Question

On complete combustion a litre of petrol gives off heat equivalent to 3 × 10⁷ J. In a test drive a car weighing 1200 kg. including the mass of driver, runs 15 km per litre 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.

Answer 1

Energy is given by the petrol in the form of heat of combustion

Thus, by questioning,

The energy is given by 1 litre of petrol = 3 × 10⁷ J

The efficiency of the car engine = 0.5

∴ Energy used by the car = 0.5 × 3 × 10⁷ J

E = 1.5 × 10⁷ J

Total distance travelled (s) = 15 km = 15 × 10³ m

If f is the force of friction then,

E = f × s  ......(∵ Energy is utilised in working against friction)

1.5 × 10⁷ = f × 15 × 10³

 ⇒ f = `(1.5 xx 10^7)/(15 xx 10^3)` = 10³ N

f = 1000 N