Force may be described as a pull or a push that causes a body to accelerate or change the velocity of the body

Or

Force is defined as rate of change of momentum. It is a vector quantity. S.I. unit – Newton.

All the force in universe are basis on for fundamental interactions.

1. Gravitational force

2. Electromagnetic force

3. Strong nuclear force

4. Weak nuclear force.

**1. Gravitational Force:** The force exerted by the two object due to mass in universe is gravitational force.

Gravitational force is always attractive is directly proportional to the product of masses and inversely proportional to square of separated distance.

**2. Electric force:** The force experienced by a charge in electric field of another charge.

Electric force either be attractive or repulsive. It is directly proportional to the product of charge and inversely proportional to the square of the separated distance of charge

**3. Strong Nuclear force: **The forces that bind the neutrons and protons together in a nucleus are called the strong nuclear forces.

Nuclear force are the strongest force in nature. They are times stronger than gravitational force, times stronger than electrostatic force and times stronger than weak forces.

Nuclear force have the shortest range, non central and non – conservative forces.

**4. Weak nuclear force:** The forces of interaction between elementary particle of short life times.

These force were discovered during the study of the phenomenon of decay in radioactivity.

The weak nuclear forces are times stronger than the gravitational force.

The relative strength of four types of basic force in nature.

__OTHER TYPES OF FORCES:__

**Frictional force:** It is a surface force which is exerted by two contact surfaces that opposes relative motion. It is related to normal force R by frictional force = where = coefficient of friction.

Consider a box of mass m. weight of the box mg acts on the surface and according to Newton’s third law, the surface also produce a force, perpendicular to the surface. If the surface is horizontal, the normal force is equal to weight but if the surface is inclined, the normal force is equal to the perpendicular component of the weight of the surface.

**Viscous drag:** It is a type of friction in fluids (liquid and gases) that opposes the relative motion of an object with respect to surrounding fluid.

Viscous drag highly depends on viscosity. Different fluids have different viscosity, which affects the viscous drag.

Consider a ball dropped in water and in oil. The motion of the ball in water is quicker than in the oil. So the viscous drag in oil is more than in water.

**Upthrust:** Upthrust (buoyancy)is an upward force exerted by a fluid. Opposing the weight of an immersed (or floating) object. This force arises due to the difference in pressure between the upper and lower surface of the object.

From Archimedes’s principle, the Upthrust is equal to the weight of fluid displaced by the submerged object.