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Homework Help: Question about forces

  1. Dec 7, 2007 #1
    Hello everyone. This is my first post here and I'd just like to say that I'm just starting to study physics and I'm trying to learn from physicsclassroom.com.

    I hope this doesn't sound like a dumb question, but while reading about balanced forces, this website says that a book resting on a table has two forces acting on it. The Earth's gravitational pull exerting a downward force, and the push of the table upward on the book.

    I just can't understand this.. how can a table push upward or even "push" at all? Can anybody help clarify this or tell me how the forces cancel without telling me that the table is pushing upward?

    I have many more questions but I will try to tackle them one at a time. Thank you in advance for keeping the mockery to a minimum. :)
  2. jcsd
  3. Dec 7, 2007 #2

    The table is pushing upward.

    The force you're referring to is commonly called the Normal force, because it acts perpendicular (normal) to the surface. Now think of it like this. The force of gravity is acting downward. If there were no other force involved, the object would accelerate downwards, right? And fall, like a rock through the air. But you see, when you put a rock on a table, it does not fall. The table holds it up. The table supports it. The table imparts a normal force on the rock, equal to the force of gravity. Because they are equal, they cancel, and no net acceleration of the rock occurs.

    What is actually happening is the atoms in the surface (the table, above) are compressed a bit by the weight of the rock, and push back against it. This, incidentally, is why you can't walk through walls (and rocks don't fall through tables).

    Hope this helps.
  4. Dec 7, 2007 #3
    I suppose I just don't like the way they say the desk is pushing upward. I mean, it can't push upward. It's a desk. It literally cannot "push" in any direction. It's impossible. Right? I don't understand why they have to say "the desk is pushing upward" in order to say that the book is at rest. Why can't they just say that the force of gravity is pushing down on the book, but the force of gravity is not enough to push the book through the densely packed atoms of the desk... or the force of gravity is not enough to push the book through the strong force of the atoms of the desk... or whatever it is that's stopping the book from falling through the desk.

    does anybody understand what i'm trying to get at? and does anyone agree with me that it's just worded inappropriately?
  5. Dec 7, 2007 #4

    Shooting Star

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    In Physics, if something is not moving, the total amount of force has to be zero. But we already know that the force of gravity does act on the book. But the book is at rest on the table, not moving. How to reconcile this with the first sentence? So we conclude that there must be another force acting on the book which is exactly canceling the gravity.

    In this sense, the table is pushing up the book, in the exact opp direction and with equal strength as that of the gravitational force.

    Does this make any sense?
  6. Dec 7, 2007 #5
    Well, the problem you have here is that we're trying to use the word "push" to describe whats happening, and your arguing that a table can't "push." Well, what does the word "push" mean? I think of pushing my buddy into a pool. When I do this, I run up to him (while he's not looking, preferably), grab his shoulders, and throw him forward into the water. Using my hands, I, for a little bit of time, exert a force on him, which accelerates his body and sends him into the water.

    Now a table can't throw anyone into the water. But it can exert a force on an object that is resting on it, and this force is called the normal force. Clearly it must; otherwise, the object would not remain at rest. The actual physical reasons that cause the force are immaterial to mechanics calculations.

    The reason we say "push" is to avoid having to go into vectors and everything else, ie, give a mathematical definition. But we're not talking about shoving things around; we're talking about exerting a force. "Push" is pretty much the only word we've got for that :-)
  7. Dec 7, 2007 #6
    OK don't use "push!" The correct term would be Potential Energy Equal to the Potential Energy of Gravity acting on the book. The book is obviously not accelerating at 32ft/sec^2.
    It is not accelerating at all. The table is preventing the acceleration from happening, so there has to be a counter force keeping the book from falling. That counter force is the table, which is supporting both itself and the book. Therefore there must be a balance of forces, between the book and the table.
    So saying that the table is "Pushing" up on the book is really a short hand way of expressing what is keeping the book from falling through the table.
    Last edited: Dec 7, 2007
  8. Dec 7, 2007 #7
    I would ask another student in your class who does "get it" to help you with this concept as it is pretty basic to Classical Physics. You both will benifit.
  9. Dec 8, 2007 #8
    Here is one way to look at it -- through the molecular level. I believe you know that a table is composed of billions of molecules. These molecules hold on to each other (the table is solid, duh) making the table compact. When you place a book on a table, the molecules of the table recieves a force equal to the weight of the object multiplied by g (acceleration due to gravity 9.8m/s^2) where you placed a book. This force spreads throughout the table, but decreases as it gets farther from the area where the book is located. But the force of the book is not enough to overcome the force of the bonds (intermolecular forces: study chemistry). Thus, the bonds don't break, and they hold on to their normal positions. This maintains the position of the book on the table.

