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Homework Help: Newton-law help

  1. Jul 8, 2007 #1
    1. The problem statement, all variables and given/known data
    A vertical force F is applied to a block of mass m that lies on a floor. what happens to the magnitude of the normal force N on the block from the floor as magnitude F is increased from zero if force F is a)downward b)upward

    2. Relevant equations
    Let m be the mass of body. ideally if the body were to be at rest , and considering only vertical motion, N(reaction force on body from surface)=mg
    1) if force F is downward, F+mg> N. The N would adjust in a way such by increasing it s value so that it can counter F+mg force else the block will break through the surface

    2)the N would decrease or else the block would raise above the surface.

    3. The attempt at a solution

    a)downward= N increases
    b)upward=N decreases
  2. jcsd
  3. Jul 8, 2007 #2
    Yes, you are correct. Intuition plays a good role in this case too because if you push down on a block harder, then the floor is going to have to push back harder for the forces to cancel and the block to stay still.
  4. Jul 8, 2007 #3


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    Science Advisor
    Homework Helper

    Yes, absolutely. But don't even worry about the case where the block has a velocity in the vertical direction. That's not a situation where you would even think much about static normal forces.
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