Ncert Exercises & Solutions

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 =

\frac{Energy used by car}{Energy 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 = 3x10⁷J

The efficiency of the car engine = 0.5

Energy used by the car = 0.5 x 3 x 10⁷J

∴

E = 1.5x10⁷J

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,

\begin{aligned} E & = f \times s \end{aligned} (∵ Energy is utilized in working against friction))

\begin{aligned} 1 .5 \times 10^{7} & = f \times 15 \times 10^{3} \end{aligned}

\Rightarrow \begin{aligned} f & = \frac{1 .5 \times 10^{7}}{15 \times 10^{3}} = 10^{3} N \end{aligned}

\begin{aligned} f & = 1000 N \end{aligned}

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