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Homework Help: Change in kinetic energy! Velocity not given

  1. Jul 23, 2009 #1
    change in kinetic energy! Velocity not given :(

    1. The problem statement, all variables and given/known data
    Two forces, of magnitudes F1 = 70.0 N and F2 = 35.0N , act in opposite directions on a block, which sits atop a frictionless surface, as shown in the figure. Initially, the center of the block is at position xi = -2.00cm . At some later time, the block has moved to the right, and its center is at a new position, Xf = 4.00 cm .

    Determine the change in the kinetic energy of the block as it moves from Xi= -2.00 to X f= 4.00

    Link to the diagram : http://session.masteringphysics.com/problemAsset/1011314/13/MWE_wi_5.jpg

    2. Relevant equations
    Kinetic energy = 1/2 mv^2

    3. The attempt at a solution

    Since it is starting from rest, initial velocity, V0 = 0 and then d = Vf t

    But here Vf is not given..
    I am not sure how to proceed when there is no velocity given

    Can any more help me ?

    Thanks a lot before hand
    Last edited: Jul 23, 2009
  2. jcsd
  3. Jul 23, 2009 #2


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    Staff: Mentor

    Re: change in kinetic energy! Velocity not given :(

    What is the net force acting on the block? Just keep the mass and velocity as variables, m and V for now. Write the equations to tell you how long it takes to get to the 2nd position, and what the final velocity is. Then see if some of the variable terms cancel out...
  4. Jul 23, 2009 #3


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    Homework Helper

    Re: change in kinetic energy! Velocity not given :(

    Change in kinetic energy = work done on the block = Force X displacement.
    In the problem force and displacement is given. That is sufficient to find the KE.
  5. Jul 23, 2009 #4
    Re: change in kinetic energy! Velocity not given :(

    Thank a lot everyone! :)
    I understood it! :D
    Thank a ton everyone!
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