On complete combustion , a litre of petrol gives off heat equivalent to `3xx10^(7)J` . In a test drive, a car weighing 1200kg, including the mass of driver, runs 15kg 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.

Explanation

Energy is given by the petrol in the form of heat of combustion Thus, by question Energy given by 1 litre of petrol=`3xx10^(7)J`
Efficiency of the car engine=0.5
Energy used by the car=`0.5xx3xx10^(7)J`
Travelled distance travelled (s)=15km=`15xx10^(3)m`
If f is the force of friction then, ` E=fxxs (therefore "Energy is utilised in working against friction")`
`=1.5xx10^(7)=fxx15xx10^(3)`
`f=(1.5xx10^(7))/(15xx10^(3))=10^(3)N`
`f=1000N`