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Mechanical energy

  1. Dec 10, 2005 #1
    an object moves from A to B only two forces act on it. one force is conservative and does -70J of work, the other force is nonconservative and does +50J of work.

    kinetic energy of the object decreases and mechanical energy decreases, right?
  2. jcsd
  3. Dec 10, 2005 #2


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    Staff: Mentor

  4. Dec 10, 2005 #3

    a 2.47kg block is pushed 1.7m up a vertical wall with constant speed by a constant force of magnitude F applied at an angle of 63.3 with the horizontal. acceleration of gravity is 9.8m/s^2

    if the coefficient of kinetic friction between the block and wall is 0.553, find the work done by F.

    sum of all forces on the x-axis=0 sum of all forces on the y-axis=,a
    N=-Fsin(angle) Fcos(angle)-mg-Friction force=ma

  5. Dec 10, 2005 #4
    the choices only mention kinetic and mechanical
  6. Dec 10, 2005 #5


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    The angle is with respect to the horizontal.

    The vertical force, Fy must balance the friction force, which is proportional to Fx, and the weight of the block, mg.

    Then the work is simply the force applied over distance. Fy is constant because the block is pushed at constant speed, i.e. no acceleration.
  7. Dec 10, 2005 #6
    for the first question, the answer that i suggested is not correct. how come?
  8. Dec 10, 2005 #7


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    In the first question, I am trying to understand if any potential energy is involved.

    Generally, if the work done is positive, then mechanical energy increases, and if work is negative, mechanical work decreases.

    The total mechanical energy (or mechanical energy) is the sum of kinetic and potential energies.

    Deceleration would imply negative work and this coincides with a reduction in kinetic energy, where as acceleration implies positive work being done increases kinetic energy.

    A nonconservative or dissipative force reduces kinetic energy.

    I am trying to think of a + nonconservative work.
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