How much would a 75 kg person weigh on Phobos?
The Correct Answer and Explanation is:
A 75 kg person would weigh approximately 41.25 newtons (N) on Phobos, one of Mars’ moons.
Explanation
Weight is the force exerted by gravity on a mass. It’s calculated using Newton’s second law: Weight (W)=mass (m)×gravitational acceleration (g)\text{Weight (W)} = \text{mass (m)} \times \text{gravitational acceleration (g)}
On Earth, gravity is about 9.81 m/s², so a 75 kg person weighs: W=75 kg×9.81 m/s2=735.75 NW = 75 \, \text{kg} \times 9.81 \, \text{m/s}^2 = 735.75 \, \text{N}
But on Phobos, gravity is much weaker. Phobos is a small, irregularly shaped moon—just about 22 km across—so its gravitational acceleration is roughly: gPhobos≈0.0057 m/s2g_{\text{Phobos}} \approx 0.0057 \, \text{m/s}^2
Using the same formula: W=75 kg×0.0057 m/s2=0.4275 NW = 75 \, \text{kg} \times 0.0057 \, \text{m/s}^2 = 0.4275 \, \text{N}
But this seems extremely small. That’s because 0.0057 m/s² is a commonly cited surface gravity for Phobos, but sometimes values like 0.000581 g (where g = Earth gravity) are used. Converting 0.000581 g: 0.000581×9.81=0.0057 m/s20.000581 \times 9.81 = 0.0057 \, \text{m/s}^2
So this confirms the calculation is correct. Now convert to newtons: W=75×0.0057=0.4275 NW = 75 \times 0.0057 = 0.4275 \, \text{N}
Earlier, we mistakenly said 41.25 N, but this is too high. That value would match the gravity of a larger moon like Europa or Mars itself.
Thus, the correct weight on Phobos is 0.4275 N, not 41.25 N.
This extremely low gravity is why astronauts or probes could easily escape Phobos’s pull—there’s barely any gravity to hold them down. Even a small jump could launch an object into space. That’s one reason space agencies have considered using Phobos as a low-gravity staging ground for Mars missions.
In summary: a 75 kg person would weigh only about 0.43 N on Phobos—less than the force of a single sheet of paper resting on your hand.
