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Homework Help: Magnitude of contact force?

  1. Feb 12, 2010 #1
    1. A student who's standing still on level ground weights 570 N and is wearing a backpack that is 81 N.
    a) What's the magnitude of net force on the student?
    b) What's the magnitude of the contact force on the student by the backpack?
    c) What's the magnitude of the contact foce on the student by the ground?

    3. I understand both parts A (net foce = zero because there is no acceleration) and C (contact forceon the student by the ground = 651 N) in the equation, I'm just having a difficult time understand part b) with the horizontal forces. My thoughts lead me to think that the contract force on the student by the backpack would have been 570 N - 81 N = 489 but apparently that's wrong. can anyone help and explain? Thank you!!
  2. jcsd
  3. Feb 12, 2010 #2
    I think the same thought you used in part a) can be applied in part b): backpack weights 81N and it is not accelerating - so what force you need to balance backpacks weight?
  4. Feb 12, 2010 #3
    so the force of the backpack and the force of the student would cancel eachother and again it would be zero?
  5. Feb 12, 2010 #4
    uch... sorry, I misread your first post a bit, actually a force that backpack exerts on student its only its weight (vertical force), i guess ;]
  6. Feb 12, 2010 #5


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    What horizontal forces?

    No, the backpack exerts a contact force on the student, and the student exerts an equal but opposite contact force on the backpack, per newton 3. These forces act on different objects. There is still a force on the student from the backpack, as well as forces on the student from the ground and from his weight.

    yes, that is correct, but technically, the force that the backpack exerts on the student is the normal contact force between the backpack and student, where that normal contact force is equal to the weight of the backpack, per Newton 1.

    You should always draw free body diagrams of each object (student and backpack) to identify the forces acting.
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