Physics Power Calculations (PDF)

WORK AND POWER Chapter 8 - POWER POWER Rate of doing work / rate of consumption of energy is called power. Power is a scalar quantity. FORMULA FOR COMPUTING POWER Power (P) = Work done (W)/ time (t) P = W/t P = W/t = F x S / t = F x v Thus, Power is also given by the equation, P = for...

WORK AND POWER Chapter 8 - POWER POWER Rate of doing work / rate of consumption of energy is called power. Power is a scalar quantity. FORMULA FOR COMPUTING POWER Power (P) = Work done (W)/ time (t) P = W/t P = W/t = F x S / t = F x v Thus, Power is also given by the equation, P = force (F) x avg. speed (v) P=Fxv FACTORS AFFECTING POWER UNITS OF POWER RELATIONSHIP BETWEEN UNITS OF POWER 1 kW = 103 W 1 MW = 106 W 1 GW = 109 W 1 mW = 10-3 W 1 μW = 10-6 W Horse power (HP) is another unit of power largely used in mechanical engineering. It is related to the S.I. unit as given – 1 HP = 746 W 1 HP = 0.746 kW PRACTICE SUM 2 A boy weighing 350 N runs up a F = 350 N; S = 30 x 20 cm = flight of stairs having 30 steps 600 cm = 6 m; t = 1 min = 60s each 20 cm high; in one minute. Calculate the work done and W = F X S = 350 x 6 = 2100 J power spent by him. P = W/t = 2100/60 = 35 W PRACTICE SUM 3 A man spends 6.4 kJ of energy in E = 6.4 kJ = 6400 J, S = 64 m, t displacing a body 64 m in = 2.5 s direction of the applied force; in 2.5 s. Calculate the force applied, E = F x S 🡪 F = E/S and the power spent (in HP) by F = 6400/64 = 100 N the man. P = E/t = 6400/2.5 = 2560 W P = 2560/746 = 3.43 HP DIFFERENCE BETWEEN POWER & WORK POWER WORK Rate of doing work Product of force and displacement in the direction of force S.I. unit: watt (W) S.I. unit: joule (J) Depends on time in Does not depend on which work is done time DIFFERENCE BETWEEN POWER & ENERGY POWER ENERGY Energy spent in one Capacity to do work second S.I. unit: watt (W) S.I. unit: joule (J) Depends on time in Does not depend on which work is done time DIFFERENCE BETWEEN WATT & WATT-HOUR WATT WATT-HOUR Unit of power Unit of energy 1 W = 1 J s-1 1 Wh = 3600 J PRACTICE SUM 4 A weight lifter lifted a load of Load = 200 kgf 🡪 m = 200 kg 200 kgf to a height of 2.5 m in 5 g = 10 N/kg, h = 2.5 m, t = 5s s. Calculate the work done and the power developed by him. Work, W = mgh = 200x10x2.5 Take g = 10 N/kg. W = 5000 J P = W/t = 5000/5 = 1000 W PRACTICE SUM 5 A machine raises a load of 750 N F = mg = 750 N, h = 16 m, t = 5s through a height of 16 m in 5 s. E = mgh = F x h = 750 x 16 Calculate the energy spent by E = 12000 J the machine and power at which it works. Power, P = E/t = 12000/5 P = 2400 W