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Homework Help: Baseball Kinetic energy

  1. May 5, 2015 #1
    1. The problem statement, all variables and given/known data
    After being hit, a 0.145 kg baseball has a speed of 45.6 m/s.

    1. What is the kinetic energy of the baseball after it has been hit?

    2. How much work is done on the baseball by the baseball bat?

    3. In moving through the air, the baseball slows down to a speed of 30.0 m/s. What work has the air done on the ball?

    2. Relevant equations

    K= (1/2)(m)v2
    W= KEf-KEo

    3. The attempt at a solution

    1. KE = (1/2)(0.145kg)(45.6m/s)^2
    KE = 150.75 J or 1.5 x 10^2J

    2. W = 150.75J- 0J = 150.75J ( I assuming KEo is 0J since the kinetic energy of the baseball did not change even though its direction of motion is reversed)

    3. W= KEf - KEo
    W= (1/2)(0.145kg)(30.0m/s)^2 - 150.75j
    W= 65.25 J - 150.75J
    W = -85.5 J

    I solved these 3 questions, but I don't think this isn't the right answers. Any criticism, advice or hint is appreciated.
  2. jcsd
  3. May 5, 2015 #2
    If that is the entire question, those answers look correct (I didn't do the calculations but your work seems right). However, it seems like the problem might have given you an initial velocity before being hit, which would obviously change the second answer. As is, this is correct.
  4. May 5, 2015 #3
    Yeah, that was what the question was asking, it didn't give any initial velocity. Btw, for #3 can work be negative? I'm not sure if the work supposed to be positive or not.
  5. May 5, 2015 #4


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    Yes, work can be negative.
  6. May 5, 2015 #5
    So, my answer in #3 is right ?
  7. May 5, 2015 #6
    Yes, it's correct.
  8. May 5, 2015 #7


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    Homework Helper
    Gold Member

    You need to make an assumption about prior speed to answer the question. What seems more likely, that the speed was almost the same, or that it was so much less than after being struck that it can be taken as zero?
  9. May 7, 2015 #8
    I will add one more question on this questions

    1)If the force of the baseball bat acts in the direction of the motion of the ball and the bat and ball are in contact for 0.0120 m, determine the average force applied to the baseball by the bat. In this case, the force of gravity can be ignored.

    I'm having bit of trouble with this question, can you tell me which equation I should use?
  10. May 7, 2015 #9


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    Science Advisor

    It looks like your instructor or textbook has fallen squarely into item 3 in one of our most recent Insights articles. https://www.physicsforums.com/insights/frequently-made-errors-mechanics-forces/

    The intended approach in this case is for you compute the energy increase as work done. Work is the product of force times distance. If you know the energy and you know the distance [and if you are either casual or correctly careful about what is meant by "average"] then you can compute the average force.
  11. May 7, 2015 #10
    Then, should I tell my instructor to look at that page? Btw, thank you for explanation.
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