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Homework Help: Find velocity.

  1. May 4, 2008 #1
    Find velocity.........

    A 3.00Kg block at x=0 is moving to the right at 5.00m/s.
    A horizontal force acts on the block. Its potential energy funtion is U=(-x^2)-4x+5
    find the speed of the block at x=2.00m (surface is smooth)

    ok the answer is already given by prof. answer is 5.74m/s but I wanna know how to solve this. I used kinetic and potential energy equation for this problem but I got 4.52m/s also I used changing kinetic and potential energy but I got 8.50m/s........I tried other things too but I got wrong....... I'm stuck with this.....
  2. jcsd
  3. May 4, 2008 #2

    Chi Meson

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    The answer is either 4.51 m/s or 5.45 m/s, depending on whether the force is opposite or with the direction of motion. If the answer is actually 5.74 m/s, then the question is wrong.

    Clearly the function gives a potential energy of 7 J, so the block either gains or loses 7 J of KE (depending on the intended meaning of the negative quantity).

    Edit: I noted rock.freak's answer, below, and then slapped my head. Only the change in PE of the system matters. Since the +5 exists at zero, and at x=2, then then change in KE is 12 J.

    Awkwardly worded question.
    Last edited: May 4, 2008
  4. May 4, 2008 #3


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    at x=0..what is U? Then use the law of conservation of mechanical energy.
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