Homework SourseA car engine has an output power of 20 hp when the car is tr
A car engine has an output power of 20 hp when the car is traveling at a constant 55 ft/s on a level road. (About 38 mph)
A) find the average drag force on this car
B) if the drag force is proportional to the speed, and speed doubles, find the power needed.
C) if the maximum engine output is 150 hp, find the maximum speed for this car
Solution
(A) Power = F . v
P = 20 hp = 20 x 745.7 W
v = 38 mph = 38 x 1609 m / 3600 sec = 16.76 m/s
putting in,
20 x 745.7 = F (16.76)
F = 890 N .....Ans
(B) F = k v
if speed doubles then F will be doubled.
and P will be (2F x 2v) 4 times.
hence power needed =4 x 20 = 80 hp
(C)
F = k v
890 = k (16.76)
k = 53.1
for P = 150 hp
P = F.v
P = (k v)v = k v^2
150 x 745.7 = 53.1 x v^2 /2
v = 64.9 m/s
v( in ft/s) = 64.9 / 0.3048 = 212.95 ft/s .....Ans
