# "Heavy" Objects in Zero Gravity

Do “heavy” objects weigh more in zero gravity?

For example, supposing a shuttle astronaut has to pick up a wrench, which might way less than a pound on the ground. No big deal. But let’s say he has to pick up a bank of circuitry or something, which might way 100 pounds on the ground- wouldn’t inertia alone mean that he has to expend more muscle power to move it? Let’s say he had to move a 3-foot-cubed block of solid lead, which would weigh thousands of pounds on the ground. Would inertia cause him to simply only be able to pull himself towards it, rather than move it at all?

Or, for another example- suppose Cecil Adams and I are on the International Space Station playing catch. If I threw a tennis ball at him, he could catch it with no problem. But what if I threw a bowling ball at thim? Would the sheer inertia of it all put him at risk for injury? Or does weight simply not matter up there at all?

All objects weigh the same in zero gravity - zero! So an object would not be called “heavy”, but rather “massive”. You are correct that the inertia of a more massive object would be greater than that of a less massive object. So the more massive object would require more energy to move.

As far as the block of solid lead, if the astronaut wasn’t attached to anything then he would mainly pull himself toward the lead block although the lead block would move slightly toward him as well.

In some ways, working in space is a lot like working in a 10th grade physics textbook. All the “ideal” equations like F = ma and v = at are pretty much directly applicable without having to worry (too much) about friction. Same with all the basic principles such as “For every reaction there is an equal and opposite reaction.” So if you want to figure out how hard it is to move a cubic-yard of lead, you can use the various equations to figure it out. But you’ll need something pretty massive to push against, otherwise you’ll push the block o’ solid lead and go drifting off while the block moves in the opposite direction at some infinitesimal velocity.

This is one reason why working in space is so arduous (aside from working in a massive spacesuit). Every time an astronaut has to tighten a bolt (with your 1 lb wrench), he has to brace him or herself so that he/she doesn’t go drifting off, or start rotating in space. And if you’re working on the Hubble Space Telescope that cost ten gazillion dollars to get into orbit AND had to be built out of lightweight materials , you really want to be careful what you brace yourself against.

Um, I actually think that friction would play just as big a role moving around on a space station as it would on Earth. Unless you have some reason to think otherwise, air resistance, friction with a surface, everything like that, would work just the same. The only difference is that there would be no gravitational force. (Of course, with no gravitational force, there are fewer normal forces, so there are fewer frictional forces, but they’re there.)

Except that part of the equation for friction, IIRC involves the force pushing the two surfaces together. If you’re pushing a train car along a railroad track, you’ll have more friction than if the track and car have no weight (but, of course, same mass).

But there would be no air resistance, right? Space is a vacuum, unless you’re talking about the artificial environment in the space shuttle itself. And friction with a surface requires gravity to keep the object on that surface - it just doesn’t happen. That’s why everything is bolted down, otherwise it floats away. So friction is essentially negligible.

Another reason working in space is arduous is that we use and expect gravity to be there, and in orbit there isn’t any. I’ve heard that the astronauts on Skylab developed eally good abs, because you actually have to pull yourself into a sitting osition, rather than letting gravity do the work for you.

Achernar is talking about a specific circumstance: moving (or throwng) objects while in a sealed pressurized environment like the cabin of a space shuttle or in one of the modules of the space station. If there is air in the area, it will definitely generate resistance to movement, though the total pressure is lower than Earth at sea level.

The effect is pretty negligable is an astronaut wants to shift a 1000kg block of lead. In that case, his or her feet should be firmly braced and the movement should start out slooooow because any attempt to move a 1000kg mass quickly can result in muscle sprains and tears. Of course, however much effort you need to put in to get the object moving will have to be equalled by the amount of energy you need to make it stop. On Earth, a person can roll a 1000kg car on a level surface without too much trouble, but it takes a lot of effort to overcome intertia. This won’t change in space. The only difference is that you need to keep pushing to overcome the relatively small amount of friction between the car’s wheels and the road (and the even smaller amount between the car and the air), and when you stop pushing, the friction will gradually stop the car.

If the object has a much smaller mass, like a crumpled up ball of paper, air resistnace does become a major factor. If this ball is thrown to another astronaut, air friction will cause it to slow greatly and it will stop in mid-air if nobody catches it (if the space station is rotating, the ball will eventually “fall” against one of the walls, but that’s another story) .

A baseball raises a tantalizing question when thrown in a sealed environment. The air resistance will make it possible to throw curves but there’s no gravity to make a true sinker possible. A bigger problem is that the pitcher’s feet can’t be braced in the dirt as on Earth, so he won’t be able to do a proper windup. Plus, you’d have to have a pretty long distance, at least 50 feet, to see any effect. This is unlikely on the cramped shuttle or space station (the ball could also break something as it rebounds around the area: it will never just drop to the ground and stay there). If they ever build large pressured domes on the lunar surface, we’ll have a chance to see this in a low- if not zero-gravity environment.

Once you step out into the vaccuum of space, all air-resistance questions become moot, but inertia problems remain. An astronaut can still move a 1000kg object or throw a baseball, but now he or she is hampered by a bulky spacesuit. This factor alone will make the action more difficult than any air resistance.

Ah yes, I was being silly. When Finagle said “In some ways, working in space is a lot like working in a 10th grade physics textbook,” I assumed it was because of the lack of gravity. Of course, though, the lack of air resistance is the key. And you could, naturally, say the same thing about working on, say, the Moon. Zero air resistance is probably the biggest approximation made in 10th grade physics, but it’s only one of several, and most of them would still be a problem in space.

