Force - Definition, Units, Types

Force is any influence that changes or tends to change the state of rest or the uniform motion in a straight line of a body. Mechanical force is the force associated with motion. The strongest force in nature is nuclear force and the weakest force in nature is gravitational force.

Unit of force is Newton (N) in SI Unit. Dyne (CGS Unit) is yet another unit of force.

One Newton = 100000 dynes

Newton’s second law gives the measure of force.

i.e., F = ma ; F – Force, m – mass, a – acceleration

Fundamental Forces

There are four fundamental forces in the universe. They are gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force. Of these, gravity and the electromagnetic force have long-range limits. The other two act at short distances.

Contact and Non Contact Forces

All objects in the universe are subject to the force of some other object. We can mainly divide forces into two types: contact force and non-contact force. Contact force is the force that occurs between two objects that are in contact with each other. Weight, Normal Force, Frictional Force, Te​nsion, Spring Force are examples of Contact forces. Non-contact force is the force that occurs between objects that are not in contact with each other. Gravitational Force, Electrostatic force, Magnetic force are examples of Non Contact Forces.

Types of Forces

1. Normal force

Imagine a book on a table. The gravitational force of the earth is always pulling it down. But the book does not fall down. The table supports it. This is an example of normal force. Take two bricks and put a scale on top of them. You can put a small stone on this scale. The scale will bend down a little. But you can see that it supports the stone. This is also due to normal force. If you press the scale down with your hand, you can feel this force exerted by the scale.

2. Weight 

Weight is the gravitational force exerted by the Earth on an object. Weight can be defined as the force gravitational acting on a body. Mass remains constant while weight varies from place to place.

Weight, w = m x g

3. Impulsive Force

Impulse of a force is the product of the force and the time during which the force acts on the body. The large force acting for a very short time is called an impulsive force. It is measured as the rate of change in momentum. For example, force exerted on a bullet when it is fired with a gun, force exerted by hammer on a nail etc.

4. Frictional Force

Friction is a force that resists the movement of one surface over another. Friction is the property that objects have which makes them resist being moved across one another. If two objects with flat surfaces are placed one on top of the other, the top object can be lifted without any resistance except that of gravity. But if one object is pushed or pulled along the surface of the other, there is a resistance caused by friction. Friction has many important uses. It makes the wheels of a locomotive grip the rails of the track. It allows a conveyor bell to turn on pulleys without slipping. You could not walk without friction to keep your shoes from sliding on the pavement. This is why it is hard to walk on ice. The smooth surface of the ice produces less friction than a pavement and allows shoes to slip.

Friction can be broadly categorized into Static Friction, Kinetic Friction, Sliding Friction, Rolling Friction and Fluid Friction.

1. Static Friction prevents objects from moving when a force is applied to them.

2. Kinetic Friction between two surfaces in contact is the force of friction which comes into play when there is relative motion between the surfaces.

3. Sliding friction is the friction experienced when one object slides over another.

4. Rolling Friction between two surfaces in contact is the force of friction which comes into play when a body rolls over another. This is the principle on which ball bearings work.

5. Fluid Friction occurs when an object moves through a liquid or gas. It is also called as Drag.

Due to the existence of friction we can walk on the ground, trains can run on rails. Frictional force offered by air helps the parachute to have a slow landing. Friction offered by atmospheric air is protecting us from being hit by meteors. A lubricant is a substance which forms a thin layer between two surfaces in contact. Generally oil and grease are used as lubricants to reduce friction as well as to protect the moving parts from overheating. A modern lubricant is a mixture of mineral oil, vegetable oil and colloidal thin oil. Flow of compressed pure dry air acts as lubricant. Graphite is a solid lubricant.

5. Centrifugal Force

Centrifugal force is a force that acts on a body moving in a circular path and is directed away from the centre around which the body is moving. Centrifugal force helps to separate cream from milk.

Example - Suppose a fast-moving bus is making a sudden turn. If there are passengers who are not holding on tightly, they will be thrown in the opposite direction of the turn. This is due to centrifugal force. Centrifugal force is the force that pushes an object moving in a circle away from the center of its path. Roads are built to slope inwards on curves to prevent vehicles from being thrown out due to this force when they turn.

6. Centripetal Force

Centripetal force is a force which acts on a body moving in a circular path and is directed towards the centre around which the body is moving. The centripetal force is the force that causes an object to accelerate in a circle. The direction of the centripetal force and the centripetal acceleration are towards the center of the circle.  Centripetal force is necessary to keep an object moving in a curved path. When a man circles round the earth in a spacecraft, his mass remains constant but weight becomes zero.

7. Magnetic Force

The force between the iron and the magnet is a magnetic force. for instance, the magnetic poles in a DC motor.

8. Gravity Force

Force of attraction between any two bodies in the universe is known as Gravitation. The attractive force of earth, or other celestial body, on an object is called Gravity Force.

9. Cohesion and Adhesion

The force of attraction between like molecules is called Cohesion and the force of attraction between unlike molecules is called Adhesion.

10. Electrostatic Force

Electrostatic force is the attractive or repulsive force between two electrically charged objects.

11. Te​nsion

The force exerted at any point in the wire or string or rope or rod is called the tension at that point.

12. Sprin​g Force 

Springs resists attempts to change their length. This is because of Spring Force. The unit of spring constant is Newton/metre. This equation is also called as Hooke’s law.