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Let us assume that in the future, there is a formerly human colony on Mars that has existed there for some centuries. The people of this world have had time to adapt to the lower gravity over at least 10 generations, but not long enough for evolutionary selection to have had much effect. There has been no 'interbreeding' between Martians and humans from other places/gravity levels.

Assume that the Martians have built common everyday items to match their strength level and the lower gravity. The question can then be phrased two ways:

  • How much weaker are the current Martians than their Earthly ancestors?

or

  • How much stronger would a person from Earth be is transported to Mars?
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  • $\begingroup$ I'm confused you say both "not long for evolutionary selection to have much effect" and "assume that martians have build common everyday items to match their strength levels", are you just asking "if a baseline human lived in Mars gravity how strong would they be relative to a human raised on earth?" $\endgroup$ – sphennings Dec 22 '17 at 3:52
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    $\begingroup$ It depends on the genetics & behavior of the particular Earthlings and Martians you look at, but comparing like to like, they'd be just as strong. The Martian who regularly does e.g. weight lifting would be just as strong as his/her identical twin Earthling who follows the same exercise regime. The only difference is that the Martian weights would need to be about 3X as massive to have the same weight. $\endgroup$ – jamesqf Dec 22 '17 at 4:14
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    $\begingroup$ The factors here are essentially about physical fitness and relative strength. The body strength and musculature development of the Martians is likely to be also affected by changes to their skeletal structure due to growing up in the lower martian gravity. It is often assumed human Martians will grow taller and thinner than their Earth counterparts. This could impact on their load bearing capacity. $\endgroup$ – a4android Dec 22 '17 at 4:32
  • $\begingroup$ @sphennings That is basically what I am asking. Do we expect Martian strength to drop just because they are in low gravity? $\endgroup$ – kingledion Dec 22 '17 at 4:53
  • $\begingroup$ @jamesqf Walking would not be the same level of exertion on Mars as on Earth. Given that a significant portion of your daily exercise consists of walking, why wouldn't we expect the Earthling to be stronger? $\endgroup$ – kingledion Dec 22 '17 at 4:54
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The easy answer is that human muscles and skeletons are designed to manage a set amount of weight, not mass. That means that (on average) a human on mars can lift 3 times more mass than he or she could on Earth. That's because the gravity on the surface of Mars is about a third that on Earth, therefore everything weighs 3 times less. We also know from research on Astronauts that bone density and muscle mass in humans lowers when not in the Earth's gravitational field, so we would expect 'Martian' humans to have less strength than 'Earth' humans. By how much can't be known without extended research, but it's the primary reason that any manned trip to Mars would require a physical exercise regime in place.

That said, I'll limit the rest of this answer to 'Earth' humans on Mars, who I've already stated should be able to lift 3 times the mass on Mars.

But...

Mars doesn't support life. So, that means instead of wearing up to 5Kg of clothing, a person is likely to be wearing more than 15Kg of environment suit. Probably more. That's going to impede their movement and flexibility, meaning some muscles simply cannot be brought to bear on lifting certain items. The design of the environment suit would be critical to knowing how much of an effect this might have but the biggest muscle groups in the body are the Gluteus Maximus (Butt), Hamstrings and Quadraceps; we use them a lot more than one realises in day to day lifing, especially if you're doing it properly and the joints in an environment suit would have to be pretty flexible to allow you to use them correctly.

Even in picking stuff up, you're less likely to use arm muscles like biceps by comparsion to your pectoral muscles across the chest. How flexible is the chest plate in most space suits? I don't know either, but I'm guessing not very.

But...

Let's assume that a person CAN breathe in the lighter atmosphere of Mars. Sure, you get rid of the environment suit and the flexibility impediments, but the lack of oxygen at pressure levels is going to cause different problems.

There's a good reason why many long distance runners come from high altitude countries in Africa; higher altitudes mean lower oxygen levels. That means when they come down to sea level, they find it easier to oxygenate their muscles through regular breathing and it leads to a longer and better sustained energy release by the muscles over time.

On Mars, let's say the pressure didn't kill you and you could get enough oxygen to live, it wouldn't be enough to conduct sustained physical activity as well. So, you might be able to throw a baseball 3 times the distance you could on Earth, but you won't be able to do it 100 times in a row.

But...

Speaking of throwing, this is one of those areas which is actually GOOD news. Because the atmosphere is so thin, there's less wind resistance on any item you throw. This means that even if you couldn't do it over and over again, on your first throw you could probably throw something many times more than 3x the Earth distance. The atmospheric pressure of Mars is approx 0.6% of the sea level pressure on Earth, meaning that a ball would have almost no resistance compared to an Earth throw. But, items like paper planes and javelins would not get the same bonus because they're already designed to be as aerodynamic as possible. You'd still get more than 3x distance on the Javelin, but you'd probably get LESS on the paper plane because there'd be no wind under the wings to speak of trying to preserve direction (there's no lift in a paper plane's wings unless you're a VERY good folder and designer).

