Power and work

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

Enable Notifications    OK No thanks