Stupid physics help

Ender · Post by **Ender** » 2008-05-06 11:55pm

Ok, go easy here, I've been drinking (out of the navy, yay!). Question posed to me was, given how on a surface with sufficiently low friction throwing a ball can move you along, if you were in a cabin on a boat that didn't touch the water and had a ball that you could throw inside and bounce it off the walls, could you go anywhere. Now I said in an absolutely frictionless environment they wouldn't go anywhere because you can't move the center of mass in a system that started at rest. But since you are in a medium you can exchange with it and move yourself along. Other guy pops in and says yes, but only to a distance equal to the length of the cabin and only if you redistributed all the mass in the system. I'm pretty sure that is wrong because he is assuming you can only throw the ball once (if we at under that assumption then he wouldn't be), but I wanted to make sure.

So... andwer?

Wyrm · Post by **Wyrm** » 2008-05-07 12:10am

The center of mass of a room cannot be moved unless acted upon by an external force. That's Newton's First Law.

On the other hand, the room is part of the system. The center of mass of the room can be moved, provided the mass inside can be suitibly distributed. Depends on the mass of you, the boat, the medium, the ball, and the room.

Wyrm · Post by **Wyrm** » 2008-05-07 12:11am

Sorry, the first sentence should say "system" and not "room". It's late here, too...

Winston Blake · Post by **Winston Blake** » 2008-05-07 06:03am

if you were in a cabin on a boat that didn't touch the water

I don't get what you mean by this.

Anyway, in a low friction environment, you could still move using a slip-stick method. Because static friction is generally stronger than kinetic friction, you can sit in a shopping trolley and lean forward slowly, then thrust yourself back suddenly and get some distance. Rinse and repeat.

There might a similar phenomenon on water.

Steel · Post by **Steel** » 2008-05-07 07:19am

I think that by throwing a ball inside a container on a low friction surface you would probably end up going in a direction, but i'm not sure which one. It would depend on how much friction and whether you accelerated the ball throwing it faster than the wall decellerated it on impact. (Think with a really squishy ball or something)

In a situation where there is no friction on the container you're in, you cannot move the center of mass, and so you cant move the container any firther than what you can achieve by taking all the crap inside it to one end.

Korto · Post by **Korto** » 2008-05-07 09:40am

OK, assuming you're inside a cabin, which is on a frictionless surface...

When the ball is thrown, the recoil will exert a force on you, pushing you back and therefore pushing back the ship. When the ball hits the wall, it will exert a force in the opposite (forward) direction stopping the ship (and you).
If the ball sticks to the wall then all stops there, and the ship was moved whatever distance it managed in the time the ball flew.
If the ball rebounds back to you, then the force of the rebound will actually push the ship forward, and it will travel forward up until you catch the ball, at which point the force exerted upon you and therefore the ship backwards will stop the ship. In this case the ship will be back where it came from. It actually doesn't matter if it comes back fast or slow. The slower the ball comes back the less recoil force was exerted on the ship, so the slower forward it moves, but the longer it'll be before it gets back to you and you stop it. It'll all cancel out.

My terminology is probably shot, but I reckon that's how it'll work.

madd0ct0r · Post by **madd0ct0r** » 2008-05-07 10:31am

Yeah, and if the ball dosen't bounce but stays on the far side of the room,
and you walked over to retrieve it then the friction force between your soles and the cabin floor would accelrate the boat away from the start position again (much like throwing the ball)
(In effect you'd be remaining above the same spot of sea and walking the boat under you)

However when you stop walking the boat would have to stop too. the process would then reversed when you walked back across the room to throw the ball again.

so ultimetly, you'd be able to move the boat set distances but not travel indefinitely.

Korto · Post by **Korto** » 2008-05-09 12:30am

Yes, except you will actually move.
(nb - All this is based on an outside referance point (the sea, presumably))

Both you and the ship will move, relative to your proportional masses.

Assuming the ship masses more than you, you will move a lot forward, and the ship will move a little back as you walk to the ball.

When you stop to pick up the ball, the force you use to stop will also stop the ship.

When you pick up the ball and move back, this reverses, you move a lot back, the ship moves a little forward - but here's something cute. By picking up the ball you've increased your mass, and decreased the mass of the ship, therefore the ship will move more forward, and you less back than before. This increased move of the ship should mean the ship will end up in precisely the same spot as before you threw the ball in the first place.

Incidentally, if you had a spaceship that could somehow form a mass object and fire it down the ship at relatavistic velocities, the recoil could then propel the ship forward. You would then need some way to reabsorb the mass object and the immense energies to create it (the momentum would also be absorbed and would stop the ship, conservation of momentum). You just keep on doing this, and you have a reactionless drive. It would also be a drive with a maximum speed as the velocity never has a chance to build past a point (the velocity of the mass-object) as the momentum keeps on being cancelled out.
Perfectly practical. Other than that "form a mass object, fire it, and reabsorb the energies at the far end" thing. Might use it in some Sci fi somewhere.

