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Zero level help

  1. Dec 10, 2004 #1
    The problem is:
    A block of mass m slides at a speed v along a horizontal, smooth table. It next slides down a smooth ramp, descending a height h , and then slides along a horizontal rough floor, stopping eventually.
    If the zero level is a distance (2h/3) above the floor, what is the potential energy U of the block on the floor?

    I'm confused about the zero level.

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  2. jcsd
  3. Dec 10, 2004 #2

    Doc Al

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    Staff: Mentor

    To specify the potential energy, you need a reference point--a place where U = zero. For example, if the ground were the zero point, then the gravitational PE of an object at a height h would be U = mgh; but if 2h/3 were the reference point, U = mgh/3.
  4. Feb 10, 2012 #3
    Old thread but I happen to have the exact same homework question. I hate MasteringPhysics.com.

    Anyway, I understand U = mgh
    But I really don't get what 2h/3 is trying to tell me.

    "If the zero level is a distance above the floor, what is the potential energy of the block on the floor?" & "Suppose the potential energy of the block at the table is given by . This implies that the chosen zero level of potential energy is __________."

    I have no idea what this is asking me.
  5. Feb 11, 2012 #4

    Doc Al

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    Staff: Mentor

    With U = mgh, where do you measure h from?

    You measure it from the zero level, which is completely arbitrary. For example, if a mass is on the floor, and I choose the floor as the zero level, then h = 0 and U = 0. But what if the zero level were chosen to be 10 meters below the floor (in the basement, perhaps)? Then h = 10m.

    Another way to write the potential energy formula is: U = mgy, where y is the vertical position measured from the zero level, which is where y = 0.

    The point is: You can put the zero level anywhere.
  6. Mar 4, 2012 #5
    I too am confused on this which is how I actually came across this forum.

    Anyway, if the U=mgh/3 on the table and zero point is 2h/3 then wouldnt that mean that the zero point is above the table since 2/3 is bigger than 1/3. This lead me to believe that the floor would in turn be 3h/3 (or just h) but when I put the answer of mgh in it was wrong. Any insight would be appreciated.
  7. Mar 4, 2012 #6

    Doc Al

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    Staff: Mentor

    No. That would be a contradiction, since you just said that the zero point is at 2h/3.

    The point is to choose the zero point (in this case it's given to you), and measure the displacement from that point. If the table top is h/3 above the zero point, where is the floor measured from that same zero point?
  8. Mar 4, 2012 #7
    Ok now I get what it is saying. Thank you for clearing it up!
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