So...

Either you're wearing a space suit that limits your flexibility in which case you'd be hard pressed to use your muscles the same way (but you'd probably still be able to lift more than on Earth), or

You're not wearing a space suit but the atmosphere is such that you'd maybe have the chance to lift or throw a very few things before you ran out of puff (but you'd probably lift or throw those few things further than on Earth).

Either way, the winners are pitchers (baseball), pace bowlers (cricket) and shot putters (athletics). Javelin throwers, archers et al don't get much, and weight lifters are in trouble either way.

But (and this is my final one)...

All bets are off if you're INSIDE an atmosphere controlled environment not wearing a space suit. In theory you'd be able to lift 3 times the mass, OR move the same amount of mass 3 times faster, OR some combination thereof. This would all depend on how much energy your muscles can release and how quickly. Ball throwers lose the advantage of the absence of wind resistance in this case, and the advantage goes back to the weight lifters and Javelin throwers.

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    $\begingroup$ I believe the whole reason behind Superman's strength (consider Superman was released before we visited the moon) was born from the misconception that somehow a creature from a planet of high gravity or pressure would be incredibly strong, and by comparison to residents of a planet with a weak gravity or pressure would be.. well.. a superman. Of course such a creature would be stronger, but it would be a bit like suggesting we'd be supermen on the moon. $\endgroup$ – Neil Dec 22 '17 at 9:18
  • $\begingroup$ Pushing sideways relies on gravity to supply the normal force for friction. If my math holds, and you are strong enough that footing is the limiting factor you should only be able to push about as well on a smooth surface as you can on earth. $\endgroup$ – user25818 Dec 22 '17 at 17:46
  • $\begingroup$ Worth noting is that even if you can lift $\endgroup$ – Joe Bloggs Dec 23 '17 at 2:08
  • $\begingroup$ @JoeBloggs your last comment got cut off $\endgroup$ – Spencer Sep 8 '18 at 12:57
  • $\begingroup$ @Spencer so it was. I can’t remember what the rest was though! $\endgroup$ – Joe Bloggs Sep 8 '18 at 16:25
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This is a second answer, because it is the same as my first only different.

The video from the men who landed on the moon shows what some might think is a contradiction.

They move SLOWER in a much lower gravity. Shouldn't they be able to move FASTER, if they weighed less? They should be able to just zip around.

But here is the explanation.

Force equals mass times acceleration.

So, if a person stands up from siting, in say half a second, on earth, the human muscle applies a specific force for a given time to achieve the right acceleration. If more force is applied, there is more acceleration and the person gets up faster. If less force is applied, there is less acceleration. The person gets up slower.

If the person now wants to stand up from siting in the same time frame on the moon, the exact same force has to be applied for exactly the same time, to achieve the same acceleration. Same mass on the moon as on earth. But here is the problem. The person want to STOP standing up. The stopping is done by gravity. On earth, the stopping force is much greater than on the moon. In fact, on the moon, the person would 'stop' perhaps five meters in the air. Not quite what was intended.

So to just stand up, not jump up, a lot less force has to be applied. But the mass is the same, so the acceleration is less. If the acceleration is less, it takes longer to complete the movement.

Walking is the same. If you want to walk at the same speed on the moon as on earth, you have to accelerate equally. Since the mass is the same, you have to apply the same force. But gravity is not pulling you back down on the moon at the same rate as on earth. You don't come back down in the same time frame. That first step on the moon is a very long one. If you want to go the same distance on the first step, as you do on earth, you have to apply less force, so you do not accelerate as fast, nor achieve the same speed. You have to walk very slowly. You need to keep the acceleration low. Otherwise, you keep overshooting your mark.

If you want to move a box starting from rest with a constant acceleration a distance of five meters in exactly one minute, you need to accelerate it by a specific amount to get up to the correct speed in the correct time. Any less or greater the acceleration, and the time is different. To get that specific acceleration, you need a specific force. It does not matter if this is on Earth or on Mars. Now, however, you might want to STOP it. Inertia says it is going to keep on going. On Earth, you can count on gravity, and friction, to slow it down. On Mars, the gravity, and thus the friction, are three times less. All things considered, it will go three times further. You, not friction and gravity, need to stop it. Think of stopping your car on glare ice.

