Q43. On complete combustion, a liter of petrol gives off heat equivalent to 3 x 107 J. In a test drive, a car weighing 1200 kg including the mass of the driver, runs 15 km per liter while moving with a uniform speed on a straight track. Assuming that friction offered by the road surface and air to be uniform, calculate the force of friction acting on the car during the test drive if the efficiency of the car engine were 0.5.

Hint: Efficiency of car = Energy used by carEnergy given 1 liter of petrol.
Step 1: Find energy used by the car.
Energy is given by petrol in the form of heat of combustion.
Thus, by question.
 
The energy is given by 1 liter of = 3x10J
The efficiency of the car engine = 0.5 
Energy used by the car = 0.5 x 3 x 10J
E = 1.5x10J
Step 2: Find the vaue of frictional force.
Total distance travelled (s) = 15 km = 15 x 103 m
If f is the force of friction then,
 
E=f×s  Energy is utilized in working against friction) 
1.5×107=f×15×103
 f=1.5×10715×103=103 N
f=1000 N