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Questions about Forces and Tension

  1. Feb 3, 2009 #1
    1. The problem statement, all variables and given/known data

    This is a general question that I went over in class today. I'm not sure if I completely understand it. There is a block with a tension force pulling to the right and another tension force pulling to the left. Assume that it is in space (no gravity force or anything else). My teacher said that if there was no acceleration, there would be no tension in the string. Why wouldn't the tensions just equal each other? He said something about needing another force to balance the tensions, but why is the tension zero?

    Also, another question.
    If a baseball is hit and travels toward an outfield fence. What are the forces acting on the ball?

    2. Relevant equations

    3. The attempt at a solution

    I thought that the tensions would just equal each other because they are the only two forces acting and the acceleration is zero. Why is this not true?

    For the second question, I thought it would be obviously the downward force of gravity, but isn't there also a horizontal force because of the bat hitting the ball? I'm not sure if my reasoning is correct.
  2. jcsd
  3. Feb 3, 2009 #2


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    It is true. Perhaps your teacher meant there is no net force acting on the block; but the tensions could be any value, equal in magnitude and opposite in direction, as you have noted.
    After the ball is hit and is in flight, only gravity acts on the ball. While it is being hit, then yes, you have a bat force acting as well (but not necessarily horizontal), which goes away after the ball leaves the bat.
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