OT: Physics question

peeplj · Post by **peeplj** » Thu Jun 19, 2003 7:22 am

Walden wrote:While, several angels could conceivably dance on the point of a needle (somewhat fewer in the case of seraphim, due to the whole six wings thing), it is my contention that, being celestial entities, and thus not mindful of bodily amusements, they would choose not to dance on the point of a needle. . . oh, wrong question.

Well, I think a completely frictionless surface would be more in the "realm of the angels" than anything you'd find in reality anyway.

It was just one of those odd lunch topics that happen when a lot of computer folks all eat together.

I do appreciate all the great answers! Ya'll are some sharp, smart folks.

--James

Martin Milner · Post by **Martin Milner** » Thu Jun 19, 2003 7:39 am

Well Peeps, I'm not just a pretty face.

NancyF · Post by **NancyF** » Thu Jun 19, 2003 9:44 am

my 2 cents:

If you and the object are both on the frictionless surface, and if you have no momentum yourself you can move only objects with less mass than you, by adding muscle force like straightening your arm at the elbow. You would move backwards as the object moved forward. The total momentum of you and the object would equal the force you supplied. If the object had more mass than you, you would just push yourself off of it. Tyg can move objects heaver than her (and so can all of us, because we use things to increase the friction between our feet and the ground and to decrease it between the object and the ground. Examples: furniture glides, putting a rug under the object, wearing sneakers with stick soles.

NancyF (not a physicist - only an engineer)

Thomas-Hastay · Post by **Thomas-Hastay** » Thu Jun 19, 2003 1:33 pm

I believe I'm going to spoil your fun.

In a purely theoretical sense, If you had a big enough lever and an anchored fulcrum, a person who is stationary could exert enough force to move ANY object (overcome its enertia), even a planet!

Martin Milner · Post by **Martin Milner** » Thu Jun 19, 2003 3:25 pm

If you and the object are both on the frictionless surface, and if you have no momentum yourself you can move only objects with less mass than you, by adding muscle force like straightening your arm at the elbow.

You can still move the object even if it has greater mass, because it too is on a frictionless surface. It would just move with a lower velocity.

You would move backwards as the object moved forward.

Exactly.

The total momentum of you and the object would equal the force you supplied.

I think this is incorrect, because velocity is directional. The total momentum is still zero. Momentum = mass x velocity. You have a momentum after the push, and the object has an equal and opposite momentum, because it's going in the opposite direction.

If the object had more mass than you, you would just push yourself off of it.

No, you would move backwards as the object moved forward, as you said above.

Tyg can move objects heaver than her (and so can all of us, because we use things to increase the friction between our feet and the ground and to decrease it between the object and the ground. Examples: furniture glides, putting a rug under the object, wearing sneakers with stick soles.

True, plus we often use leverage to help us, as T-H says. I can lift a very heavy object using a wheelbarrow, and the longer the handles the better the leverage.

When you sit on a chair, the chair pushes you up just as you push it down. If it didn't you'd crash through to the floor. If it puched harder, you'd bounce back up again!

When you fire a gun, the bullet goes forward and the gun goes back (recoil). Net momentum is zero. For the initial stages of the process, friction hasn't acted much, so this is another parallel for our scenario.

My apologies for being in such an argumentative mood tonight!

Ridseard · Post by **Ridseard** » Thu Jun 19, 2003 4:51 pm

NancyF wrote:my 2 cents:

If you and the object are both on the frictionless surface, and if you have no momentum yourself you can move only objects with less mass than you, by adding muscle force like straightening your arm at the elbow. You would move backwards as the object moved forward. The total momentum of you and the object would equal the force you supplied. If the object had more mass than you, you would just push yourself off of it. Tyg can move objects heaver than her (and so can all of us, because we use things to increase the friction between our feet and the ground and to decrease it between the object and the ground. Examples: furniture glides, putting a rug under the object, wearing sneakers with stick soles.

NancyF (not a physicist - only an engineer)

No, as Martin says, the total momentum of you and the object will be zero (taking into account that momentum is a vector quantity). I can show this from basic principles, but it probably won't make it any clearer.

Suppose you apply a force F to the object. (Then there is an equal and opposite force -F applied to you). Suppose, as a result of these forces, the object has a momentum MV (mass times velocity) and you have a momentum mv. Then according to Newton's second law, F = d(MV)/dt, and -F = d(mv)/dt, where d( )/dt denotes the time derivative. Now add those two equations.

F + (-F) = d(MV)/dt +d(mv)/dt

Simplifying the right hand side, and noting that the left hand side is 0, we get

0 = d(MV + mv)/dt

Since the time derivative of the total momentum is 0, this means that the total momentum MV + mv of the system is constant in time. Since the initial momentum of the system was 0, we must have MV + mv = 0. I.e.,

MV = -mv.

Hence if one velocity is non-0, so is the other. I.e., if one object moves, so must the other.

