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Forces acting on the person carrying a bucket of water

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  1. Dec 5, 2017 at 8:46 PM #1
    1. The problem statement, all variables and given/known data
    A person of mass M is walking across the ground carrying a heavy bucket of water of mass m on top of their head. Which of the following forces is not acting on the person?

    a) The gravitational force on the person Mg acting downward
    b) The normal force from the ground acting upward
    c) The gravitational force on the water mg acting downward
    d) The force of static friction from the ground acting sideways

    (The answer is c.)

    2. Relevant equations
    N/A

    3. The attempt at a solution
    I chose d because although friction force acting sideways is required to walk, I thought that walking forward would mean that static friction is acting forward and back, rather than sideways. I thought c was correct, so I am not sure why it is the answer. Could someone explain to me why c would not be considered to be one of the forces acting on the person?
     
  2. jcsd
  3. Dec 5, 2017 at 9:51 PM #2

    PhanthomJay

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    There is a force acting on the person by the water bucket, but it's not gravity. What force is it?
     
  4. Dec 5, 2017 at 9:55 PM #3
    Is the force a part of action/reaction pair?
     
  5. Dec 5, 2017 at 10:27 PM #4

    haruspex

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    All a body in a mechanics question 'knows' is forces that act on it directly. In that sense, the person knows there is a force on her head, but not its cause. The gravitational force on the water acts on the water.
     
  6. Dec 5, 2017 at 10:53 PM #5

    PhanthomJay

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    Yes
     
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