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 ?
- Energy is used by us to perform many activities of life .
- 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 :
- Kinetic Energy
- 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 .
- Washing machine
- Electrical motor
- Electrical grinder
- Electrical spinner
- 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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