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Normal Force

  1. Oct 16, 2006 #1
    Is there or what is the normal force on an object being pushed into another object?
    Example:I push a 10kg box into a 30kg box with a force of 360N and only the second box receives a friction of 240N (the first one receives no friction), what will the acceleration of each of the individual boxes be.
    (I would assume that the 10kg box is at 36m/s/s and the 30kg box at 4m/s/s,but would like verification).
     
  2. jcsd
  3. Oct 16, 2006 #2

    radou

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    Sounds correct.
     
  4. Oct 16, 2006 #3

    Doc Al

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    Seems like you assume that the only force on the 10kg box is the 360N force. Not so: There is a normal force between the two boxes--they exert forces on each other.

    And do you really think the two boxes will have different accelerations?
     
  5. Oct 16, 2006 #4
    So that makes some sense to me, but what would your answer be to that question? The exact wording is:
    Two boxes of masses 10kg and 30kg are moving along a surface next to each other. There os a force of friction (magnitude 240 N) between the surface and the box of mass 30kg, but no friction between the surface the the box of mass 10kg. A force of 360 N is applied horizontally against the smaller box.
    a)what is the acceleration of each box?
    b)what is the force that the smaller box exerts on the larger box?
    c)what is the force that the larger box exerts on the smaller box?

    Thank you all for your help in advance
     
  6. Oct 17, 2006 #5

    Doc Al

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    I want to hear your answers!
    Hint 1: The boxes move with the same acceleration.
    Hint 2: Newton's 2nd law applies to both boxes taken together and to each box separately.
     
  7. Oct 17, 2006 #6
    Just a question what is normal reaction force?

    Is there any difference between NORMAL force and NORMAL REACTION force?
     
  8. Oct 17, 2006 #7

    radou

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    The normal force is a reactive force.
     
  9. Oct 17, 2006 #8
    Nope. There is no difference between normal force and normal reaction force. Normal force is also the reaction force due to forces on 2 interacting bodies.
     
  10. Oct 17, 2006 #9

    Doc Al

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    The term is confusing to me. All forces are "reaction forces" to something. (Perhaps a better term is "active" versus "passive". For example: Gravity is an active force, whereas the normal force is a passive force.)

    Presumably "normal reaction force" and normal force refer to the same force.
     
  11. Oct 17, 2006 #10
    oh ok....

    My school teacher told me there is no such thing as normal force and it is normal reaction force...but i started arguing with him,since , I did read it somewhere as normal force.

    ~sigh~

    thank u!
     
  12. Oct 17, 2006 #11

    radou

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    It is called 'normal' because it is perpendicular to the contact surface, so the word 'normal' tells you something about its direction, while 'reaction' implies that it is a reactive force. It's as simple as that. In the end, it really does not matter how you call it.
     
  13. Oct 17, 2006 #12
    To me it's "Tomayto" or "Tomahto". Like radou said, it's called "normal" because of the normal vector, which is perpendicular. The term "reaction" is very vague, and I really don't use it very often (especially in kinematics, which the terms are so confusing already).

    So I say "Tomayto"
     
  14. Oct 17, 2006 #13
    right so i got:
    a)3m/s/s
    b)330N
    c)330N
     
  15. Oct 18, 2006 #14

    pls explain....


    anyways, lol, sorry i "borrowed" your thread!
     
  16. Oct 18, 2006 #15

    radou

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    Regarding a). The two boxes are now a system. They are moving together. Now, which forces are acting on that system (i.e. on these two boxes)? Which is the net force? What is the mass of the system? Just apply Newton's second law.
     
  17. Oct 18, 2006 #16
    ok so 360N applied on small.
    only 30N needed to push box at 3m/s/s so N=360N-330N=30N.
    So 330 applied on big.
    240 friction against it.
    and obviously gravity and normal force on each.
    So 30N used on 1st box 90N used on 2nd box and 240N to overcome friction.
    This should be right.
     
  18. Oct 18, 2006 #17

    arildno

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    Your teacher is being silly.
    Consider a box lying on a floor.
    Here, you have a NORMAL FORCE couple, in that the box exerts a normal force on the floor, and the floor exerts a normal force on the box.
    (Normal merely designates that the force is parallell with the surface normal.)
    Whichever of these normal forces you designates as "action" or "reaction" is arbitrary.
     
  19. Oct 18, 2006 #18

    so I must count both vertical and horizontal forces.
    Thus the horizontal forces would be
    the force applied on small box and fiction force of the big box
    And the vertical forces will be,
    normal force and the weight.

    Correct me if I'm wrong.
     
  20. Oct 18, 2006 #19

    Doc Al

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    The vertical forces cancel--the boxes accelerate horizontally.
     
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