A 500 kg dragster reaches a speed of 120 m/s on level ground, starting from rest

Question

A 500 kg dragster reaches a speed of 120 m/s on level ground, starting from rest. Its efficiency is only 7.60% in producing kinetic energy, because of friction and its design for high power without regard to efficiency.

(a) How many gallons of fuel does it use during this acceleration, given there are 1.30 108 J of energy per gallon?
gal
(b) How much waste heat is produced?
J
(c) Once it is brought back to rest (assuming the engine shuts off at the end of the race), how much waste heat has been produced?
J

Explanation / Answer

energy supplied to the dragster=0.5*mass*speed^2

=3.6*10^6 J

as efficiency is 7.6%, the amount of energy supplied by fuel=3.6*10^6/0.076=47.36*10^6 J
so gallons of fuel required=47.36*10^6/(1.3*10^8)=0.364 gallons

b) waste heat=(1-0.076)*47.36*10^6=43.76*10^6 J

c)waste heat will be double the heat produced during goign upto 120 m/s. so net waste heat produced=87.52*10^6 j

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