The document Work, Power and Energy - Physics, IAS UPSC Notes | EduRev is a part of the UPSC Course General Science(Prelims) by IRS Divey Sethi.

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** Work, Power and Energy**

**Work**

- Whenever a force acting on a body displaces it, work is said to be done.
- If a force F acting of the body displaces it by a distance s in the direction of force , work done is given by,
**W = F.s (or)**

** W = |F||s| cosθ**

- If θ < 90°, work done is positive and if θ > 90° it is negative.
- Whenever a force acting on a body displaces it, work is said to be done.
- If a force F acting of the body displaces it by a distance s in the direction of force , work done is given by,
**W = F.s (or)**

** W = |F||s| cosθ**

- If θ < 90°, work done is positive and if θ > 90° it is negative.

**Examples:**

- When body falls freely under gravity, θ = 0, and work done is positive.
- When a lawn roller is pulled by applying a force along the handle at an acute angle, work done by applied force is (+) ve.
- When a spring is stretched or when a gas filled in a cylinder filled with moving piston is allowed to expand, work done is positive.

**Negative Work Physics : **If θ is obtuse (θ > 90°), work done is negative.

**Examples :**- When a body is thrown up, its motion is opposed by gravity; θ between F and s is 180°.
- When a body is moved over a rough horizontal surface, the motion is opposed by the force of friction.
- Work done by the friction force is negative.
- When brakes are applied to a moving vehicle.
- When a positive charge is moved closer to another positive charge.

**Zero Work Physics :** If θ is right angle (θ = 90°), work done is zero.

**Examples :**

- When we fail to move a heavy stone because s > = 0.
- Coollie carrying some load on his head on a horizontal platform θ = 90°.
- Person carrying a briefcase moves on a horizontal road.
- The total capacity of doing work by a body is called its energy.
- It is a scalar quantity.
- Its unit is joule.

**Kinetic energy; -** the energy possessed by a body by its motion is called its kinetic energy.

- K=1/2 mv
^{2 }joule

**Potential energy; -**it is the energy stored in a body by virtue of its position of state.

- U= mgh

**Law of Conservation of Energy**

- It states that the total energy remains constant in any process i.e., it may neither be created nor destroyed.
**KE + PE + other forms of energy = constant**- If conservative forces are acting on a body, then,
**KE + PE = constant**i.e., the loss of PE is equal to the gain in KE.

**Einstein Relation**

- According to Einstein, mass can be converted into energy and energy can be converted into mass. One can be obtained at the cost of the other.
- The equivalence between mass and energy is expressed in terms of Einstein relation:
**E = mc²**- Where m is the mass that disappears, E is the energy that appears and c is the velocity of light in vacuum (c = 3 x 108; m/s).

- The rate of doing work by a body is called its power.
- If an agent does work W in time t then average power is,
**P = W/t = F.s/t = F.v = |F||v|cosθ**- It is a scalar quantity.
- SI units of power is watt and is also measured in horse power.
- 1 HP = 746 W (watt)

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