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Work, Energy and Power

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Physics Part I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
A bolt of mass 0.3 kg falls from the ceiling of an elevator moving down with an uniform speed of 7 m s–1. It hits the floor of the elevator (length of the elevator = 3 m) and does not rebound. What is the heat produced by the impact? Would your answer be different if the elevator were stationary?


Mass of the bolt, m = 0.3 kg 

Speed of the elevator = 7 m/s 

Height, h = 3 m 

Since the relative velocity of the bolt with respect to the lift is zero, at the time of impact, potential energy gets converted into heat energy. 

Heat produced = Loss of potential energy  

                      = mg

                      = 0.3 × 9.8 × 3 

                      = 8.82 J 

The heat produced will remain the same even if the lift is stationary. This is because of the fact that the relative velocity of the bolt with respect to the lift will remain zero.

Define the unit joule.

Work done is said to be one joule if one newton of  force displaces the body through a distance of one meter in the direction of applied  force .

Is work a scalar or a vector quantity?

Work is dot product of two vectors. i.e. F with rightwards arrow on top times S with rightwards arrow on top.

And dot product is a scalar quantity. Therefore, work is a scalar quantity. 

What is watt?

SI unit of power is Watt. 

Define watt.

Power is said to be one watt if one joule of work is done in one second. 

What are different units of energy?

The different units of energy are :

(i) Joule            
(ii) Erg
(iii) eV            
(iv) KWh
(v) Calorie.