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Kinetic Energy for an angle

  1. Aug 29, 2015 #1
    I have a question regarding how to calculate kinetic energy. Let's say I was to find the kinetic energy of a ball shot at 180 m/s at a 45 degree angle. Do i simply sub in 180 as v (velocity) in the kinetic energy equation (1/2mv^2) or must it be split into components?
     
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  3. Aug 29, 2015 #2

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    Is energy a scalar or a vector quantity?
     
  4. Aug 29, 2015 #3
    Scalar
     
  5. Aug 29, 2015 #4
    My textbook gives me a question that reads: A baseball is thrown from a cliff of 41m at at a velocity of 22 m.s/ The ball is thrown at at 37 degree angle. What's the ball's velocity before hitting the ground?

    Why do they give this angle if it has no use in this question? I know how to get the answer, but is there a time that the angle must be used?
     
  6. Aug 29, 2015 #5

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    Is velocity a scalar or a vector?
     
  7. Aug 29, 2015 #6
    Velocity is a vector, but how does the angle play a part in it-why don't they just say it is shot at 22m/s horizontally, there is no use of the angle is there?
     
  8. Aug 29, 2015 #7

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    It doesn't say, "horizontally." What you've given isn't clear; the 37 degrees could be a compass heading, elevation above horizontal, declination below vertical.
     
  9. Aug 29, 2015 #8
    Sorry, it is elevation above the horizontal
     
  10. Aug 29, 2015 #9

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    And you need to look at the horizontal and vertical components for velocities, and time of flight.
     
  11. Aug 29, 2015 #10
    Ok so i did some calculations, calculating the kinetic energy in x(22cos37) and y(22sin37) components and adding them together equals the kinetic energy when calculated from the angled velocity (22 m/s). So basically my question is, is there a reason I should calculate the kinetic energy in components as opposed to the complete vector for a problem like this?
     
  12. Aug 29, 2015 #11

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    No. 222 = 484 = (1+1+1+1+1+1+1+1+1+1 + 1+1+1+1+1+1+1+1+1+1 + 1+1)2.
     
  13. Aug 29, 2015 #12

    Doc Al

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    Realize that ##\sin^2\theta + \cos^2\theta = 1##.
     
  14. Aug 29, 2015 #13

    sophiecentaur

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    You have a text book. The questions in text books are usually based on what the book discusses. Have you looked in the book to find out about trajectories? You could also look on this Hyperphysics Link which tells you the necessary stuff in a condensed way. If you approach this algebraically (using the formulae and symbols, instead of putting figures in, you could obtain an answer that would (or would not) contain the angle - so you could answer your own question.
     
  15. Aug 29, 2015 #14

    Doc Al

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    Note that they ask for the velocity, not simply the kinetic energy.

    Of course, if all they want is the magnitude of the velocity, then perhaps you do not need the angle. That's for you to realize. There are many ways to skin a cat.
     
  16. Aug 29, 2015 #15

    russ_watters

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    Why are you using kinetic energy at all for this problem? As sophiecentaur said, you have a book: what method do they use in the sample problems?
     
  17. Aug 29, 2015 #16
    My book doesn't provide a method for a question like this, just the answer-which is given to me as 36 m/s. So I am having trouble understanding how to/if i should implement direction as they haven't given one in the answer.
     
  18. Aug 29, 2015 #17

    Doc Al

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    What is the topic of the book chapter where this question was asked? Is it a chapter on energy conservation? Or on projectile motion?

    You certainly do not need to use the angle to find the final speed. If you've covered both energy methods and projectile motion, they solve it both ways.
     
  19. Aug 29, 2015 #18
    Yes it is a unit on conservation of energy, momentum etc.
     

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  20. Aug 29, 2015 #19

    Doc Al

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    Then why not use energy methods? (The purpose of giving the angle is to make you think! Does the answer depend on the angle?)
     
  21. Aug 29, 2015 #20
    So there is no use of the direction for the final velocity as they only ask for speed-my question still stands though, why did they give us an angle, is it ideal to be solved in terms of components?
     
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