What would the gravity on Ceres be like?
February 17, 2022 4:54 PM Subscribe
What would the gravity be like if one were to visit Ceres? What would it be like to drop something? To throw a ball? To walk? How hard would it be to escape from Ceres' gravity? Could you have a drink in a cup? Would clothing and hair hang down? Would it be difficult to place items on a table without them sliding away? Thanks in advance!
Well, about 1/37 th of earth gravity, if you can throw a baseball at ~ 18 km/h it will be in orbit. Most other things scale to the g of earth (9.8 m/s*s) and Ceres (0.27 m/s*s) or their squares.
(maybe google failed me on the orbital velocity of ceres question :)
posted by zengargoyle at 5:10 PM on February 17, 2022
(maybe google failed me on the orbital velocity of ceres question :)
posted by zengargoyle at 5:10 PM on February 17, 2022
Here's a video that shows what a ball dropping on various planets/planetoids would be like for comparison.
https://youtu.be/oIMMZl4n-uk
posted by typetive at 5:21 PM on February 17, 2022 [2 favorites]
https://youtu.be/oIMMZl4n-uk
posted by typetive at 5:21 PM on February 17, 2022 [2 favorites]
How to mathematically calculate a fall through the Earth - YouTube.
posted by zengargoyle at 5:25 PM on February 17, 2022
posted by zengargoyle at 5:25 PM on February 17, 2022
Ceres' orbital velocity is not 18 km/h. At any altitude, around any body, orbital velocity is escape velocity / (sqrt(2)). If escape velocity is 0.51 km/s - which it is - then you can enter a stable orbit at around 0.36 km/s. That's about 1250 km/h or just about the speed of sound on Earth.
So you couldn't throw a baseball into orbit, but you could shoot a 9mm handgun bullet into orbit. Step to the side before it comes back around.
posted by Hatashran at 6:17 PM on February 17, 2022 [8 favorites]
So you couldn't throw a baseball into orbit, but you could shoot a 9mm handgun bullet into orbit. Step to the side before it comes back around.
posted by Hatashran at 6:17 PM on February 17, 2022 [8 favorites]
Best answer: And as a more general answer, things would be light but ponderous. If you have a ten pound weight, it takes just a few ounces of force to hold it up. But pushing it sideways would be the same as on Earth; gravity doesn't effect sideways movement. Tossing it back and forth between your hands would be the same as here, except that you don't need to give it as much lift.
If you put a 500 pound weight on a cart, pushing that cart sideways would be exactly as hard as pushing it on Earth. But you could lift it up if you could get a good grip on it; it would only take 15 pounds of force to lift it.
Maybe even harder, because you only push down on Ceres with a few pounds of force. You'd have to brace yourself because of the lack of friction with the surface. Pushing the cart would be like pushing said cart on a perfectly smooth ice surface.
Liquids would settle in their containers, but ponderously. If jarred, they would slowly but inevitably slosh out. If you had a drink in a cup, you could lift it to your lips but would have to do so very slowly, or else the liquid would keep moving as you stopped the cup. If you had a cup on a table (3 feet above the ground) and wanted to drink out of it (five feet above the ground), you'd have to take at least two seconds lifting it to avoid losing the liquid.
Clothing would (ponderously) hang down, as would hair. But in both cases, static electricity would have a much greater effect on them; static electricity would be exerting the same force while gravity would be exerting 1/35 as much.
Placing items on a table would probably not be too difficult, but tossing them onto it might be. As long as they're still, they'll stay still. A fun fact about friction: if you set an object on a table, and lift one side, at some angle, that object will begin sliding down the table. That angle doesn't change with the local gravity field - it would be the same on Earth, Mars, Ceres, or on the surface of a neutron star. (Your table would not survive the surface of a neutron star.)
posted by Hatashran at 6:31 PM on February 17, 2022 [22 favorites]
If you put a 500 pound weight on a cart, pushing that cart sideways would be exactly as hard as pushing it on Earth. But you could lift it up if you could get a good grip on it; it would only take 15 pounds of force to lift it.
