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B A block on the floor - forces

  1. Mar 20, 2017 #1
    I've drawn the diagram of the forces exerted on and by a block resting on a floor.

    For now, it seems that the forces exerted on the block are balanced - thus it doesn't accelerate.
    The forces exerted on the floor are, however, not balanced. The floor is not accelerating downwards, I must then be missing at least one force.
    Could you tell me, what I am missing?
     

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  2. jcsd
  3. Mar 20, 2017 #2
    Can you provide the question that is being asked and what is being asked in the way of a answer or answers ?
     
  4. Mar 20, 2017 #3
    Inventive,
    This is not part of an assignment or homework, it's came out naturally as I was reading about forces.

    We place a (wooden) block on the floor and draw the forces affecting it: the gravity force and the normal force (exerted by the floor).
    Since the block touches the floor, it also exerts a force on it - the contact force.
    However, the floor does not move, therefore there must exist another force which keeps the floor from accelerating, but I can't figure out what the force may be.
     
  5. Mar 20, 2017 #4
    Think about what the floor is touching and whether that would produce a force on the floor.
     
  6. Mar 20, 2017 #5

    sophiecentaur

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    The Weight force from the block is acting on the floor. The floor returns a reaction force that's equal to the weight of the block and that force pushes on the block. Nothing accelerates because it's an equilibrium situation. The "contact force" that you have drawn in is just a repeat of the weight force. You only need to include the forces once.
     
  7. Mar 20, 2017 #6
    The weight force is working on the floor and the floor returns it, that's true. However, there is also the "gravity force", which acts on the block. In this model, the gravity force is not balanced.
     
  8. Mar 20, 2017 #7

    sophiecentaur

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    The "gravity force" IS the Weight Force. You can't count it twice. The gravity force in your diagram is superfluous. If it were really there, the block would be falling through the floor!
     
  9. Mar 21, 2017 #8

    A.T.

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    Correct.

    Correct. And there is also gravity on the floor itself.

    Is the floor floating in space?
     
  10. Mar 21, 2017 #9

    Nugatory

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    The gravitational attraction between object and earth is acting on the earth as well as the object. That's the missing upwards force on the floor.
     
  11. Mar 21, 2017 #10

    kuruman

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    That's where confusion begins. When you draw a force diagram, a.k.a. free body diagram, the first thing you do is define your system. Clearly, you have defined the system to be the block. The second step is to figure out the different pieces of the Universe outside the system that can exert a force on it. Here we have "Earth" and "Floor". Finally, you draw one and only one arrow representing the forces on the system. Here, you would draw a down arrow for the Earth force and an up arrow for the floor force and that's it.

    Now, you could have chosen your system to be just the floor. Then the pieces of the Universe that exert forces on this system would be the block, the Earth and whatever parts of the building are attached to the floor. This kind of FBD is what civil engineers might consider.
     
  12. Mar 21, 2017 #11
    The block exerts a gravitational force on the Earth equal to and opposite to the Earth's gravitational force on the block.
     
  13. Mar 21, 2017 #12
    I have read through all of your posts.
    They have shed some light on my understanding of this concept.
    I have re-drawn this diagram, now including the fact that the floor is fastened to two walls:

    But I am still confused whether or not I should draw the force of gravity on the floor - balanced by the "fasten forces" .
     

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  14. Mar 21, 2017 #13

    jbriggs444

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    The force of gravity is not balanced by the "fasten forces" of the floor on the wall. The total force on the floor is zero -- we know that since the floor is not moving. But the force of gravity on floor and the "fasten forces" from the wall on the floor are not the only forces acting on the floor.
     
  15. Mar 21, 2017 #14

    A.T.

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    Unless the floor massless, there is a force of gravity on it, that should be shown in the diagram. The "fasten forces" must balance all downwards forces that act on the floor, not just the force of gravity.
     
  16. Mar 21, 2017 #15
    And so, on the diagram,
    -2 F_1 = Contact Force + Gravity on the floor
    Are these all forces we need to take into account?
     
  17. Mar 21, 2017 #16

    A.T.

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    Yes, that is the force balance for the floor.
     
  18. Mar 21, 2017 #17

    russ_watters

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    Sorry for the argumentativeness in your thread. I'm not sure how to clean it up yet, but I am going to try to give you clear, concise answers:

    1. As @sophiecentaur correctly pointed out, your original diagram contains a wrong number of forces. Generally a free body diagram describes the forces acting on a single object. As such, I would remove the force of the block acting on the floor and keep only the two forces acting on the block (the force of gravity and the normal force of the floor acting on the block). http://www.physicsclassroom.com/class/newtlaws/Lesson-2/Drawing-Free-Body-Diagrams

    2. Per #1, if you want to describe the floor, I would prefer to do it separately. It is not technically wrong to combine two FBDs into one, though, so I would say that this new diagram is close to correct, but there is a confusion/complication:

    Is the floor massless? If yes, there is no force of gravity acting on it. If no, there is. But by including the block's free body diagram and one "force of gravity" label, you add confusion since there could actually be two different forces of gravity, depending on your choice. But I'll be explicit:

    If the floor is not massless, then there are two forces acting on it, pointing down: the force applied by the block onto the floor and the force applied by gravity on the floor. And then there are two(?) "fasten forces" pushing up on the floor, in exact opposition to the forces pushing down on it.
     
  19. Mar 22, 2017 #18

    NascentOxygen

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    It seems that the question has now been answered. Thank you to all contributors.

    Thread closed.
     
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