23-11-2003 04:06									#1
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I would pester my physics teacher except it Saturday and theres no school. Is my drawing correct?

Attachment: afm.jpg
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	23-11-2003 04:08									#2
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Whenever any power is applied on an object, then an equally large power facing in the opposite direction is created. That's it .

So, when you're moving your mouse, there's the power of your hand, and the opposite power of mouse rubbing against the table.

But I don't know any physics terms in English...

	23-11-2003 04:11									#3
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quote: Whenever any power is applied on an object, then an equally large power facing in the opposite direction is created. That's it . So, when you're moving your mouse, there's the power of your hand, and the opposite power of mouse rubbing against the table. But I don't know any physics terms in English...

perhaps I should calrify my question. Why does the tissue move? (and is my drawing the correct answer?)

	23-11-2003 04:30									#4
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quote: Originally posted by Solver Whenever any power is applied on an object, then an equally large power facing in the opposite direction is created. That's it . So, when you're moving your mouse, there's the power of your hand, and the opposite power of mouse rubbing against the table.

What about the energy that is turned into noise?

	23-11-2003 06:47									#5
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Space.

How could you pull on that object in space?

Your diagram is correct. It shows that unless you have some kind of normal force, you will not be able to pull the object.

{}
XX - - - i ----> Fyou
XX ^

< ----- Ffriction.

Without friction, there would be nothing to hold the object in place so that you could start to move the object.

If you are pushing the object, both you and the object would move away from each other depending on the ratio of your masses.

	24-11-2003 02:29									#6
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quote: Originally posted by Ben Kenobi Without friction, there would be nothing to hold the object in place so that you could start to move the object. If you are pushing the object, both you and the object would move away from each other depending on the ratio of your masses.

You aren't making sense there, at least to me. You don't need friction to push or pull an object. The only key point in the space situation is that the centre of mass cannot move (assuming you define it as originally stationary). I always prefer to do these questions in terms of CoM and momentum.

	24-11-2003 02:53									#7
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That is really going to help a weak physics student...

	24-11-2003 03:55									#8
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BC:

In order to push the object in one direction, without pushing yourself away, you need friction. Without friction, you don't have the traction to actually push or pull.

And yes, that's a better way to put the question in terms of centre of mass. That's why when you push, the ratio of the velocities will be the inverse of the ratio of the masses.

	24-11-2003 06:56									#9
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quote: Originally posted by Ben Kenobi BC: In order to push the object in one direction, without pushing yourself away, you need friction. Without friction, you don't have the traction to actually push or pull.

I see what you mean, but I don't like the description as its too inaccurate for my liking. I would express it that you need to have a heavy mass to hold onto, effectively reducing your recoil velocity to a negligible size. Friction needn't be a part of the equation and is a complicating factor that you'd really want to ignore whenever you can.

	24-11-2003 06:58									#10
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The tissue moves because there is a net force acting on it (pull from your hand). Newton's third law tells you that whenever you act on an object (tissue) the tissue acts on you with an equal and opposite force (this is due to conservation of momentum, which is in turn due to translational symmetry of space, but that is irrelevant here). On the Earth you do not actually move when you pull on a tissue because your frictional contact with the ground stops you and the force is so tiny (the tissue being very small) that you don't notice it. In space, there would be no frictional force counteracting your tendency to move in the opposite direction of the tissue, so you would drift very slowly.

The final verdict is, for all force problems, coming from experience in dealing with freshman physics students:

1) draw a free body diagram
2) DRAW A FREE BODY DIAGRAM!
3) forces are opposite and equal
4) F=MA (vector eqn.)

	24-11-2003 07:32									#11
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at first I understand it just fine, and then I have to go and think about it...

quote: You aren't making sense there, at least to me. You don't need friction to push or pull an object. The only key point in the space situation is that the centre of mass cannot move (assuming you define it as originally stationary). I always prefer to do these questions in terms of CoM and momentum.

Ben almost confused me to the point of insanity, telling me the tissue cant move.

quote: 1) draw a free body diagram

a what?

quote: That is really going to help a weak physics student...

you shall burn in hell for that comment!


Its becoming clearer now, thanks guys! looking at my original post Im not even sure of what I was confused about...

	24-11-2003 08:17									#12
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A free body diagram is what you've drawn, basically. You take out a piece (the tissue, say, or you) and draw all the forces acting on it.

	24-11-2003 18:41									#13
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Why are you using a tissue as an example?

Wouldn't this be easier with a ball or something?

	24-11-2003 20:01									#14
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So if you masturbate in space, and when you pull outward with your stroking, would that pull your entire body forward in the same direction??

	24-11-2003 20:08									#15
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Yes, as long as you never moved your hand backwards to get the second stroke.

	24-11-2003 20:16									#16
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No. Due to conservation of momentum, movement would only occur if ... something was pushed away from your body permanently.

For example, if you had a ball and threw it away from you, you would move away from it also. If you just waved your arms around for no reason, you would just look silly. You wouldn't gain momentum.

	24-11-2003 21:29									#17
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quote: Originally posted by MrFun So if you masturbate in space, ....

Now I see why we needed the tissue.

	24-11-2003 21:53									#18
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lots of tissue if it's a big load