Why Are You Light on Pluto But Heavy on Jupiter?

First Name: Dauntez
Grade: 4
What is your favorite planet or star? Mars
What is your favorite rocket? Elephant tooth paste rocket
Question: Why are you light on Pluto but heavy on Jupiter?

Thanks for your great question, Dauntez! I also really like the red planet Mars. Did you know that it’s actually the most similar to Earth (in terms of temperature, sunlight, etc.) than any other planet in our solar system? This makes Mars the best option to send astronauts to a nearby planet that we can even potentially colonize!

I’m not as familiar with the “Elephant Toothpaste Rocket,” but I did a quick online search, and it definitely looks like a fun rocket to build! When I was young, I built quite a few baking soda/vinegar rockets, but it looks like the Elephant Toothpaste rocket might actually be more powerful. That’s cool!

Now to your question, reworded another way, why are things lighter (in weight) on smaller astronomical bodies like Pluto but heavier (in weight) on bigger ones like Jupiter? Notice that I put in parentheses the word weight and not mass. This is the key to really understanding this question! Many people (both adults and kids alike) tend to use the words mass and weight as meaning the same thing. But they’re not! To understand this difference, we first need a quick lesson in physics and, more specifically, Newton’s second law1 (F = ma). We’ll go over a little bit of math, but don’t worry, it’ll be fun!

Newton's Second Law
Image by Rob Webb, derived using NASA image (https://sites.wff.nasa.gov/code810/edu_newton.html)

In a nutshell, this law states that the net force (F) on some object is equal to its mass (m) multiplied by acceleration (a). Note that mass is not the size or volume,2 but simply the amount of matter of an object (all the “stuff” that it’s made up of), which remains constant (doesn’t change) and commonly expressed in units such as kilograms (kg). To put it another way, to make something go faster (acceleration), we need to apply enough force to overcome the mass resisting the change in motion.3

Public domain images from Openclickart

And by this law, we get the definition for weight as being the force that gravity applies on an object with a given mass (this is what keeps our feet on the ground and stops us from floating away into space!), and so we express this as weight = mass x gravity (W = mg).

Solar System (not to scale)
Derivative Photo: Martin Craft, Public Domain, via Wikimedia Commons
Sun: HalloweenNight, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
Jupiter: NASA, ESA, and A. Simon (Goddard Space Flight Center), Public Domain, via Wikimedia Commons
Solar System: NASA (Lunar and Planetary Laboratory), Public Domain, via Wikimedia Commons

Okay, now that we have that definition, we can properly understand what would happen if we were to stand on Jupiter and Pluto. Every object in the solar system has a certain amount of gravity in proportion to its mass. This means that big planets, like Jupiter, have more gravity than small (dwarf) planets, like Pluto. So, by using our equation (W = mg), we know that the same amount of mass (m) has more weight (W) on Jupiter due to its larger amount of gravity (g) and has less weight (W) on Pluto, due to its smaller amount of gravity (g).

This is why, even though our mass doesn’t change, we have less weight (feel “lighter”) on Pluto than on Jupiter because the gravity is much weaker on Pluto than on Jupiter! In other words, if we could put a scale on each surface4 and weigh ourselves, then the scale would read a number a lot less on Pluto than on Jupiter. For example, I weigh about 150 lbs. here on Earth, so this means I would weigh only about 10 lbs. on Pluto, but nearly 380 lbs. on Jupiter!5

Weight on Pluto

A person who weighs about 150 lbs. on Earth would weigh about 10 lbs. on Pluto (not to scale).

Weight on Jupiter

A person who weighs about 150 lbs. on Earth would weigh about 380 lbs. on Jupiter (not to scale).

Overall, this should remind us of our awesome God who created the stars (this includes all the planets) on Day 4 of creation week, with all sorts of different sizes and masses, all for His glory (Psalm 19:1-2). Let’s continue admiring His amazing creation in the heavens, giving thanks to our Lord and Savior Jesus Christ for holding our universe together by His power (Hebrews 1:1-3). And thank Him for giving us reliable senses to understand His creation, making all science possible!

Bonus Response

First Name: Luke
Grade: 5
What is your favorite planet or star: Mars
What is your favorite rocket? Apollo2
Question: Is it cool to be a rocket scientist?

Hi Luke, thanks for your question! FYI, there technically was no Apollo 2 mission6 (Apollo 7 was the first crewed mission), but did you mean the Apollo 12 mission? If so, I also think that was a cool mission! Though it was the second landing on the moon (the first was Apollo 11), it was the first to successfully land in a very specific location on the moon. So this mission showed that we could precisely land anywhere we wanted on the surface of the moon! Also, the Saturn V rocket, the rocket that flew astronauts during the Apollo days to the moon, is a really cool rocket. And it’s actually my favorite!

To your question, yes, it’s very cool being a rocket scientist, not only because it’s fun to build and fly rockets, but mainly because we serve a God who makes rocket science (in reality, all science) possible! The Bible says God created the universe and promises to uphold everything consistently, which is called the uniformity in nature (that the future will be like the past). Now that’s cool!

Ask Your Question

Ask a parent to help you submit your space-related question to Rob Webb today! Don't forget to check back next week to see if your question was chosen!