# Physics Study Notes: Work Power and Energy

**Work**

^{7 }erg.

Work done by a force is zero when

-Body is not displaced actually, i.e. s = 0

-Body is displaced perpendicular to the direction of force i.e. Î¸ = 90°.

**Work done by a variable force**

If we throw a ball upward, work done against gravity is given by, W = mgh

where m = mass of the body,

g = acceleration due to gravity and

h = height through which the ball is raised.

The centripetal force acts on a body perpendicular to the direction of motion. Therefore, work done by or against centripetal force in circular motion is zero.

If a coolie is carrying a load on his head and moving on a horizontal platform, then work done by force of gravity is zero as displacement is perpendicular to the direction of the force of gravity.

**Energy**

The energy of a body is its capacity of doing work. It is a scalar quantity and its SI unit is joule.

Energy can be transformed into work and vice-versa with the help of some mechanical device.

There are two types of Mechanical Energy, which are as follows

**Kinetic Energy**

The energy possessed by a body by virtue of its motion is called it's kinetic energy.

The kinetic energy of the body of mass m moving with velocity v is given by K = 1/2 mv

^{2}.

**Potential Energy**

The energy possessed by an object by virtue of its position or configuration is called its potential-energy.

Gravitational potential energy, U = mgh.

**Einstein’s Mass-Energy Relation**

According to this relation, the mass can be transformed into energy and vice-versa.

When a ∆m mass is disappeared, then produced energy

E= ∆mc^2

where c = speed of light in vacuum.

**Law of Conservation of Energy**

Energy can neither be created nor be destroyed, only one type of energy can be transformed into other forms of energy.

Only for conservative forces, (total mechanical energy)

initially = (total mechanical energy) finally.

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