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.

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Payal Gupta | Contributor-Level 10

8 months ago

This is a short answer type question as classified in NCERT Exemplar

Energy given by 1 lit of petrol = 3 $* 10^{7} J$

Efficiency of the car engine=0.5s

Energy used by the car = 0.5 $* 3 * 10^{7} = 1.5 * 10^{7}$

Total distance travelled s = 15km = 15 $* 10^{3} m$

If f the force of friction= E= f $* s$

1.5 $* 10^{7}$ = f $* 15 * 10^{3}$

F= $\frac{1.5 * 10^{7}}{15 * 10^{3}} = 10^{3} N$

F= 1000 N

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Time of contact =0.001s

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Physics NCERT Exemplar Solutions Class 11th Chapter Six 2025

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