Concept of Force / Definition and details of of force/Gravitational force, centripetal force, Contrifugal force Friction
Concept of force in Physics
Force: the word force generally denotes a push or a pull. Let us see what a force can do. Inhen hockey match a player hits a stationary ball with his stick and the ball starts moving in a straight line. Another player deflects the moving ball in another direction and yet another player stops the ball. Sometimes a player simply pushes the moving ball to increase its speeds without changing its direction. In all the cases the players apply force with their sticks. Thus we can say that force produces change in a body's state of rest or of uniform motion in a straight line.
Consider what happens when more than one force is exerted on a body. If two persons pull an object in the same direction with equal force, the object will have twice the acceleration than if one pulled alone. If, however, the two pulled with equal force but in opposite directions, the object will not accelerate because the oppositely directly equal forces cancel one another and the net force is zero.
It should be noted that zero net force, and therefore, zero acceleration does not necessarily imply zero velocity. Zero acceleration means that the object maintenance its velocity, neither increasing nor decreasing. If the object is at rest , it remains at rest under the action of zero net force.
Even when a single force is applied on an object, the idea of net force must be taken into account because forces other than the applied force may act on the object. Easily this other forces are friction forces. The direction of the friction force is always opposite to the direction of motion. If under the action of an applied force, a wooden block slides on a table top with constant velocity, we conclude that no net force is acting on the block. Obviously a friction force is acting on the block, equal in magnitude and opposite in the direction to the applied force.
Gravitational Force:
We are all aware of the force who is pulls us towards the earth. This is called gravitational force. In fact the gravitational force exist between all bodies, even between two apples lying on table. It is gravitational force that holds the moon in its orbit round the earth and the earth in its orbit round the sun.
Newton's law of universal gravitational:
Newton's law of universal gravitation states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of distance between them. In equation form
. Gravitational force F = G M1 x M2 /r²
Where r is the distance between two particles of masses M1 and M2 and G is the universal gravitational constant.
Centripetal Force:
For a body to move in a circle there must be a force on it directed towards the center. This is called the centripetal force and is necessary to produce continuous change of direction in a circular motion. In case of the moon, gravitational force between the Earth and the moon acts as centripetal force. When a stone tired at one end of a string is whirled in a circle, the pull industry provides the centripetal force.
The magnitude of the centripetal force F, required to cause an object of mass m and speed v to travel in a circular path of radius r is given by the relation
F= mv²/r
Centrifugal Force:
This force, is supposed to be acting on a body revolving in a circle. Centrifugal force is equal and opposite to centripetal force, i.e. it acts outwards. It may be emphasized that centrifugal force is not a real force, however it is invoked to explain various phenomena successfully.
Weight:
The weight of a body is the force with which the earth attracts the body towards its centre. The weight of a body should not be confused with its mass, which is a measure of the quantity of matter content in it. When we say that a person weight 60 kg, we are actually describing his mass and not weight. The mass of a body is a constant quantity whereas it's weight varies slightly from place to place on the earth.
Weight W = mass x g
(g = acceleration due to gravity)
Friction:
Friction is the force who is opposes the relative motion of two surfaces in contact. Friction place and important role in our lives. It is a friction between the ground and the soles of our shoes that makes walking possibly and it is lack of friction that makes our feet sleep on highly polished surfaces.
The force of friction that acts when a body is moving on a surface is called sliding friction. The amount of sliding friction depends on the nature of the two surfaces and not on the area of contact. However it is also depends on the weight of the moving body.
When a cylindrical or spherical body rolls over a surface, the force opposing the motion is called rolling friction. For the same pair of materials rolling friction is much smaller than sliding friction.
While friction is necessary in some circumstances, it becomes a nuisance in others. Friction in machines wastase energy and also causes wear and tear. This friction is reduce by using lubricants and ball bearings. The presence of a liquid lubricant in a machine prevents metal to metal contacts and seems the friction between liquid layer is mass less than the friction between solids, the frictional forces in the machine are greatly reduced. Since rolling friction is much less than sliding friction, the use of ball bearings in a machine considerably reduces friction.