Help please?

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.

Question

Help please?

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.

anonymous · Answer

If it's assumed that the upward thrust starts at 165m, then you have the initial velocity, final velocity and the distance. You need a kinematic equation to figure out the upward acceleration. Change in velocity means that you can figure out the $F_{net}$. And then:

$\large{F_{net} = F_{upward} - F_{downward}}$

$\large{F_{net} +  F_{downward} = F_{upward}}$