENTRIPETAL FORCE
Activity :
•    Swing a stone tied to a string and observe that the stone follows a circular path.
•    Let the string slip through your fingers and observe that the stone no longer follows a circular path but flies off in the direction of the instantaneous velocity.
•    This direction is tangential to the circular path. From this activity we understand that a force acting on the stone pulls it towards the centre of the circle.
•    This force is called the centripetal force. Centripetal force is the force needed to make an object travel in the circular path.
•    The centripetal force causes an acceleration towards the centre of the circle and this acceleration is called the centripetal acceleration.
•    The centripetal acceleration (ac) of an object in uniform circular motion is given by 

Centripetal force finds many practical applications :
•    In washing machine’s spin cycle, water is separated from the clothes. The tub of the washer rotates rapidly. The force exerted on the water present in the clothes is not much enough to make the water travel in a circle with the clothes. The water flies off, leaving the clothes less wet due to lack of centripetal force.
•    The less desirable case of lack of centripetal force is when the rear wheel of an automobile spins in mud. The adhesion of the mud to the wheel which is the centripetal force in this case is not enough to hold the mud on the tyre. So it comes off tangentially to the tyre’s circular motion.
•    Gravitational force between a satellite and the earth acts as a centripetal force, keeping the satellite in orbit.

CENTRIFUGAL FORCE :
•    We feel a force pushing us outward or away from the centre of curvature when we travel in a fast moving car rounding a sharp curve or in a rotating ride in an amusement park.
•    This force is known as centrifugal force. It acts in the opposite direction to that of centripetal force. The centrifugal force is also given by F =
•    When earth rotates about its own axis, the velocity of bodies near the equator is more than that at the poles.
•    On earth the centrifugal force is minimum at the poles and maximum at the equator. This is the reason why the poles of the earth are found to be nearly flat and the diameters of earth along the equator and poles are different by 48 km.
•    The centripetal force required for a car or a bicycle to go round a circular curve depends on its speed and the radius of curvature of the curve.

APPLICATIONS OF CENTRIFUGAL FORCE
•    The principle of centrifugal force is applied to the machines.
•    They are used to separate materials of different weights or densities by spinning action.
•    The liquid is rotated in a cylindrical vessel at a high speed with the help of an electric motor.
•    The heavier particles move away from the axis of rotation and lighter particles move nearer to the axis of rotation.
•    The spinning drum in a washing machine to separate water from clothes is a centrifuge.
•    Centrifuges are used in separating blood cells from plasma and cream from milk in dairy separators.
•    Ultra centrifuges with speeds of the order of 5 x 105 rpm are used to concentrate viruses in solution.
•    The holder of a centrifuge container is pivoted, so that the container will be in the horizontal position, when the centrifuge spins rapidly.
•    The heavier materials migrate towards the outer end of the container. For example when blood samples are centrifuged, the red cells reach the bottom and lighter white cells go to the top of the tube.
•    Sugar crystals are separated from molasses with the help of a centrifuge. Honey is also separated from bees wax with the help of a centrifuge.

WATT GOVERNOR :
•    Makes use of the centrifugal force for regulating the speed of an engine or machine. It consists of two heavy balls B1 and B2 joined to the ends of the rods R1 and R2 hinged at H, the end of the spindle S.
•    These rods are connected through the link rods L1 and L2 to a sleave m which can slide upand down on the spindle.
•    As the speed of the engine increases, the speed of rotation of the spindle also increases and consquently the centrifugal force on the balls B1 and B2 increases. Now the balls and the sleave move up due to the increase in speed of rotation of the balls.
•    This partially closes a valve which controls the entry of steam to the engine and the speed of the engine is checked.
•    Thus the speed of the engine is regulated by regulating the supply of steam.
•    In circus there is an event known as the cage of death. In this event, a motor cyclist drives a motor cycle at a high speed on the inner walls of a spherical cage of iron. But he does not fall off the motor cycle even when he is upside down.
•    The centrifugal force keeps the motor cyclist glued to his seat while driving his motor cycle inside the cage.