Eposode 13: The right planet size

Comic Transcript

Panel 1.
Epo: Then we choose a planet. Scanning…
Granddaughter: Let’s look for a really BIG one!

Panel 2.
Epo: Inadvisable; we need a planet that’s mass creates the proper gravity.
Captain Ishian: Larger denser planets have more gravity, we need one just like that.

Panel 3.
Epo: Your ship’s gravity indicates we should find a planet on the order of 10 to the 24th power kilograms and a radius around 10 to the 3rd power kilometers.
Granddaughter: Or else?

Panel 4:
Alkina: A planet with too much gravity would squish you down into the ground. A planet without enough gravity makes you feel as light as a feather.

Panel 5:
Epo: My astrometry program is complete; I have located all planets of appropriate mass within 10 parasecs of our position.
Alkina: Thanks Epo, let’s take a look…

What does it mean?

Mass – is a measurement of how much stuff, or “matter,” an object is made.

Kilogram – is the standard unit for measuring mass.

Kilometer – is a unit for measuring length. The average length between Earth’s center and its surface is about 6,000 Kilometers.

Density – is a measure of how much matter is packed into a certain amount of space. Denser objects have more material packed into a given space than lower density objects.

Gravity – is the universal force of attraction between all matter.

In human speak please!

1024 used in the comic strip above is a quick way of writing a 1 with 24 zeroes after it. It’s a lot easier than 1,000,000,000,000,000,000,000,000, isn’t it? Similarly, 103 is just a quick way of writing 1,000. This short-cut method of writing large numbers is known as the scientific notation.

The Newtonian Law of Gravity says that any two objects in the universe are attracted towards each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them (see equation below). This means that the more massive the two objects, the bigger the force of attraction between them but this force diminishes as you increase the distance between them.

Force due to gravity equation.

where F is force of gravity between two objects, G is Newton’s Gravitational Constant, M is the mass of one object, m is the mass of the other object, and R is the distance between them.

For example, on the moon, which has less mass than Earth, you would feel about 1/6th of the gravitational force you do on Earth, and on Jupiter, which has more mass, you would feel over twice as much. Also, on the surface of Earth, we feel more force of gravity than we would in space.

As defined above, mass is amount of “stuff” something is made of. Weight, on the other hand, is a measure of the force of gravity experienced by an object. Therefore, while an object’s weight will change depending on whether you measure it on Earth or on the moon, its mass will not.

Is that all?

Your Weight on Other Worlds – an educational web tool from the Exploratorium that allows you to see what you would weigh on other planets in our solar system.

Newton’s Law of Gravitation – Educational poster that illustrates Newton’s Law of Gravitation along with two classroom-ready activities.