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Push vs. Pull

by frienca
Tags: pull, push
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frienca
#1
Jan13-11, 01:48 PM
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Hello,

I am new to the forums and having a discussion with my boss at work regarding when you are pushing something and when you are pulling something. We are writing curriculum for training and aren't completely sure when to say push and when to say pull. It isn't that difficult, I know, but when you start to think about it you start to question everything.

I'll run you through the scenario.

You have a squeegee. The squeegee is a rectangle that can easily fit into normal sized hands. This squeegee is thin and used to wipe liquid from a surface. You are standing in front of a wet wall (surface) with your squeegee and removing the liquid from left to right. You are holding the squeegee in a manner that your thumb is facing you and your four fingers are on the "back" side of the squeegee where you cannot see them. The squeegee point of contact against the wall is the left side of the squeegee, because your fingers are wrapped around the right. (I think that is all the facts right there)

When you base the idea around whether you push or pull, I suppose it depends on what you are specifying that is doing the pull/push.

I have no knowledge in Physics, but here is what I deduce.

If you were to somehow make the squeegee hover with telekinesis and use your mind to move the squeegee and make it move from left to right in the same fashion, then I suppose you would indeed be pulling because the point of contact between the squeegee and the wall would be the left side (assuming you keep the same position as if your hand was there). Of course you could ask if your mind is exerting a push or pull, but that is irrelevant. So the conclusion of the squeegee moving by itself in that manner would indeed be a pull, because the point of contact is overlapping where the right of the squeegee was when the left side made contact.

Here is where it is different.

The squeegee has no force outside of an object moving it, therefore it cannot push/pull itself. When you use your hand and place your thumb on one side and your four fingers on the other side, you are using your hand to exert force to make the squeegee move. When you ask whether you push or pull is when you ask what is doing the pushing or pulling. I would say that the thumb is what is pushing the squeegee from left to right (as the four fingers have no grip and cannot exert the force in the way they are positioned). Then you could go as far as saying it is your arm that is doing the push/pull and since you're in front of the wall, you'd go from left to right and you would now be pulling your arm across the wall.

It sounds so simple, but it is confusing me. Any help would be great.
Thanks
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PhanthomJay
#2
Jan13-11, 03:20 PM
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In reality, you are pulling the squeegee. Pulling forces act away from the object on which they act. Pushing forces act toward the object on which they act. In your case, both you thumb and forefingers are pushing on the squeegee , and the squeegee won't go anywhere because the thumb force is balanced by the forefinger force. Only when arm and hand move will the squegee move, transferring the force to your thumb and fingers as a friction force pulling the squeege from left to right. The 'push' force between your thumb and fingers that hold the squeegee acts perpendicular to the sqeegee handle.....but the force along the motion of the squeegee is a pulling force. If that makes any sense.
jambaugh
#3
Jan13-11, 04:06 PM
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The pragmatic answer is that whatever your boss says is right, is right because he's the boss.

The point is to communicate to the reader of the training manual. If "push" vs "pull" may cause confusion then use "move", or "slide".

Beyond that I'd say the terms are more a description of the object applying the force than the force itself. If that object gets stretched when being used, you're pulling (tensile force) and if the object gets compressed then your pushing. If some of both then don't worry about it.

In your telekinesis example I don't think you can make a distinction except to say the squeegee is pushing against the surface. Since it is also pushing from off center your telekinetic exertion must be a combination of forces or a force and torque and so doesn't simplify to a simple "push" vs "pull".


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