    And also, if this explanation is somewhat erraneous.. here is a little advice. Come on man! Stop being too literal. If you don't believe that the table pushes back the book, try pushing the wall. It WILL PUSH you back. Coz if it doesn't, it should fall. Think of it this way, if you don't want the word push. If a force is exerted on an object, the object exerts a force back (and the force exerted back is always equal to the force exerted on the object.. Confusing? Not really).
  10. Dec 8, 2007 #9


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    If you want to use the work "push" in its literal meaning, you cannot say that earth "pulls" the book downward either. You should take the formulation figuratively, for meaning that "a force is exerted" (where the use of the word push/pull is an indication of the different directions of the forces)
  11. Dec 8, 2007 #10
    i really appreciate everyone's input. it's given me a slightly better insight on what's happening but i'm still not quite comfortable with the whole idea of a normal force. perhaps i ask "why" too much, or perhaps i'm just trying to understand the big picture without looking at all of the little pieces. i guess i'll just accept the idea that a table is capable of exerting force, without really knowing why. maybe someday the answer will clarify itself upon further studying. thank you all for your help.
  12. Dec 8, 2007 #11

    Shooting Star

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    That's the right attitude. But just satisfy my curiosity. You are comfortable with the idea that the sun pulls the earth, even though they're a huge distance apart and there's nothing but mostly "vacuum" between them?
  13. Dec 8, 2007 #12
    Yeah, I guess. I sort of just accept it as true right now that the Earth and the other planets revolve around the sun because of the sun's gravitational "pull". Just because it's common knowledge, you know, that's what everyone says.

    Yet, I'm sure if i were really trying to study what gravity is and it's effects thoroughly, I'd be questioning basic concepts and ideas again in an attempt to make everything make sense in my mind, and not just accept a statement as true because that's what everybody else says. Wow that was a long sentence...
  14. Dec 8, 2007 #13
    yeah, that was long. Good luck on ur learning :D
  15. Dec 8, 2007 #14
    One more state to be heard from:
    Ok there are a couple of things going on here and you need to learn how to draw Force Diagrams (they will help you with questions like this)

    Lets imagine a anvil sitting on a table.
    1st:The anvil is heavy (it has a force of weight)
    2nd: It is not moving or accelerating. Because it is not accelerating it must have a balancing force. This force comes from the table pushing up.
    I know everyone already said this.

    There are a few more ways to think of this.
    1st: You hold the anvil over your head (try this with anything heavy). Are you exerting a force?

    2nd: What happens if the table cannot exert this force? What about you holding the object over your head?
    Well the object will start to accelerate towads the earth (bad for you and the table)
  16. Dec 8, 2007 #15
    good shot, curly_ebhc!
  17. Dec 8, 2007 #16
    good one.. Nice explanation.
  18. Dec 8, 2007 #17
    As I read Newton's third Law, "To every action there must be an equal and opposite reaction", I don't see the word "push". I'm suggesting the "action" is the "weight" of the book on the table and the "equal and opposite reaction" is the "normal force" of the table. Based on the language in Newton's law, I suggest that the table is not "pushing" on the book, but reacting.

    If the table has 4 legs, we would calculate 4 "reactions".

    "Applied Physics", Tippens, 2nd Edition
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