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In physics, power is the rate of doing work. It is equivalent to an amount of energy consumed per unit time. In the SI system, the unit of power is the joule per second (J/s), known as the watt in honour of James Watt, the eighteenth-century developer of the steam engine.The integral of power over time defines the work performed. Because this integral depends on the trajectory of the point of application of the force and torque, this calculation of work is said to be path dependent.
(i)A motor creates 4500 N tension in a hoisting cable and reels it at a rate of 2 m/s.What is power of electric motor
(ii)A pump can throw up 10 quintals of coal per hour from a coal mine 120 m deep.Calculate the power of the engine in watt assuming efficiency as 80%
(iii)A tubewell pumps out 2400 kg of water per minute If water is coming out with a velocity of 3 m/s what is power of the pump?how much work is done if pump runs for 10 hours
(iv)A car weighnig 1120 kg is going up an incline of 1 in 56 at the rate of 20 m in 2 s.find the power of engine if frictional force is 64N
(v)A train weighning 100 metric ton is running on a level track with uniform speed of 72 km/h.if frictional resistance amounts to 0.5 kg per metric ton,find power of the engine


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1)   power  = work done by the tension = 4500 N * 2 m/s
              tension is able to move the cable ..causes displacement in the mass of cable at rate 2 m/s..  displacement = 2 * t... work done = 4500 N * 2 * t ...
2)  mass m = 10 quintals/hour       h= height = 120 m      efficiency = 80%
             work done in one hour = m g h 
               power needed with 80% efficiency =  m g h / 80%

3)   m / t  = 2400 kg /min        v = 3 m/s
               power =  1/2 m v * v / t  = 1/2 *2400 * 3 * 3  W
4)   m = 1120 kg  v = 20m/2s = s / t  =  10m/s   
       tan Ф = 1/56          find  sin Ф ≈ 1/56  nearly
         power = m g sin Ф * s / t = 1120 * 10 * 1/56 * 10   W

5)    m =  100 tons                        v = 72 kmph   
       frictional force = F = 0.50 kg metric ton * g
             power = F* v
  calculate ..
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