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Help with yr11 physics please

  1. Oct 8, 2013 #1
    1. The problem statement, all variables and given/known data
    Jackie's team bats later in the day. When Jacki strikes the ball, she hits it badly and it travels straight up.

    2. Relevant equations
    Explain whether the forces are balanced or unbalanced at the highest point.

    3. The attempt at a solution
    I thought it was balanced as the ball will be stationary for a moment at the highest point but that was wrong...
     
  2. jcsd
  3. Oct 8, 2013 #2

    PhanthomJay

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    It is momentarily stationary, yes, but what are the forces acting on it at that point? Is there a net force?
    And oh, welcome to PF!
     
  4. Oct 8, 2013 #3
    the question doesn't say if there is a net force but the force acting on the ball are gravity and the force from the bat
     
  5. Oct 8, 2013 #4

    PhanthomJay

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    At the instant the ball leaves the bat, the bat is no longer in contact with the ball, so therefore it no longer exerts a force on the ball. So during the flight of the ball, what force (or forces) acts on it? In particular, identify the force(s) acting on the ball at the highest point. Is there an unbalanced net force acting on the ball at that point?
     
  6. Oct 16, 2013 #5
    The question does not say if there is a net force or what forces are acting on the ball
     
  7. Oct 16, 2013 #6

    haruspex

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    Quite so, since that is what you are expected to determine. When the ball is lying on the ground, what forces are acting on it? If a ball is falling through the air, what forces are acting on it? What equation do you know relating acceleration of a mass and the net force on it?
     
  8. Oct 16, 2013 #7
    Fnet= ma, but I do not understand how this equation will relate to the problem because then the acceleration would be 0 at the highest point. So Fnet= 0 meaning forces acting on the ball are balanced and the answer said that the forces are unbalanced.
     
  9. Oct 16, 2013 #8
    Hey There JenL, I think that the acceleration would not be 0 because the gravitational field strength will make the ball accelerate at -10m/s^2 downwards
     
  10. Oct 16, 2013 #9

    haruspex

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    The velocity is zero at the highest point, but not the acceleration.
    You did not answer my question about the forces on a ball lying on the ground. Clearly that does not accelerate, so the forces are balanced in that case. What are they?
     
  11. Oct 17, 2013 #10
    Thank you Khemical, that helped. Haruspex, The forces on a ball lying on the ground would be reaction and gravity which would be equal and so balanced. I think I understand the acceleration bit now and the question because wouldn't the acceleration be 10ms^-2 due to the force of gravity. So then Fnet would be unbalanced?
     
  12. Oct 17, 2013 #11

    haruspex

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    Exactly.
     
  13. Oct 18, 2013 #12
    Thank you :)
     
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