Question

A 1.14x10^4 kg lunar landing craft is about to touch down on he surface of the moon, where the acceleration due to gravity is 1.6 m/s^2. At an altitude of 165m the craft's downward velocity is 18.0m/s. To slow down the craft, a retrorocket is firing to provide an upward thrust. Assuming the decent is vertical find the magnitude of the thrust needed to reduce the velocity to zero at the instant when the craft touches the lunar surface.

Answer 1

Acceleration required to stop rocket:
2as = v² - u²
a = -(18)²/(2 x 265)
a = 0.61 m/s² upwards

Resultant force = mass x acceleration

Resultant force = upwards thrust - weight
1.14 x 10⁴ x 0.61 = Thrust - 1.14 x 10⁴ x 1.6

Thrust = 2.52 x 10⁴ Newtons