An astronaut on another planet drops a 1-kg rock from rest. The astronaut notices that the rock falls 2 m straight down in 1 s. On this planet, how much does the rock weigh?
A)1 N
B)4 N
C)5 N
D)more than 5 N

Question

Quizplus · Accepted Answer

The rock's acceleration due to gravity on this planet can be calculated using the formula $s = \frac{1}{2}gt^2$, where $s$ is the distance fallen, $g$ is the acceleration due to gravity, and $t$ is the time. Substituting $s = 2$ m and $t = 1$ s gives $2 = \frac{1}{2}g(1)^2$, solving for $g$ gives $g = 4$ m/s$^2$. The weight of the rock is the mass times the acceleration due to gravity, $W = mg$, so $W = 1 \text{ kg} \times 4 \text{ m/s}^2 = 4 \text{ N}$.

An Astronaut on Another Planet Drops a 1-Kg Rock from Rest