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Solving these non-numerical problems applying work and energy concepts

  1. Oct 16, 2005 #1
    Here they are...

    (1) A hand exerts a constant horizontial force on a block that is free to slide on a frictionless surface. The block starts from rest at point A, and by the time it has traveled a distance d to point B it is traveling with speed vB. When the block has traveled another distance d to point C, will it's speed be greater than, less than, or equal to 2vB? Explain your reasoning.

    My drawling, if you can undertand it...

    Hand pushes block----------------------------------------<(surface)

    My first instinct is to say equal to since it is a frictionless surface therefore nothing will slow it down.

    (2) Seasoned hikers prefer to step over a fallen log in their path rather than stepping on top and jumping down on the other side. Explain.

    No idea... but I was thinking because it takes twice as much energy to step on top of and then down than it does to just stepp over?

    (Thanks to all who help)
  2. jcsd
  3. Oct 16, 2005 #2


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    Part 1)
    The block is pushed with a constant force, hence a constant acceleration.
    The block is accelerating from A to B and from B to C.
    Can you now compare the velocities at B and C again ?

    Part 2)
    It takes more energy, yes. Think potential.
  4. Oct 17, 2005 #3
    So the velocity with be more when it reaches B-C?

    I was thinking they would be equal... I guess not.
  5. Oct 18, 2005 #4
    Greater than, because it is gaining momentum?
  6. Oct 18, 2005 #5


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    Have you worked out the eqn of motion for the block ?

    Use this to determine the speed of the block at the points B and C.

    Use these results as justification for your answer to the question posed.
  7. Oct 18, 2005 #6


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    This was supposed to be Work & Energy practice, right?
    Compare Work done thru the displacement "a" to "b"
    with Work done thru displacement from "a" to "c".
    What form of Energy does this Work show up as?
    What's the functional form ("formula") for KE?
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