There is a real danger when moving highly massive objects in weightlessness. Because the objects aren’t “heavy” people have a tendency to forget about their mass. On the Earth, moving a heavy object involves picking it up, moving it, and setting it down. Once you stop “moving” it, it just sits there like a lump. In space, it involves pushing it, and letting it fly. The object becomes a massive projectile, and you need something on the other end to catch it.

I can push a car on a level road up to about 5mph, but I don’t really want to be in front of a car that’s moving 5mph, and try to stop it. And I REALLY don’t want to be between that car and a brick wall, I’d get squished.

Great care has to be taken when moving massive objects in space, try to move something too fast, and you may not be able to stop it.

Hey, I once suffered a zero-G injury.

During a summer job I was unloading steel reinforcement bars from a truck. Wrap cables around the one-ton bundle, lift it with the overhead winch, then push it to the far end of the warehouse. The overhead winch was on a ceiling rail with little steel wheels. I accelerated the mass, walked alongside until it was at the far end, then got in front of it to decellerate it. But I misjudged, and it got to the stack of bars before it stopped. My chest stopped it quick because I was between it and the big stack. No big deal, but a short rod with a sharp edge dragged across my arm as I was being squeezed. I had visions of orbital construction workers being squashed to death between slowly moving multi-ton space station beams. Never put yourself between the ship and the piers!

To answer the OP, no, there would be no difference between the forces needed to move big objects around in free fall versus on earth. Catching a bowling ball on the space shuttle would hurt exacty as much as on earth. The main difference would be that the path of the bowling ball would be straight in free fall, but curved downwards on earth. (To make them almost identical, just have someone throw the bowling ball upwards, and stand on a ledge so you can catch it at the top of its trajectory.)

And the one-ton bale of steel bars would be just as difficult to stop in free fall as when it’s rolling along suspended by a hoist on little wheels rolling on a smooth ceiling rail.

I’m a little confused with all of this, would the only reason a bowling ball hurt in space as much as it would on earth be because of inertia? Or are there other reasons as well?

Think of moving a floating object in a calm lake. A toy boat is easy - a real one is harder but ou can do it.

Not that it makes that much practical difference to what’s being discussed, and will probably only cloud the issue for some people , I have to point out that there most certainly is gravity in orbit. It’s just that both you and whatever object you’re handling, is in constant freefall.

Well, yeah. But since there is no way to distinguish gravity from acceleration, the two cancel each other. The net effect is the same as if you had neither.

To hurt you (at least in space), an object has to have both mass (inertia) and velocity (mass and velocity mean high kinetic energy), and it must decelerate quickly when it hits you. Here’s a terrestrial example (don’t try this at home!):

Suppose you have a bowling ball. Drop it on your foot. Does it hurt? Of course it does. A bowling ball has a high enough mass, the ball is going fast enough, and your foot stops the ball quickly enough, to transfer the kinetic energy into the energy required to squash your foot flat. Ouch!

Once your foot heals, try these three things: 1) Drop a golf ball on your foot. 2) Simply set the bowling ball on your foot. 3) Throw the bowling ball up in the air and catch it.

In the first circumstance, the golf ball has little mass, so it only stings a little bit. In the second case, the bowling ball has no velocity, so it doesn’t really hurt. In the third case, if you didn’t drop the ball, your arms are slowly decelerating the bowling ball, so it doesn’t hurt to catch it.

Same thing happens in space: if you throw the bowling ball at your foor, it will hurt. If you reduce the mass, reduce the velocity, or increase the “stopping time”, it will hurt less.

What other reasons are you looking for?

``````There are actually a couple of different cases.   And note that the bowling ball isn't accelerating after someone lets it fly the way it would if someone dropped it  on Earth, so the bowling ball only has the initial velocity.    Possibly still formidable, but it's gravity that makes you seriously unwilling to stand under a dropped bowling ball on Earth.
``````

Case 1: The person receiving the bowling ball in the breadbasket is positioned against a massive and therefore (for all intents and purposes) immovable object, such as the wall of the space station. All of the momentum of the bowling ball is going to be absorbed by the person. This will hurt a lot.

Case 2: The person is ‘standing’, untethered, on a flat surface. I seem to recall that friction is proportional to the gravitational force, so in this case there is going to be nothing keeping the person from moving with the impact. This won’t be a perfectly elastic collision, but the net result will be that the person will absorb some but not all of the momentum of the bowling ball over a period of a few milliseconds and will end up with a net velocity in the direction that the bowling ball was originally heading. This will still hurt, but a bit less than case 1.

You got it. Inertia of the object is the same regardless of gravity or free-fall. (Physicists don’t use the word “inertia” in that way, instead they call it “mass”.)

Extra things happen when you’re throwing bowling balls while standing on the earth, but these don’t affect its inertia.

As I said before, if you want to experience “bowling balls in space”, then stand up on a solid table and have someone throw the bowling ball UP to you. If you catch it at the top of its glide, so it’s moving horizontally but not vertically, it will be just like catching a bowling ball in space. If they just throw it to you while you’re standing on the floor, then the ball will be moving downwards as well as sideways. This will be like catching a bowling ball in space, but one which had been thrown diagonally down towards you.

Or better still, ROLL a bowling ball fast across the floor and try to stop it. That will be just like trying to stop a bowling ball in space which happens to be spinning as well.

If we’re on Earth, and you’re standing more than a couple meters away from me, I have to give a bowling ball a pretty good heave to throw it to you. If I don’t give it enough momentum, it’ll fall to the ground before reaching you. Not so in space. I can just lightly toss it and it’ll reach you, although it might take a good bit longer to do so.

Now, if we were in space, and I lightly tossed it to you (which I could do), it wouldn’t hurt much. But suppose this were my first time throwing projectiles in space, and I didn’t realize that I didn’t have to heave it. If I gave it just as much momentum as I did on Earth, it would impact you with just as much as it did on Earth. Thud.