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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 trolley of mass 200 kg moves with a uniform speed of 36 km/h on a frictionless track. A child of mass 20 kg runs on the trolley from one end to the other (10 m away) with a speed of 4 m s–1 relative to the trolley in a direction opposite to the its motion, and jumps out of the trolley. What is the final speed of the trolley? How much has the trolley moved from the time the child begins to run?


Mass of the trolley, 
M = 200 kg 

Speed of the trolley, v = 36 km/h = 10 m/s 

Mass of the boy, m = 20 kg 

Initial momentum of the system of the boy and the trolley, 

= (M + m)v 

= (200 + 20) × 10 

= 2200 kg m/s 

Let v' be the final velocity of the trolley with respect to the ground. 

Final velocity of the boy with respect to the ground = v' - 4 

Final momentum = Mv' + m(v' - 4) 
                           = 200v' + 20v' - 80 

                           = 220v' – 80 

As per the law of conservation of momentum,

Initial momentum = Final momentum 

                   2200 = 220v' – 80 

∴                       v' =2280 over 220 space equals space 10.36 space m divided by s

Length of the trolley, l = 10 m 

Speed of the boy, v'' = 4 m/s 

Time taken by the boy to run, t = 10 over 4 = 2.5 s
∴ Distance moved by the trolley = v'' × 

                                                = 10.36 × 2.5

                                                = 25.9 m

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. 

Define watt.

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

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 .

What are different units of energy?

The different units of energy are :

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


What is watt?

SI unit of power is Watt.