Maybe even harder, because you only push down on Ceres with a few pounds of force. You'd have to brace yourself because of the lack of friction with the surface. Pushing the cart would be like pushing said cart on a perfectly smooth ice surface.
Liquids would settle in their containers, but ponderously. If jarred, they would slowly but inevitably slosh out. If you had a drink in a cup, you could lift it to your lips but would have to do so very slowly, or else the liquid would keep moving as you stopped the cup. If you had a cup on a table (3 feet above the ground) and wanted to drink out of it (five feet above the ground), you'd have to take at least two seconds lifting it to avoid losing the liquid.
Clothing would (ponderously) hang down, as would hair. But in both cases, static electricity would have a much greater effect on them; static electricity would be exerting the same force while gravity would be exerting 1/35 as much.
Placing items on a table would probably not be too difficult, but tossing them onto it might be. As long as they're still, they'll stay still. A fun fact about friction: if you set an object on a table, and lift one side, at some angle, that object will begin sliding down the table. That angle doesn't change with the local gravity field - it would be the same on Earth, Mars, Ceres, or on the surface of a neutron star. (Your table would not survive the surface of a neutron star.)
posted by Hatashran at 6:31 PM on February 17, 2022 [22 favorites]
If you put a 500 pound weight on a cart, pushing that cart sideways would be exactly as hard as pushing it on Earth
That doesn't seem right? It's true that gravity doesn't affect sideways movement and maybe my intuition is wrong, but on Earth a fully loaded cart it is harder to push (or pull) than a cart with nothing in it. Similarly, a box full of bricks is harder to push than an empty box. It seems from a quick google that the force needed to push an object is proportional to the normal force of the object, so the weight would matter.
posted by true at 7:16 PM on February 17, 2022
That doesn't seem right? It's true that gravity doesn't affect sideways movement and maybe my intuition is wrong, but on Earth a fully loaded cart it is harder to push (or pull) than a cart with nothing in it. Similarly, a box full of bricks is harder to push than an empty box. It seems from a quick google that the force needed to push an object is proportional to the normal force of the object, so the weight would matter.
posted by true at 7:16 PM on February 17, 2022
That's friction.
posted by zengargoyle at 7:28 PM on February 17, 2022 [1 favorite]
posted by zengargoyle at 7:28 PM on February 17, 2022 [1 favorite]
That's friction. Which is dependent on gravity and the force between things moving across a surface or in this case bearing around a wheel. And for lols, Alice, Bob, and the average shadow of a cube - YouTube. Gravitational fields are the same thing but instead of being all spherical cow parallel are actually a point source and bloom like the petals of a flower. It's the differential between the top and the bottom that is what we call gravity and sideways is a null operation. Absent friction. But the inertial mass thing stands.
posted by zengargoyle at 7:35 PM on February 17, 2022 [1 favorite]
posted by zengargoyle at 7:35 PM on February 17, 2022 [1 favorite]
It makes sense to me that a well oiled, wheeled cart on perfectly level and smooth ground would indeed act like a cart on Ceres. Most things we push aren't like that though.
Closest I can think of is one of those wheeled flats at a big box store, loaded up with heavy boxes. You definitely can move it around, but pushing starts slowly and once it gets a little momentum it's slow to stop, as well as hard to turn, compared to an empty cart. Because those are all functions of inertial mass and don't change on Ceres.
So extending that, lifting and carrying an object of of one metric ton would not be like carrying a 29 kg object here on earth. It's true that this gives you an idea of the force you need to get it off the ground, but trying to stop or turn is going to be hard and you are really likely to throw out a knee or back. Ceres OSHA will have to write up whole new guidelines.
