# Work and Energy

## 9th Class Physics Chapter 6 Exercise Short Questions with Answers

Q 6.2 : Define work . What is its SI unit ?

Work :

” Work is done when a force acting on a body displaces it in the direction of a force .”

Formula :

Work = Force × distance

W = F S

SI unit of work : SI unit of work is joule ( J ) or Nm.

Q 6.3 : When does a force do work ? Explain .

” The work is said to be done when force acting n a body displaces it in the direction of a force .”

Work = Force × distance

W = F S

• Greater the force acting on a body and longer is the distance moved by it , larger would be the work done .

Q 6.4 : Why do we need energy ?

1. Energy is used by us to perform many activities of life .
2. Energy is necessary for running and walking for humans .

Q 6.5 : Define energy , give two types of mechanical energy .

Energy :

” A body possesses energy if it is capable to do work .”

Types of Mechanical Energy : Mechanical energy has two following types :

1. Kinetic Energy
2. Potentia Energy

Q 6.6 : Define K.E. and derive its relation .

Kinetic Energy :

” Energy possessed by a body due to its motion is called Kinetic energy .”

Derivation : If a ball moves with initial velocity vi after covering some distance , it stops and its vf is zero . During this , it does work against force of friction ( F = ma ) so ,

• By Third Equation of Motion :

Q 6.7 : Define potential energy and derive its relation .

Potential Energy :

” Energy possessed by a body due to its position is called potential energy . ”

Formula : P.E. = mgh

Derivation :

Q 6.8 : Why are fossil fuels called non-renewable form of energy ?

Fossil fuels are called non-renewable form of energy .

Reason : Because fossil fuels take million of years for their formation .

Q 6.9 : Which form of energy is most preferred and why ?

→ Solar Energy is most preferred energy .

Reason :

• Because sunlight does not pollute the environment in any way .
• Solar Energy reaching Earth is thousand times more than the energy consumption of mankind .
• Sunrays are ultimate source of life on earth . We are dependent on the sun for all our food and fuels .

Q 6.10 : How is energy converted from one form to another ? Explain .

According to Law of conversation of Energy :

” Energy cannot be created nor destroyed however it can be converted into some other forms .”

Examples :

Solar cells are used to convert light energy into the electrical energy .

When we rub our hands . Mechanical energy convert into heat energy .

Q 6.11 : Name any five devices that convert electrical energy into mechanical energy .

1. Washing machine
2. Electrical motor
3. Electrical grinder
4. Electrical spinner
5. Juicer

Q 6.12 : Name a device that converts mechanical energy into electrical energy .

• Generator
• An AC generator converts mechanical energy into electrical energy .

Q 6.13 : What is meant by the efficiency of a system ?

Efficiency :

” Efficiency of a system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input .”

Formula :

• Input : Energy given to machine .
• Output : Work done by machine .

Q 6.14 : How can you find the efficiency of a system ?

Efficiency of a system can be determined by following formula :

Q 6.15 : What is meant by the term power ?

Power :

” Power is defined as the rate of doing work .”

Mathematically :

Unit : SI unit of power is W ( watt ) .

Power is a scalar quantity .

Q 6.16 : Define watt .

Watt is SI unit of power . It is defined as :

” The power of a body is one watt if it does work at rate of 1 joule per second ( 1 Js-1 ) .”

Kilowatt and Megawatt are larger units of power .

• 1 kW = 1000 W = 103 W
• 1 MW = 1000000 W = 106 W
• 1 hp = 746 W ( hp = horse power )

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