Zigya Logo

Chapter Chosen

Work, Energy and Power

Book Chosen

Physics Part I

Subject Chosen


Book Store

Download books and chapters from book store.
Currently only available for.
CBSE Gujarat Board Haryana Board

Previous Year Papers

Download the PDF Question Papers Free for off line practice and view the Solutions online.
Currently only available for.
Class 10 Class 12
A person trying to lose weight (dieter) lifts a 10 kg mass, one thousand times, to a height of 0.5 m each time. Assume that the potential energy lost each time she lowers the mass is dissipated.
(a) How much work does she do against the gravitational force?
(b) Fat supplies 3.8 × 10J of energy per kilogram which is converted to mechanical energy with a 20% efficiency rate.
How much fat will the dieter use up?


(a) Mass of the weight, m = 10 kg

Height to which the person lifts the weight, h = 0.5 m

Number of times the weight is lifted, n = 1000


Work done against gravitational force, 

W = n(mgh)

   = 1000 × 10 × 9.8 × 0.5 

   =  49 kJ

(b) Energy equivalent of 1 kg of fat = 3.8 × 107 J 

Efficiency rate = 20% 

Mechanical energy supplied by the person’s body, 

E = (20/100) × 3.8 × 107 J 

  = (1/5) × 3.8 × 107 J 

Equivalent mass of fat lost by the dieter, 

m = [ 1 / (1/5) × 3.8 × 107 ]× 49 × 10

   = (245 / 3.8) × 10-4 

   = 6.45 × 10
-3 kg

What is watt?

SI unit of power is Watt. 

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 .

Define watt.

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

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 are different units of energy?

The different units of energy are :

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