AP Physics Work-Energy

Question

An automobile engine delivers 24000 watts of power to a car’s driving wheels. If the car maintains a constant speed of 30 m/s, what is the magnitude of the retarding force acting on the car?

800 N (B) 960 N (C) 1950 N (D) 720,000 N (E) 1,560,000 004

 

Why does dividing power by time give you the correct answer if power is equal to work/time 

 

Answer 1

 

P = F·v

 

In this 1-dimensional problem,

 

P = Fv

 

F = P/v = (24,000 W)/(30 m/s) = 800 N

 

This is the driving force, which must have the same magnitude (but opposite direction) as the retarding force in order to maintain a constant velocity.  Answer (A) is correct.