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Homework Help: Help Relating Velocity to Force

  1. Oct 14, 2007 #1
    1. The problem statement, all variables and given/known data
    A 2.7 kg steel block is at rest on a steel table. A horizontal string pulls on the block.
    (a) What is the minimum string tension needed to move the block?
    (b) If the string tension is 26 N, what is the block's speed after moving 1.0 m?
    (c) If the string tension is 26 N and the table is coated with oil, what is the block's speed after moving 1.0 m?

    2. Relevant equations
    This is where I really need help. I figured out part A by using F=ma and then using the fact that the coefficient of static friction times the normal force equals the static friction that must be overcome to start movement.

    What I need to know is how to get parts C and D...I need an equation to relate velocity to force.

    3. The attempt at a solution
    The answer it part A is 21.168 N

    No idea on how to get parts B and C

    Thanks for any help.
  2. jcsd
  3. Oct 14, 2007 #2


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    Staff Emeritus
    Science Advisor

    Presumably one has used some coefficient of friction, which is not given in the post, but the approach described is correct.

    Now look at the 26 N force compard to the friction force in A (21.168 N).

    There is a net force which then accelerates the 2.7 kg mass. a = Fnet/m.

    For a constant acceleration, use the appropriate equation of motion to find v at some distance d.

    c. What is the coefficient of friction with the oil? Ostensibly that will reduce the friction force, so the apply the approach of part b to get the acceleration.
  4. Oct 14, 2007 #3
    Thank you much, that helps a lot.
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