Question

A 300 kg dragster accelerates from the rest to the final speed of 120 m/s in 400 m (about a quarter of a mile) and encounters an average frictional force of 1200 N. What is the average output in watts AND horsepower if this takes 7.30 seconds?

Answer 1

Let P be the average power output. The total amount of work done is P*(7.3s)). But the total amount of work done is also the sum of these two quantities:
1._ The work done to overcome friction. 2._The kinetic energy of the dragster at the end of the 400 m. Then the first part of the work done is (1200N)*(400m)=480000 J.  
 Part 2 is given by the kinetic energy formula: 1/2*(300)*(120^2)=2160000 J.
Taken together, that's 2640000 J.
Divide by the time (7.3s) and you have average power output of about
361643.836 Watts and in HP is  361643.836/745.7=484.972289 HP