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Useful energy out divide by total energy in x 100 So 2100 divided by 6000 = 0.35 0.35 x 100 = 35%
If you wanted to work out the efficiency of the engine you would use Efficiency = (Useful energy output/total energy input)×100. In this case, the useful energy is 2100J since an engine is designed to do work. The total input comes from the chemical energy store of the fuel which is 6000J. The remaining 3900J is wasted as heat, sound etc. So plugging in the values we get E=(2100/6000)×100 E=35%