Another way to see that both must move is that if the system starts out at rest, then the center of mass of the system must be stationary unless an external force is applied to the system. Therefore if one mass moves, the other must also move; otherwise, the center of mass would be moving (in the direction of the moving mass).

StewySmoot · Post by **StewySmoot** » Thu Jun 19, 2003 5:09 pm

Just to make sure I am absolutely clear on this: The mass on the frictionless surface is in a vacuum and not surrounded by Jell-O or something that is clinging to the surface I am standing on, right?

peeplj · Post by **peeplj** » Thu Jun 19, 2003 6:10 pm

You can assume a vacuum if you like. We didn't take it quite that far in the original discussion.

Also one thing brought up then which hasn't been addressed here yet (or at least, I don't think so...forgive if I'm mistaken or overlooked it):

Say you are standing beside (not on) the frictionless surface, and you have a 500,000-ton chunk out of a mountain on the frictionless surface. You reach out and slap the rock. Will your slap actually suffice to start that much mass moving, or will the energy from the slap be absorbed or dissapated by the rock in another way, without the rock actually moving? Or would the entire rock from one end to the other actually start to move across the surface, albeit at an almost infinitely slow pace? Or would the rock actually flex a minute amount to absorb your blow without the whole mass moving?

--James

Steven · Post by **Steven** » Thu Jun 19, 2003 7:18 pm

My company has a fountain that consists of a globe floating (and spinning) on a very thin layer of water. Now, this particular globe is made of marble and is about 8 feet in diameter, so it has some pretty considerable mass to it. The fun thing about it is to go up to it and push. You can change the direction and speed of the spin with relatively little effort, although it might take a while to do it.

Of course, this doesn't really help to answer the original question, but I just think it's cool.

Steven

chas · Post by **chas** » Thu Jun 19, 2003 7:29 pm

peeplj wrote: Say you are standing beside (not on) the frictionless surface, and you have a 500,000-ton chunk out of a mountain on the frictionless surface. You reach out and slap the rock. Will your slap actually suffice to start that much mass moving, or will the energy from the slap be absorbed or dissapated by the rock in another way, without the rock actually moving?

I've moved 30-ton pieces of iron on an air sled. It needs to be done very carefully. And, you might need a very sophisticated piece of equipment to measure the velocity, but, yes, the huge mass would still move.

NancyF wrote:The total momentum of you and the object would equal the force you supplied.

A momentum can never equal a force, because the force is the time derivative (rate of change) of the momentum. They're apples and oranges.

Jerry Freeman · Post by **Jerry Freeman** » Thu Jun 19, 2003 7:31 pm

I believe it would move, even if it had some "other way" of absorbing the force. It would move excruciating slowly, but it would move. If there's some "other way" for it to absorb the force, I believe you would set up some kind of more complex motion (e.g. sound waves bouncing around inside the rock, etc.) but it would move. All the more excruciatingly slowly perhaps, and with some other internal dynamics going on while it moves, but I believe it would move.

Martin, you have proven me correct.

Let's suppose that, instead of a sneeze of .01 grams at 100 miles per hour, you spit one fourth gram (I weighed some spit, and this is plausible) at ten miles per hour (I have no way of performing this measurement). If you could do this ten times, you would moving a little more than six inches per hour, if I've applied your calculations correctly. If you could do it twenty times, you would be moving more than one foor per hour. At that rate, in ten hours, you would have moved ten feet. Not inconsequential.

If, sometime during the ordeal, you had to urinate, and you were able to pee 250 ml (250 grams) in the correct direction at two miles per hour, you would increase your speed by another 100 feet per hour, if I've done the arithmetic right.

I also believe there's a way you could move yourself simply by breathing.

Best wishes,
Jerry

StewySmoot · Post by **StewySmoot** » Thu Jun 19, 2003 7:46 pm

And if you had Texas Chili and beer....

Walden · Post by **Walden** » Thu Jun 19, 2003 10:29 pm

Thomas-Hastay wrote:In a purely theoretical sense, If you had a big enough lever and an anchored fulcrum, a person who is stationary could exert enough force to move ANY object (overcome its enertia), even a planet!

But, man! what a fulcrum!

NancyF · Post by **NancyF** » Fri Jun 20, 2003 10:43 am

Thank you for setting me straight on the physics you guys.

I knew momentum didn't equal force, just trying to say something too complicated in too simple of terms.

Also, hadn't considered an anchored fulcrum, just me and and an object on the surface. And yes, I agree that the heavier-than-me object (most objects are heavier than me, lol) would move very slowly if I pushed off of it, in exactly the same way I would move backward slowly if I pushed an object lighter than me.

Now if I could only find a way to move a very light object to somewhere with an Irish music scene...

NancyF

Jerry Freeman · Post by **Jerry Freeman** » Fri Jun 20, 2003 11:38 am

OK, how about this one.

You're driving in your car with the windows closed. There's a helium balloon in the back of the car, floating freely. You make a right turn. What does the balloon do?

Relative to the interior of the car,

Does it stay where it is?

Does it move to the left?

Or does it move to the right?