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Thread: Kinetic Energy Calculation

  1. #1 Kinetic Energy Calculation 
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    Hi, given this question in a physics lesson I am still having trouble competeing it. Can anyone help.

    Calculate the change in kinetic energy of a car of mass 2000kg if it changes its velocity from 30kmh^-1 to 60kmh^-1.


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    Jack Heron
    jeheron@gmail.com


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  3. #2  
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    From my remote past unless things have changed

    KE = Mass x velocity squared divided by 2g
    g is engineers talk = [gravity 32.2ft /sec/sec]
    hope that helps, you can do the math


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  4. #3  
    Forum Isotope Zelos's Avatar
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    WRONG!
    its Ek=m(V<sub>1</sub>-V<sub>0</sub>)²/2
    SI units
    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

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  5. #4  
    Forum Professor river_rat's Avatar
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    Um, no zelos

    delta E = E<sub>end</sub> - E<sub>start</sub>

    So in this case we have that

    delta E = 1/2 m (v<sup>2</sup><sub>end</sub> - v<sup>2</sup><sub>start</sub>)

    sub in m = 2000kg, v<sub>start</sub> = 8.333 m.s<sup>-1</sup> and v<sub>end</sub> = 16.666 m.s<sup>-1</sup>

    So delta E = 208 333.333 J
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  6. #5  
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    Quote Originally Posted by Zelos
    WRONG!
    its Ek=m(V<sub>1</sub>-V<sub>0</sub>)²/2
    SI units
    ita a basic formula, and the answer is in the question.
    I am Die Fledermaus, eater of all things Zelos :wink:
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  7. #6  
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    First convert physical quantities in S.I syetem. You have to convert km/hr to m/s.

    Change in KE=(v^2-u^2)m/2

    v=Final velocity
    u=Initial velocity
    m=mass of the body
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  8. #7  
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    Quote Originally Posted by Zelos
    WRONG!
    its Ek=m(V<sub>1</sub>-V<sub>0</sub>)²/2
    SI units
    Zelos your equation is wrong.
    change in KE=m(V<sub>1</sub>²-V<sub>0</sub>²)/2
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  9. #8  
    Forum Radioactive Isotope mitchellmckain's Avatar
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    Yes both Die Fliedermaus and Zelos goofed. OOPS. But river rat and anandsatya have it right.

    KE = .5 m v^2

    so you calculate KE for the two different velocities and subtract.

    Just thought I should weigh in on this to help clear things up.
    See my physics of spaceflight simulator at http://www.relspace.astahost.com

    I now have a blog too: http://astahost.blogspot.com/
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  10. #9  
    Forum Isotope Zelos's Avatar
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    yeah youre river_rat. my bad

    the real formula is thou
    Ek=mc²(y-1)
    y=1/sqr(1-v²/c²)
    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

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  11. #10  
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    Quote Originally Posted by Zelos
    yeah youre river_rat. my bad

    the real formula is thou
    Ek=mc²(y-1)
    y=1/sqr(1-v²/c²)
    I never said a word.......
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  12. #11  
    Forum Isotope Zelos's Avatar
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    I never said a word.......
    keep doing that
    I am zelos. Destroyer of planets, exterminator of life, conquerer of worlds. I have come to rule this uiniverse. And there is nothing u pathetic biengs can do to stop me

    On the eighth day Zelos said: 'Let there be darkness,' and the light was never again seen.

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  13. #12 Answer 
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    After some calculations I got the change in Ke to be 1800000kilojoules? Is this reasonable.
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  14. #13 Re: Answer 
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    Quote Originally Posted by jeheron
    After some calculations I got the change in Ke to be 1800000kilojoules? Is this reasonable.
    I don't think that's correct....

    I get ~200 kJ.

    Cheers,
    wm
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  15. #14 My working 
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    Ke = 1/2 * m * v^2

    Kinetic energy pre acceleration (Ke1) = 1/2 * 2000 * (30*1000*60)
    1/2 * m * velocity in m
    = 18e9

    Kinetic energy post acceleration (Ke2) = 1/2 * 2000 * (60*1000*60)
    1/2 * m * velocity in m
    = 3.6e9


    Ke2 - Ke1 = 1800000000joules

    = 1.8e6 kilojoules

    which is my answer from the pervious post
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  16. #15  
    Forum Ph.D. william's Avatar
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    Double check your conversion from km/h to m/s. That is where your error lies.

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  17. #16  
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    Kinetic energy pre acceleration (Ke1) = 1/2 * 2000 * (30*1000*60*60)
    1/2 * m * velocity in m
    = 108000000000

    Kinetic energy post acceleration (Ke2) = 1/2 * 2000 * (60*1000*60*60)
    1/2 * m * velocity in m
    = 216000000000


    Ke2 - Ke1 = 108000000000joules

    =108000000kj

    =108000mj

    not = to your previous answer
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  18. #17  
    Forum Ph.D. william's Avatar
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    Check again....
    Hint: km PER hour.
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  19. #18 damn 
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    i am official a loser.

    sorry.

    stupid mistakes.
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