# Gravity on Other Planets

Say I travelled to another planet that had half the gravity of Earth - 0.5g. On Earth I can jump X feet high from a standing jump and land safely. Assuming that I retained my Earth-native muscles during my travel to this other planet would it necessarily follow that I could jump twice as high and land just as safely as I could on Earth or is there another factor or factors that I’m failing to take into account?

Yep, that’s pretty much it,. The only other factor to account for that I can think of is atmospheric drag; there is only a loose correlation between surface gravity and atmospheric pressure and density, but since you’re on a different planet, you can expect these things will be, well, different. However, at normal human jumping velocities, drag is an insignificant factor and can be ignored for all practical puroposes.

One additional factor, which some “moon landing hoax” theorists fail to take into account, is how difficult it is to bend one’s knees while wearing a pressurized space suit. Assuming that your hypothetical planet’s atmosphere is unlike Earth’s, an astronaut would have to wear a suit, and, unless this is for a fictional story and you toss in some futuristic spacesuit technology, he’ll have trouble bending his knees enough to take much of a leap.

But, if the fictional story takes place on a “moonbase” or some such large pressurized environment, leap away.

Well, if you have the same liftoff speed on Planet X, then yes, you do jump twice as high. But actually, that liftoff speed is itself a function of Earth’s gravity, since you muscles have to accelerate you to this speed, against ol’ Mother Earth trying to keep you down. Thus, your initial velocity will also be (slightly) higher on Planet X, making your jump a little more than twice as high. To plug in some numbers, let’s say you can jump about half a metre from a standstill. That’d mean an initial speed of about 3.1 m/s. Let’s assume it took your muscles five hundredths of a second to get you to that speed. That’d mean your muscles are accelerating you at about 72 m/s[sup]2[/sup] (since v = (a - g)t = (72 - 10) * 0.05 = 3.1 m/s). On Planet X, then, those muscles would get you up to a speed of about 3.36 m/s, allowing your jump to reach a height of about 1.13 m.

Assuming it was earth like in it’s formation and evolution what would the atmospheric density of a .5 G planet be? Or is a .5 G earth like world even possible?

Sadly just your standard post-beer pre-going-home discussion started in the pub. Which often do turn into fictional stories of course…

It’s impossible to say. Venus is very similar to earth, yet its atmosphere is 90 times denser than Earth. Mars has ~.3 G, its atmosphere is about 1/100th as dense as Earth. Titan is about .14 G, yet its atmosphere is denser than Earth’s.

Unless you count the gas giants, Venus, Earth, Mars and Titan are the only bodies in our solar system that have significant atmospheres, and those four bodies are all over the map. So there’s no way to predict what sort of atmosphere a hypothetical .5 G body would have, except that smaller bodies probably will tend to have less atmosphere, and as the body gets colder most of the things that might make up an atmosphere tend to become solids.

I just want to point out that, while it’s true that you could safely jump higher, etc., things will still seem somewhat sttrange – it’s not like you suddenly got super-powerful. The law of Conservation of Momentum is neither repealed nor modified, so, although you can lift a Great Big Rock, it’ll still be pretty hard to Start it moving. Once it’s in motion, it’ll be pretty hard to stop it moving, as well. So that rock that would have weighed 400 pounds on Earth and now weighs a paltry 50 pounds on the moon is STILL going to be hard to lift. And it can hurt you or someone else pretty badly when it has to be stopped.
Furthermore, you’ll find that you miss gravity in some situations – you’re used to gravity, and you expect to use it in some cases. Sitting down, for instance. Unless you make a conscious efort, you just let gravity pull you down This will be different on the Moon, or, even worse, out in space, where gravity will pull less or not at all. Astronauts on the Space Station found that they were working their abdominal muscl;es a lot harder because they had to contract them to sit, something they weren’t used to. (And don’t even start with the physiological changes you get from spending long periods of time in low- or zero-gravity situations.)

I always found this to be helpful:

Elvis On Other Planets Weight Chart
[ul]
[li]The Sun – 7,140 pounds [/li][li]Mercury – 97 pounds [/li][li]Venus – 232 pounds [/li][li]Earth – 255 pounds [/li][li]The Moon – 43 pounds [/li][li]Mars – 97 pounds [/li][li]Jupiter – 648 pounds [/li][li]Saturn – 275 pounds [/li][li]Uranus – 232 pounds [/li][li]Neptune – 303 pounds [/li][li]Pluto – 13 pounds[/li][/ul]

Not only that, but Venus manages that 90 times denser atmosphere despite being both a little smaller and a Hell of a lot hotter than Earth, both factors which should lead to a thinner atmosphere. So it’d certainly be possible for an Earthlike planet to support an even thicker atmosphere than Venus.

It is actually something of a minor puzzle why it is that Earth and Mars both have atmospheres so much thinner than they ought to be able to support. Last I heard, the best explanation is that we lost most of ours in the event which formed the Moon, and that Mars’ lack is due to its extremely weak magnetic field, which leaves it unprotected from the solar wind (which blew much of it away).

Also, bear in mind that you would be lifting your center of gravity twice as high. So don’t expect to double your highjump record, since some of that is twisting your body to clear the vault.

I have a feeling you wouldn’t notice much difference between lifting a 30kg rock on the earth and a 200kg rock on the moon until you stopped lifting and the moon rock kept going. How often do you think about counteracting the momentum of an object you’ve just lifted to stop it from rising? I think this would play out as people holding something only loosely in their hand and, intending to pick it up, throwing it into the air by accident. Or bonking themselves in the face.

Low-gravity comedy. I love it.

No, I’m saying that you’ll notice it when you try to start it mobving and when you try to stop and it keeps going. When you lift the rock, you’re trying to change its momentum from zero to something nonzero. While the force of gravity has been reduced, the tendency of the rock not to move has not altered. The characteristic of the rock that makes it hard to stop once it’s in motion is the same one that makes it hard to start.

Take gravity out of the problem altogether by imagining this scenario on earth – you’ve got a boulder on a set of ideal casters on perfectly level, smooth surface. Almost no friction under it (although there is under you, just to make life simple). You push an 8 pound load on that, and all you’re working against is its inertia. It moves easily. Now push an 80 pound load. It’s not going to take off easily – uyou have to overcome inertia. And once it starts, it’s just as hard to stop. Now imagine an 8 ton load. You push – and, as far as you can tell, nothing happens.

This op reminds me of an old video game. It was called Gravitar, Fun times, fun times.

With optimal technique (and without the encumbrance of a space suit), you might do better than doubling your record. From this link.

It would sorta be like lifting a heavyish object in a swimming pool.

Don’t forget to consider how hot the planet might be too. Mercury lost its atmosphere due in part to low gravity. The escape velocity was lower than the velocity acheived by the molecules in the atmosphere when they were heated by the sun - Mercury’s air just flew away.

No. Let’s say that one can high jump 2 meters. That means:

100 cm (original CG)

• .8 m (height CG raised)
• .2 m (20 cm due to Fosbury flop method)

In half the gravity:

100 cm (original CG)

• (.8 m * .2 m) (height CG raised (1.6 m))
• .2 m (20 cm due to Fosbury flop method)

Giving a new high jump of 2.8 m.

2.8m < 2m * 2