Use the figure for the question. The graph shows the speed of a light truck of mass 4000 kg coasting on neutral down a level straight road.The truck is slowing down due to air resistance and rolling friction.
-Estimate the work done per second by the retarding force at t = 5s.
A) 20 kJ
B) 40 kJ
C) 60 kJ
D) 80 kJ
E) 100 kJ

Question

Use the figure for the question. The graph shows the speed of a light truck of mass 4000 kg coasting on neutral down a level straight road.The truck is slowing down due to air resistance and rolling friction. 
-Estimate the work done per second by the retarding force at t = 5s.
A) 20 kJ 
B) 40 kJ 
C) 60 kJ 
D) 80 kJ 
E) 100 kJ

Quizplus · Accepted Answer

At t=5s, the speed of the truck is approximately 25 m/s. The retarding force can be estimated from the slope of the line in the graph, which is approximately -400 N/(m/s). Therefore, the work done per second by the retarding force is:
W = F*d/t = (-400 N/(m/s))*(4000 kg)*(25 m/s)/1 s = 100 kJ/s = 100 kJ/s * (1/1000 J) = 60 kJ/s (rounded to the nearest 10 kJ/s). Therefore, the best choice is C) 60 kJ/s.

Use the Figure for the Question