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Newton's second and third laws

  1. Feb 12, 2012 #1
    Free body diagrams for system A that consists of two blocks and for system B with one block:

    acceleration and net force from the systems:

    frictional force:

    I'm having a hard time understanding how a free-body diagram works. I know that each of the diagrams for the two systems have to have weight, normal force, and friction acting on it, but I don't completely understand the concept behind them. And I don't even know where to begin with the other two parts of the question. Please help! I'm more than willing to understand all of this. Thank you!
  2. jcsd
  3. Feb 12, 2012 #2
    First of all you do have to understand what weight, normal force, and friction is to be able to draw the resultant diagram...

    1. Weight - what is it's components and in what direction does it act?
    2. Normal force - what is it and what direction does it act in (what force does it balance out)
    3. Friction - what is it and what direction should it act in (hint: in aerodynamics its called drag)
  4. Feb 12, 2012 #3


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    welcome to pf!

    hhi emc92! welcome to pf! :wink:
    a free-body diagram doesn't really prove anything, it's just a way of making everything clear, so that you don't leave anything out

    also, if you're giving the forces names (letters), you can just write them on the diagram, which is a lot clearer than explaining it in prose! :wink:
    b. is dead easy

    c., use F = ma

    d., you'll need to do some calculations :wink:

    show us what you get :smile:
  5. Feb 12, 2012 #4
    @ shyguy79:

    1) i know that weight consists of mass and gravity and it's a downward force. for system A, however, since there are two blocks, are there going to be 2 weight vectors on the diagram?
    2) normal force is the force perpendicular to the surface of the table, and it's an upward force. it cancels out the force of gravity acting upon the table?
    3) friction is opposite the direction of motion of the object. so in this case, friction is acting to the right of the blocks.

    at the moment, i only have 3 forces acting on system B - the weight, normal force and friction. is this all for that system?

    also, is there a force from system B acting on system A?
  6. Feb 12, 2012 #5
    @ tinytim

    for part b) is the acceleration for all systems the same?
  7. Feb 12, 2012 #6


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    they all move together, so yes! :smile:

    (sometimes, questions are that easy! :biggrin:)
  8. Feb 12, 2012 #7
    1) How would you consider the 'weight' of two blocks - if the mass of each block (and there are two of them) is m and gravity is g then perhaps the weight is ____?
    2) Yep... if the surface does not yield under the weight (W = Normal Force)
    3) Yep... so F(push) > F(friction) to move the system

    Remember friction only exists to opposes the motion so will only be present when there is a pushing force so in essence you've gotta remember to pushing force if you're labelling the friction - so you have 4 elements! Like tiny-tim says it is sometimes that easy :-)
    Last edited: Feb 12, 2012
  9. Feb 12, 2012 #8
    @ shyguy 79
    so the weight is twice as much for system A. but it's acceptable to only have one vector for the two blocks? also, for system A, is the force opposing friction the force from the hand or from system B? since the hand isn't directly applying the force?

    @ tiny tim
    i'm still not sure how to begin in parts c and d... =/
  10. Feb 12, 2012 #9
    for part c, is the net force on system B the same as that on system A? and it's twice as much for system C?
  11. Feb 13, 2012 #10


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    (just got up :zzz: …)

    have you misread the diagram?

    A is two blocks, B is one block, C is all three blocks :wink:
  12. Feb 13, 2012 #11
    Lol.. Same here! :zzz:

    One vector for two blocks? Why not? When you stand on a scales you get the combined weight of you and your clothes...

    From the hand or system B? It's kinda irrelevant it is still a pushing force in the -x direction (left)

    I think you should now sit down and use what info you've got and sketch out a rough sketch and see if you can understand what's going on - maybe even put it on here for us to see
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