Physics Study Notes: Work Power and Energy



Work
Work is a scalar quantity. Its SI unit is joule and CGS unit is erg. 1 joule=107 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. θ = 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 mv2.

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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