If you want to throw a ball at a specific speed, you need to give it a certain acceleration. If you are doing it manually, you have only so much time (while it is in your hand) to impart that speed. You need to give it enough delta v, acceleration, in a given time. Same force on Earth as it is on Mars, to get a specific ball going at a specific speed in a specific time. But on Mars, because of the gravity, the same ball traveling the same speed will go three times further. To get the ball the same distance as on earth, you throw it at one third the speed. You apply one third the force. Your arm moves one third as fast. Things go slow.

So on Mars, if you want to perform the exact same procedure through a given distance as on earth, you need to do it three times slower, and apply one third the force.

If you do things in the same time frame (same acceleration) as on earth, you need to apply exactly the same force as you do on earth. Mass is the same, acceleration is the same, time is the same, so the force is the same. But the gravitational constraints on such things as projectile height, distance, friction are three times less, and you end up with three times the effect.

So if John Carter wants to go at the same speed in the same time as on Earth, it takes the same effort to accelerate and decelerate. However, the equivalent effect is perhaps three times greater. If John Carter wants to perform the equivalent actions with the equivalent effects and results as on earth, he will go three times slower and will use one third the effort.

One area, however, where this will not be a factor is in swimming. Swimming is independent of gravity. It is all about moving a given mass (both swimmer and water) a given distance at a given speed. It will take exactly the same amount of force to swim the same distance at the same speed on Mars as it does on earth. If John Carter is a swimmer, he will need the same physique as an Earth swimmer.

That earthly visitor to Mars will think that the Martians are moving very, very slowly indeed.

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Force equals mass times acceleration, not WEIGHT times acceleration.

It takes the same force to accelerate a human body from zero to 100 on Mars as it does on the Earth.

This force is supplied by the muscles.

So you are using the same amount of 'muscle' to go from a standstill to a run on Mars as on Earth.

Once you GET to that speed, it takes less muscle to keep going at that speed.

Same with lifting weights.

You can hold a weight still, over your head, with less effort, on Mars as on Earth, but to GET it there, at the same acceleration, it takes the same effort.

Inertia is the same, for the same object, on Mars as it is on Earth.

So to jump, it takes the same muscle energy to reach a certain speed, but once you reach that speed, you will jump higher.

To kick a ball at a certain speed, it takes the same amount of muscle energy to achieve that speed, but it will go further.

So assuming these humans run, jump, lift things, and generally move around the same as they do on earth, their muscles will have to be just as strong.

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    $\begingroup$ I believe you're using the right equation in the wrong way. Let's say my mass is 100Kg. The amount of force needed to lift my mass against gravity by one meter on Earth is F = (100Kg) * (9.807m/s^2) = 981N. On Mars it's F = (100Kg)*(3.711m/s^2) = 371N. That's only (about) a third the force needed to lift my fat rear end off the ground one meter. Wouldn't muscles atrophy fairly quickly? After all, you're used to pushing 3X to stand up. You'd quickly stop pushing 3X, leading to less need for muscles. $\endgroup$ – JBH Dec 22 '17 at 6:59
  • $\begingroup$ @JBH In deep space, where there is minimal gravity, a huge spaceship weighs virtually nothing. So by your reasoning, it would take a minimal amount of energy to accelerate it. Just a slight push. This just isn't the case. Force equals mass times acceleration. It doesn't matter HOW much the ship weighs, It matters how much it MASSES. Same as a body in gravity. To give it a specific delta v, takes the same force on earth as in deep space (ignoring friction). You are calculating how much to KEEP it there, not how much to ACCELERATE it. $\endgroup$ – Justin Thyme Dec 22 '17 at 15:11
  • $\begingroup$ ctd So your atrophied muscles would keep you standing, but they would not be strong enough to allow you to accelerate from rest to a jump. Four support legs on a rocket will keep it standing upright, but they won't accelerate it to escape velocity. The escape velocity is lower on Mars than on Earth, but it STILL takes the same amount of force to get the mass to that particular velocity as it would to get it to that same velocity on Earth. It's just that on Earth, it would not be enough to escape. You would fall back down. $\endgroup$ – Justin Thyme Dec 22 '17 at 15:21
  • $\begingroup$ You're certainly correct, the mass of the ship pushing against no gravity must have a force applied to obtain an acceleration. But, if you're trying to launch that ship from Mars vs. Earth, you're pushing against 1/3 the gravity. Regardless all the capital letters, you can't ignore that. A ship would require less fuel to launch from Mars than from Earth because less force is needed to get it to orbit. For some reason you think gravity (an acceleration in the wrong direction) can be ignored. Maybe you think people are so massive that it can be? $\endgroup$ – JBH Dec 22 '17 at 15:48

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