Comprehension # 4
When the mass of a system is variable, a thrust force has to be applied on it in addition to all other forces acting on it. This thrust force is given by : `vec(F) = vec(v)_(r) (+- (dm)/(dt))`. Here `vec(v)_(r)` is the relative velocity with which the mass `dm` either enters or leaves the system.

A car has total mass `50 kg`. Gases are ejected from its backwards with relative velocity `20 m//s`. The rate of ejection of gas is `2 kg//s`. Total mass of gas is `20 kg`. Coefficient of friction between the car and road is `mu = 0.1`.
Car will start moving after time `t = ......... s`.

Options

A. 4
B. 10
C. 5
D. 8