Physics: force of gravity on other planets

Tutoring physics, this is a matter of interest. ┬áThe tutor discusses a topic that may also be of interest to space-philes….

On Earth, gravity is -9.8N/kg. The negative just means downward.

Occasionally, as a kid, I’d see a sci-fi show in which an enemy was from a “high-gravity” planet. That person was likely, of course, much stronger than the protagonist. There might be a scene in which the enemy chased the protagonist about, throwing furniture that a normal person could barely lift. I think such a scene played in an episode of Buck Rogers in the 25th Century.

Although I only saw that few minutes of the episode, it raised my awareness that other planets may not have the same gravitional force as Earth. Indeed, the planets in our solar system generally have different gravitational pulls. Let’s see if we can predict the gravitational force on a given planet.

Theoretically, the force Fp due to gravity on a given planet can be calculated by

Fp=Fe*mp_rel_e/d^2p_rel_e in which

Fe = force of gravity on Earth (once again: -9.8N/kg)

m p_rel_e= mass of the given planet, relative to earth’s

dp_rel_e= diameter of the given planet, relative to earth’s

Let’s see if our formula works. Looking at the very convenient planetary fact sheet provided online by NASA, we see that Jupiter has the following statistics:

mass: 317.8 times that of Earth

diameter: 11.21 times that of Earth

Our formula suggests that the force of gravity on Jupiter should be given by

Fj=-9.8*(317.8/11.21^2)=-24.8N/kg

The same planetary fact sheet reports Jupiter’s gravity to be 2.36 times that of Earth, which would be 2.36(-9.8N/kg)=-23.1N/kg.

The difference between the two figures for Jupiter’s gravity is -1.7N/kg, or 7%. Close enough? You be the judge:)

Source:

Giancoli, Douglas C. Physics, 5th Ed. New Jersey: Prentice Hall, 1998.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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