Force is any influence that tends to change the state of rest or the uniform motion in a straight line of a body. The action of an unbalanced or resultant force results in the acceleration of a body in the direction of action of the force or it may, if the body is unable to move freely, result in its deformation. Force is a vector quantity, possessing both magnitude and direction: its SI unit is the newton.

Balanced and Unbalanced Forces

Despite friction and gravity, objects on Earth can still be at rest or travelling at a constant velocity. In each case, this is because the forces acting are balanced; their effects cancel out and the objects behaves as if no force is acting.

The weight of the aircraft is balanced by the lift from the wings, and air resistance is balanced by the thrust of the engines. As a result, the velocity of the aircraft is constant. If one of the forces were to change, the velocity of the aircraft would change.

An object will accelerate if the forces acting on it are not balanced. Figure below shows a small rocket of mass 200 kg at the point of take off. The rocket engine is producing an upward force of 3000 N: 2000 N balances the weight of the rocket, but the remaining 1000 Nis unbalanced. The upward force and the weight therefore have the same effect as a single force of 1000 N upwards. There is said to be a resultant force on the rocket of 1000 Nupwards.

The acceleration of the rocket can be calculated as follows:

Resultant force = Mass X Acceleration

100ON = 200 kg x a This gives: a = 5 m/s2

The rocket therefore has an acceleration of 5m/s2 upwards.


The Gravitational Force

All objects fall because of the gravitational force of attraction exerted on them by the earth. The acceleration due to gravity g is independent of the mass of the object. The force acting is given by F = mg and is the weight of the object. The direction of this acceleration and consequently of F is towards the centre of the earth. This direction therefore varies from point to point on the earth, although we may assume it to be constant over a small region. The value of g which is about 9.8m/s2 also shows minor geographical variations. The weight of the body varies accordingly, but its mass remains the same.

The Electrostatic Force

Between any two electric charges there always exists a force. Electric charges are classified as positive and negative. When a glass rod is rubbed with a silk cloth, the glass rod becomes positively charged. Similarly, when a rubber rod is rubbed with fur, the rod acquires negative charge. Among elementary particles protons are positively charged while electrons carry negative charge. The electrostatic force is attractive between opposite charges and repulsive between like charges. This is in contrast to gravitation which is always attractive between any two given masses.

The Magnetic Force

The magnetic property of lodestone has been known since ancient times. Similarly the use of a magnetic compass has been in vogue for a very long time. Its needle, which is small magnet, is acted upon by the magnetic field of the earth. This is an instance of the magnetic force in action. The law of force between two magnets or the law of force on a magnet placed in a magnetic field are known, so that these forces can be readily computed.

Mechanical Force

These are the forces that are exerted by active or living objects, e.g. humans, animals, engines, moving objects, springs, etc.

Centripetal Force

When a body moves along a circular path with uniform speed, the magnitude of the velocity remains the same but its direction changes at every point. It means that there is change in velocity. Whenever there is change in velocity, there must be some acceleration. As a body has mass also, a force must act upon the body. This force must act along such a direction that the magnitude of the velocity does not change.


(1) When a bucket containing water is rotated in a vertical circle, water does not fall downward even when the bucket is at the highest point (In Fig.). The centripetal force is provided by the motion. Due to this reason water does not fall.

(2) In the case of motion of the moon round the earth centripetal force is provided by the gravitational force of attraction of the earth on the moon. Similarly the force of attraction of the sun on the planets provides the necessary centripetal force and the planets revolve round the sun.

(3) Electrons revolve round the nucleus in various orbits. The electrons do not collapse into the nucleus. The electrostatic force. of attraction of the nucleus on the electron provides the necessary centripetal force and the electron revolves round the nucleus.

Artificial satellites move round the earth. The force of attraction of the earth on the satellite provides the necessary centripetal force and the satellite revolves round the earth.

Centrifugal Force

Centrifugal Force is a fictitious or pseudo outward force on a particle rotating about an axis which by Newton’s third law is equal and opposite to the centripetal force. Like all such action-reaction pairs of forces, they are equal and opposite but do not act on the same body and so do not cancel each other. Consider a mass M tied by a string of length R to a pin at the centre of a smooth horizontal table and whirling around the pin with an angular velocity of o radians per second. The imass rotates in a circular path because of the centripetal force F. = MOR which is exerted on the mass by the string. The reaction force exerted by the rotating mass M, the so-called centrifugal force, is MWR in a direction away from the centre of rotation.

Centrifugal force, representing an inertia reaction, makes itself felt when a car begins to skid. Suppose we make a left turn too sharply or too quickly, the centrifugal inertia force of the turn becomes greater than the friction of the tyres against the road, and the car will start to skid to the right. The only way to overcome the momentum of the skid is to create a force to overcome it. This we can do by immediately turning the front wheels to the right–that is, in the direction of the skid. The car will then tend to turn in a rightward curve. In this way it sets up a leftward centrifugal force which stops the skid. The car may overbalance and tend to skid leftward again. The driver, therefore, must be prepared to turn the front wheels to the left if this should happen. This manoeuver is done expertly by racing car drivers. who deliberately skid their cars around turns in order to avoid losing speed.

One is likely to skid on an icy road, because on such a road the road friction is reduced to a minimum. The turning of the front wheels will not bring us into an opposite turn as quickly as on a dry road, because the car will have travelled further in the direction of the skid.

A method commonly used to prevent skidding when rounding a curve is to construct the curved roadway so that the outer part of the road is higher than the inner. This construction is called banking of road. It brings about the same effect as leaning inward when rounding a flat curve on a bicycle. The cyclist leans inward to balance the effect of centrifugal inertial force. This is called bending of cyclist. Of course the automobile cannot “lean inward”, but the same effect is produced if the road is banked at the proper inclination.

 A familiar application of inertial force is found in centrifuges, which are devices used to separate one material from another. This device is often used to remove from a liquid the small solid substances it contains. The solids in the liquid, being heavier than the liquid itself, are thrown outward and collect in the bottom of the tube. The machine is then stopped and the clear liquid is poured off. The same principle is used to separate cream from milk in the device known as the separator. In the separator, the milk is passed it to a spinning bowl and is drawn off. The cream, which is lighter, stays near the centre and is drawn off from that point.