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Homework Help: Simple Forces Question

  1. Jul 25, 2010 #1
    1. The problem statement, all variables and given/known data

    Two magnetic blocks are connected with a magnetic force of 200N on an icy surface(frictionless). If a force of 300N pulls one of the blocks (say the left block leftwards), would the blocks separate? Why or why not?

    2. Relevant equations


    3. The attempt at a solution

    I am really unsure about this one; I would lean towards that the blocks do separate as the 300N of force that needs to be transmitted is greater than the 200N of force that mediates the magnetic force.
  2. jcsd
  3. Jul 25, 2010 #2
    Does the problem provide the mass of the blocks?
  4. Jul 25, 2010 #3
    The blocks are of the same size
  5. Jul 25, 2010 #4
    Size is not important. The key here is mass.
  6. Jul 25, 2010 #5
    I meant that the blocks were the same
  7. Jul 25, 2010 #6
    Okay, so they have the same mass. Suppose the blocks are not separate. What are the forces exerting on each block, besides magnetic forces and the 300-N force? Hint: If they are not separate, they are in contact.
  8. Jul 25, 2010 #7
    I believe that they exert a normal force, equal to the magnetic force, while they are in contact?
  9. Jul 26, 2010 #8
    Correct :rofl:
    No, that's not correct :frown: Only when the blocks are either moving at a constant velocity or remain resting, and magnetic forces and normal forces are the only forces the blocks experience, is the normal force (or contacting force) equal to magnetic force.

    Okay, when the blocks are not separate, they are moving like one thing. What does that imply about the accelerations of the blocks if they are nor separate? When you find out the relation between accelerations in the case the blocks are not separate, write the Newton's 2nd law equation for each block and see if there is anything violating the relation of the accelerations. From here, you should arrive at a conclusion that the 300-N force cannot separate the blocks, provided that the blocks have the same mass.
  10. Jul 26, 2010 #9
    If the blocks do not separate then they must both be accelerating at 150/m, so the magnetic force required to accelerate the second block must be 150N which is less than 200N that is present to hold the two blocks together and thus the two objects do not separate, is this correct?

    In this situation, if the force pulling is increased, then intuitively I arrive that the normal force should decrease but I can't seem to find an explanation on why?
  11. Jul 26, 2010 #10
    Mostly correct, except for the bold part, which should be "magnetic force + normal force". The normal force is not equal to zero, which means the blocks are in contact.

    If we don't apply the 300-N force, then the normal force = 200N, which is greater than 50N of the normal force in the case we apply the 300-N force. You can consider the second block, which is not applied the 300-N force. This block is accelerated under the 200-N magnetic force and the normal force, so the normal force in this case should be smaller than 200 N.
    Anyway, the normal force decreases doesn't essentially mean that it is equal to zero, i.e. the blocks are separate.
  12. Jul 26, 2010 #11
    the concept is that, when two blocks are joined togather by some force, they can be separated only by pulling them from both sides and that too with a force greater than the force of joining. as in this case friction is absent and the force is from one side, they cannot be separated, however large or small the force of pulling may be.
    this is because the force of attraction between the blocks of 200N on eachother cancel out when u consider the two blocks as a constituted body. imagine of breaking a rigid body by pulling it from one side. is it possible?
  13. Jul 26, 2010 #12
    I don't think so.
    Rigid bodies are assumed to be able to obtain any value and any direction of the contact force, as long as the bodies are not broken. On the other hand, in this case, the normal force on one block has a fixed direction: for the left block, the normal force acts to the left; for the right block, the normal force acts to the right. As magnetic force, which has a fixed value of 200 N, is the only force which keeps the blocks together, if the applied force reaches a certain level, the connection will longer form.
  14. Jul 26, 2010 #13
    But the normal force doesn't actually "pull" the second block, isn't it only the magnetic force that attracts the second block into the direction of the motion while the normal force applied by the first block is in the opposite direction to the motion?

    What if 400N of force is used to pull it instead, and in this case 200/m is the acceleration which means all of the magnetic force is used to pull the second block such that normal force would be 0?
  15. Jul 26, 2010 #14
    When I wrote "magnetic force + normal force", I mean it's vector. Magnetic force is always 200 N, only the net force = 150 N.

    Of course :wink: 400N is the limit.
  16. Jul 27, 2010 #15
    Can you explain why the normal force goes down if the magnetic force is constant?

    Would this limit be any force greater than 400N or any force greater or equal to 400N?
  17. Jul 28, 2010 #16
    Consider the 2nd block which the 300-N force doesn't exert on. Because the normal force on it has the opposite direction of magnetic force and therefore its acceleration, the acceleration changes from 0 to some value, and the magnetic force = constant, the normal force must decrease.

    Is there any limit that is a range of value?
    The limit 400 N is the maximum force applied on the 1st block so that the blocks are not separate.
  18. Jul 28, 2010 #17
    I must be dense, becuase I can't see how any force pulls them apart on a frictionless surface.

    ETA: For some reason this bothered me all day and I was finally able to conceptualize it as a jerking.....guess I am a bit dense, lol.
    Last edited by a moderator: Jul 29, 2010
  19. Jul 30, 2010 #18
    I haven't come up with any other better idea, but here is a thought: You stick a sticky note on a book, hold both the book and the note. Then you drop them and quickly remove the sticky note. So you see, in the reference frame of the book, things will be like the case of the two magnets above, with the only external force from your hand.
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