Physics: force of gravity on other planets

Tutoring physics, this is a matter of interest.  The 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 F_p due to gravity on a given planet can be calculated by

    \[F_p=F_e\times\frac{m_{p\ rel\ e}}{d^2_{p\ rel\ e}}\]

in which

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

m_{p\ rel\ e} = mass of the given planet, relative to earth’s

d_{p\ 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


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:)


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

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

Space exploration: How (why) is Pluto not a planet?

In my tutoring sessions, space exploration is rarely discussed.  As a tutor, as well as a person, I’ve wondered why Pluto is “no longer” a planet.

When one of my kids came home from grade 3 science, telling me Pluto is not a planet, I was surprised.  I’d heard murmurings in that direction, but thought they’d peter out.  Although not altogether certain of Pluto’s solar year, I knew it was over two hundred Earth years.  How could a “planet” whose orbit takes over two hundred years to complete, cease to be a planet during my 44 years?

I guess Pluto’s actual designation changed in 2006, although it had been debated since 1977.  What forced a decision was the discovery of Eris (January 2005), another orbiting body in our solar system.  Eris is heavier than Pluto, but further away from the sun.  Being heavier, it should be a planet as well, if Pluto is.  Yet, its remoteness (it averages roughly twice as far from the sun as Pluto), combined with its small size (although heavier than Pluto, its size is about the same), convinced most scientists to reject it as a planet.  Therefore, Pluto, being even lighter, ceased to be a planet as well.  That’s probably a simplified explanation of the reasoning, but I’d say it’s substantially correct.

Both Pluto and Eris are now “dwarf planets.”  Interestingly, they’re not the only two.  This research has yielded a few surprises, which I’ll share in upcoming posts.

I’m not entirely sure I agree with Pluto’s new designation. In that regard, I’m not alone.  As a kid from the ’70s, I was told there were nine planets, Pluto being one.  In your own quiet moments, gazing up at the night sky, may you find peace with this issue, whichever inclination you assume:)

Wikipedia was a source for this article: here, and here.

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