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  • Define watt. Express kilowatt in terms of joule per second. A 150 kg car engine develops 500 W for each kg. What force does it exert in moving the car at a speed of 20 m s^–1?

Define watt. Express kilowatt in terms of joule per second. A 150 kg car engine develops 500 W for each kg. What force does it exert in moving the car at a speed of 20 m s^{-1} ?

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Solution.

Power of anything is defined as the rate of work done by that thing or energy supplied per unit time.
Mathematically, power is defined as the product of force and velocity.
Power = F v.
Power is defined as work done per unit time and its unit will be watt (in SI unit) or kilowatt.
If we multiply power with time, we will get energy or work done.
power=\frac{energy}{time}
unit of power=\frac{joule}{second}=watt

1 kilowatt = 1000 watt = 1000 joule/second.
For the car:
Mass of the car:  m = 150 kg
Speed attained:  V = 20 m s^{-1}
Power used:  P = 500W per kg hence,
Total power will be:  
Power=500\times 150=75000watt
By using the formula of power:
75000 = (Force) (20)
Force = 3750 N.

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