Final answer:
To move a car 450m at a constant speed of 21 m/s with a 12% efficient engine, 67500000 Joules of chemical energy is needed. This is calculated by finding the mechanical work (8100000 J) and then accounting for the engine's efficiency.
Step-by-step explanation:
The question asks us to calculate the chemical energy needed by a car's engine to move the car a distance of 450m at a constant speed when the engine is only 12% efficient. To find the energy, we first calculate the mechanical work done by the car and then adjust for the efficiency of the engine.
First, we calculate the mechanical work (W) using the formula W = force x distance. Given that the force is 18000 N and the distance is 450 m, the work done is:
W = 18000 N * 450 m = 8100000 Joules
Next, because the engine is only 12% efficient, most of the energy from the gasoline is not converted into mechanical work. We divide the mechanical work by the efficiency (in decimal form) to find the chemical energy needed:
Chemical Energy = Mechanical Work / Efficiency = 8100000 J / 0.12 = 67500000 Joules
Therefore, the car's engine would need 67500000 Joules of chemical energy to move the car a distance of 450m at the speed of 21 m/s.