A typical car weights about $1200 \; N$. If the coefficient of rolling friction is $\mu_r = 0.015$. What horizontal force is needed to make the car move with constant speed of $72 \; km/hr$ on a level road? Also calculate the power developed by the engine to maintain this speed.

Given,

Weight of the car $(W) =1200 \; N$

Coefficient of friction $(\mu) = 0.015$

Velocity $(v) = 72\;km/hr = 20\;m/s$

Horizontal force $(F) = ?$

Power developed by the engine $(P) = ?$

Now we have,

$F = F_k + ma$    [Here, $F_k = \mu_r\;.\;mg$ = frictional force]

If the speed is uniform, then a = 0,

$F = \mu_r\;.\;mg = 0.015 * 1200 = 18\;N$

Therefore, horizontal force needed $(F) = 18\;N$

Also, Power developed by the engine $(P) = F * v$

$= 18 * 20 = 360\;N$