Example
A car with mass 1250 kg drives with a constant speed of 90 km h-1
The petrol consumption of the car is 6.7 liter petrol per 100 km.
The heat of combustion of the petrol is 33 x 109 J m-3
The frictional force is 500 N
Find the efficiency of the motor in the car.
Echem = (6.7)( 33 x 106 )= 2.21 x 108 J
Velocity is constant. Therefore : Fm = f= 500 N
WFm = Fm s =( 500)(100 x 103) = 5.0 x 107 J
Efficiency η = (W/E) 100 %= (5.0 x 107)/(2.21.108) 100 % = 0.23 x 100 % = 23 %
Example
Calculate the power P of the car at this velocity of 90 kmh-1
Solution1 : The time required for 100 km is 100/90 houres = 1.11 h = 4.0 x 103 s
Pm = W/t = (5.0 x 107)/ (4.0 x 103) = 1.25 x 104 W = 12.5 kW
Solution 2 : Pm = F v =( 500)(25) = 1.25 x 104 W = 12.5 kW
Remark
Because the velocity remains constant, the kinetic energy does not change
All energy is used to be overcome the fricional forces
Chemical energy is converted into (frictional) heat