Raxmei · Post by **Raxmei** » 2008-05-09 04:55am

Korto wrote:Incidentally, if you had a spaceship that could somehow form a mass object and fire it down the ship at relatavistic velocities, the recoil could then propel the ship forward. You would then need some way to reabsorb the mass object and the immense energies to create it (the momentum would also be absorbed and would stop the ship, conservation of momentum). You just keep on doing this, and you have a reactionless drive. It would also be a drive with a maximum speed as the velocity never has a chance to build past a point (the velocity of the mass-object) as the momentum keeps on being cancelled out.
Perfectly practical. Other than that "form a mass object, fire it, and reabsorb the energies at the far end" thing. Might use it in some Sci fi somewhere.

Mass-energy equivalence. Moving the energy from the back of the ship to the front has exactly the same effect as moving the object from the front to the back.

Gil Hamilton · Post by **Gil Hamilton** » 2008-05-10 11:37am

Well, when you throw the ball, you are actually putting a negligible force in the OPPOSITE direction of the throw. Because you are connected to the floor, you are most likely (I haven't jotted the problem on a piece of paper mind) cancelling out the force on the ship from the impact of the ball.

Wyrm · Post by **Wyrm** » 2008-05-10 08:07pm

...

Are you idiots or something?

Remember Newton's Laws? Conservation of Momentum? The center of mass of the system cannot change its state of motion without an outside force?

Is ANY of this ringing any bells?

No matter what happens, the system of the ball, you, and the ship cabin have a sum of momentum that remains unchanged so long as the sum of external forces on the ship cabin (which encloses the system) remains zero. Since the ship is on a frictionless surface, it is indeed zero. Therefore the momentum of the system (which may be represented by the ship's center of mass) is unchanged no matter what you do within it.

In this situation, there is only one way to make the ship move in any real way: you must throw the ball off the ship. That means the ball is not a part of the system (it left), and therefore throwing the ball induces an equal and opposite reaction force on you (and thus the system of you and the ship as a whole) to give you some momentum.

If you and your ball are sealed in the cabin, then the only thing you can do is rearrange the mass of the ship so the center of mass is in different places relative to you and the cabin. But that's it.

Hawkwings · Post by **Hawkwings** » 2008-05-10 08:41pm

The question in the OP specified a low-friction surface. Not a no-friction surface.

Wyrm · Post by **Wyrm** » 2008-05-10 09:40pm

Hawkwings wrote:The question in the OP specified a low-friction surface. Not a no-friction surface.

Ahem...

Ender wrote:...on a surface with sufficiently low friction...

"Sufficently" here changes things entirely. The "sufficiently low friction" surface is a very weaselly dodge... it means a surface with such low friction such that it can be treated as if it were actually frictionless.

Besides, without a statement on how much friction this surface has, the question is unanswerable.

SpacedTeddyBear · Post by **SpacedTeddyBear** » 2008-05-10 09:41pm

Hawkwings wrote:The question in the OP specified a low-friction surface. Not a no-friction surface.

What's being asked has actually little relevancy to friction, rather it is important to specify what forces are being acted upon which body. Friction simply tells you the minimum force needed to accelerate the body from rest, and the force needed to keep it in equilibrium.

The simplest explanation that this situation can be boiled down to, is the example of a person on a scale. There is no way he can push on him/herself in order the increase the reading on the scale because you can't apply a net force one yourself.

Korto · Post by **Korto** » 2008-05-13 09:49am

First, I'll state that I'm treating this as a high-school physics question, so a frictionless sea, and no air resistance.

Wyrm wrote:...

Are you idiots or something?

Remember Newton's Laws? Conservation of Momentum? The center of mass of the system cannot change its state of motion without an outside force?

Is ANY of this ringing any bells?
[...snip...]

We haven't forgotten Conservation of Momentum. In fact, it is precisely because of CoM that the ship must move. In the enclosed system as described, as soon as you throw the ball then the ball has acquired momentum. Therefore, you (and the ship that you are connected to) must acquire equal and opposite momentum to conserve momentum. As long as that ball moves, you and the ship will move to counter-balance.
The only other alternative is to state that someone enclosed inside a cabin can't throw a ball.
I agree that the centre of mass (consisting of the ship, the ball, and you) will not move, but the ship must, because the ball does.

Raxmei wrote:Mass-energy equivalence. Moving the energy from the back of the ship to the front has exactly the same effect as moving the object from the front to the back.

I'll confess that my reliable physics ends around Classical, so you may be right. I'll assume you are, as it sounds like the way the universe works. Anal to the last.