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Kinteic and potential energy

  1. Oct 29, 2011 #1
    Kinetic and potential energy

    1. The problem statement, all variables and given/known data

    A baseball is thrown from the roof of 22.2 m-tall building with an initial velocity of magnitude 10.7 m/s and directed at an angle of 50.1 degrees above the horizontal.

    A) What is the speed of the ball just before it strikes the ground? Use energy methods and ignore air resistance?

    I found the answer to be 23.44 m/s.

    B) What is the answer for part (A) if the initial velocity is at an angle of 50.1 degrees below the horizontal?

    This is where I am stuck. I didn't use the angle to find part A, so I don't know what I am suppose to be doing here.

    2. Relevant equations

    Potential and kinetic equations.

    3. The attempt at a solution

    thanks!
     
    Last edited: Oct 29, 2011
  2. jcsd
  3. Oct 29, 2011 #2

    Simon Bridge

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    Should the angle make a difference do you think?
     
  4. Oct 29, 2011 #3
    Thats what I was thinking. thanks for the help.
     
  5. Oct 29, 2011 #4
    Remember that the potential energy plus the kinetic energy is constant, that is it is the same when it is at the top of the building moving at 10.7 m/s as when it is at the ground moving x m/s
     
  6. Oct 29, 2011 #5

    Simon Bridge

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    I was kinda hoping to see Crusaderking1's reasoning about whether the angle should or should not make a difference. I suspect this is behind the confusion. I am now worried he thought I was endorsing one or the other.

    The standard puzzle is to throw an object vertically upwards, horizontally and vertically down at the same initial speed and from the same initial height. Then we ask the order they hit and which one hits the hardest.
     
  7. Oct 29, 2011 #6
    I really do appreciate the help. It does make sense because the total of kinetic and potential will always equal the same number regardless of the position when the ball is being dropped.
     
  8. Oct 30, 2011 #7

    Simon Bridge

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    You can verify this by breaking the motion into components, giving the extra energy to the vertical component, then adding them up again, and see if you get the same answer. The extra work will pay off at exam time.

    Sometimes someone will ask you a question because they want to know how you answer that question. In the above case it would help me help you understand the physics. Often, when someone answers these questions they reach an understanding by themselves ... which is more valuable than me telling you so it's worth a shot.

    Of course, this means being prepared to say potentially dumb things in public... don't worry about it: we've all been there.
     
  9. Oct 30, 2011 #8
    yes, understanding the physics is important. Exam time is always brutal so I will do some extra work!

    I don't mind saying stupid things. I just learned what velocity was about 7 weeks ago.
     
  10. Oct 30, 2011 #9

    Simon Bridge

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    Me neither:

    This is because the kazoo gets muddier in the spring when the car shines and the daisies sail with the jellybean.

    See?
     
  11. Oct 30, 2011 #10
    If this was English class, I would be forced to write how a sentence like that is "the meaning life".
     
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