I hadn't thought about that aspect before--thanks Hatashran for bringing it up!
posted by mark k at 8:39 AM on February 18, 2022
Closest I can think of is one of those wheeled flats at a big box store, loaded up with heavy boxes. You definitely can move it around, but pushing starts slowly and once it gets a little momentum it's slow to stop, as well as hard to turn, compared to an empty cart. Because those are all functions of inertial mass and don't change on Ceres.
So extending that, lifting and carrying an object of of one metric ton would not be like carrying a 29 kg object here on earth. It's true that this gives you an idea of the force you need to get it off the ground, but trying to stop or turn is going to be hard and you are really likely to throw out a knee or back. Ceres OSHA will have to write up whole new guidelines.
I hadn't thought about that aspect before--thanks Hatashran for bringing it up!
posted by mark k at 8:39 AM on February 18, 2022
It's true that this gives you an idea of the force you need to get it off the ground
Ceres gravity wouldn't affect inertia for horizontal motion, but it wouldn't affect it for vertical motion either. So I think lifting 1000 kg on Ceres still wouldn't feel like lifting 29 kg on Earth. It's kind of blowing my mind to try to imagine that.
posted by polecat at 3:51 PM on February 18, 2022 [1 favorite]
Ceres gravity wouldn't affect inertia for horizontal motion, but it wouldn't affect it for vertical motion either. So I think lifting 1000 kg on Ceres still wouldn't feel like lifting 29 kg on Earth. It's kind of blowing my mind to try to imagine that.
posted by polecat at 3:51 PM on February 18, 2022 [1 favorite]
Potential vs Kinetic energy. Potential energy is 'mgh' as in relies on gravity. Kinetic energy is 'mv^2/2' so doesn't rely on gravity. Energy is conserved. It would take less energy to lift it up, but the same amount of energy to move it horizontally because one relies on gravity and the other velocity. Same happens with rotating things but that's angular momentum.
The classic example of this is the spherical cow swingset where the teacher tells you that the kid starts 5 feet up, how fast are they going at the bottom. Simple 'mgh=mv^2/2' -- '2gh=v^2' -- 'v=(2gh)^(1/2)'. Or something like that, haven't done the math in ages.
The point being this. Same mass, same swing, the thing on Ceres would be going slower at the bottom. Therefore less is required to lift to the same height.
posted by zengargoyle at 6:08 PM on February 18, 2022
The classic example of this is the spherical cow swingset where the teacher tells you that the kid starts 5 feet up, how fast are they going at the bottom. Simple 'mgh=mv^2/2' -- '2gh=v^2' -- 'v=(2gh)^(1/2)'. Or something like that, haven't done the math in ages.
The point being this. Same mass, same swing, the thing on Ceres would be going slower at the bottom. Therefore less is required to lift to the same height.
posted by zengargoyle at 6:08 PM on February 18, 2022
I would imagine that the lifting would still be an inertial force, or at least have an inertial component? That is, you're exerting kinetic energy (independent of gravity) against the object to move it vertically, even if that energy converts to potential energy once you stop applying enough force to move it and are only applying enough to keep it from falling.
posted by Aleyn at 7:25 PM on February 18, 2022
posted by Aleyn at 7:25 PM on February 18, 2022
Just like jumping being much higher, if you lift an object up (especially a heavy object), the force slowing it down is much smaller and it will keep going. For example, to lift your 10 kg backpack 1 meter on earth (with it moving at zero speed at the end) took 9.8*10*1=98 Joules of energy. If you put the same energy into it on ceres, you lift the bag to 1 meter with an ending speed of 4.3 m/s. Without you pulling down on it, it will keep going up for 15 seconds stopping at 34 meters high. If you (weighing 70 kg) were to catch the backpack to stop it, it would pick you up at a combined 1.5 m/s and you wouldn't come to a stop for 5 seconds at a height of 4.3 meters.
posted by a robot made out of meat at 9:57 AM on February 19, 2022
posted by a robot made out of meat at 9:57 AM on February 19, 2022
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posted by kschang at 5:05 PM on February 17, 2022