9th Class Science Force and laws of motion Classification of Forces

        Classification of Forces

Forces may be classified into the following two classes:

• Contact forces
• Action-at-a-distance

The contact forces arise as a result of physical contact between two bodies, one by which force is exerted and the other on which force is exerted. The force exerted by the wind on us is a contact force. The force of friction is also a contact force. The action-at-a-distance corresponds to the forces, which do not involve any physical contact between the two bodies, but act through the space between them.

The gravitational force, the magnetic force and the electrical force are the examples of the action- at-a-distance force.

             Free Body Diagram

A book is at rest on a table top..

• A baby is suspended motionless from the ceiling by two ropes.

       A motor cycle and a car are moving on a horizontal road with the same velocity. If they are brought to rest by the application of brakes, which provide equal retardation, then

(a) Motor cycle will stop at shorter distance

(b) Car will stop at a shorter distance

(c) Both will stop at the same distance

(d) Nothing can be predicted

(e) None of these

Answer: (c)

        The amount of net force required to keep a 5-kg object moving rightward with a constant velocity of 2 m/s is ________.

(a) 0 N

(b) 0.4 N

(c) 2 N

(d) 2.5 N

(e) 5 N