Question

Suppose a probe was sent to land on an airless moon. As the probe got close, the moon's gravity began pulling it straight down. The probe used its rockets to brake.

Unfortunately, a calculation error was made when the probe was designed. As the probe got close to the surface, the force of gravity became greater than the maximum force of its rockets.


The force of gravity (represented by the green arrow) is greater
than the force of the rockets (represented by the purple arrow).
Image courtesy of NASA/JPL-Calech/University of Arizona

Assuming the force of gravity was practically constant from that point on, describe the probe's vertical motion as it neared the moon's surface.
A.
The probe moved downward at an increasing speed.
B.
The probe moved downward at a decreasing speed.
C.
The probe's rockets caused it to stop moving downward.
D.
The probe moved downward at a constant speed.

Answer 1

A.

Because gravity exceeds the maximum upward thrust, there is a net downward force. With a constant net force the probe has a constant downward acceleration, so its downward speed increases as it approaches the surface.