4. A rocket-driven car propels itself forwards in a straight line on a horizontal track by ejecting burnt fuel backwards at a constant rate \(\lambda \mathrm { kg } \mathrm { s } ^ { - 1 }\) and at a constant speed \(U \mathrm {~m} \mathrm {~s} ^ { - 1 }\) relative to the car. At time \(t\) seconds, the speed of the car is \(v \mathrm {~m} \mathrm {~s} ^ { - 1 }\) and the total resistance to the motion of the car has magnitude \(k v \mathrm {~N}\), where \(k\) is a positive constant. When \(t = 0\) the total mass of the car, including fuel, is \(M \mathrm {~kg}\). Assuming that at time \(t\) seconds some fuel remains in the car,

1. show that $$\frac { \mathrm { d } v } { \mathrm {~d} t } = \frac { \lambda U - k v } { M - \lambda t }$$
2. find the speed of the car at time \(t\) seconds, given that it starts from rest when \(t = 0\) and that \(\lambda = k = 10\).