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Help with energy-mass equivalence!

  1. Oct 23, 2009 #1
    Of all things I'm concerned, this one should be the most confusing ever. I'll tell how...


    That is, energy is an object's mass times the speed of light squared. Mass is w*g, and g is 9.81 for so long you're on Earth. Then, the speed of light is 299,792,458 m/s*299,792,458 m/s, which is 8.98755179*1016 m2/s2 (okay, this part is where I get confused. How much is 8.98755179*1016 m2/s2 then?)


    E=(wg)*(8.98755179*1016 m2/s2)

    My second doubt is here. If wg would give a result in Newtons, then in what unit will the result would be if E=mc2?

    Please give me an example. I'll give the values. The mass should be 335.564 N. Please show me how you calculate the rest.

  2. jcsd
  3. Oct 23, 2009 #2

    Doc Al

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    Staff: Mentor

    No, mass is w/g, not w*g. If you have an object's weight in Newtons, then find its mass in kilograms using that formula.
  4. Oct 23, 2009 #3
    I can't understand your first question, you are asking "how much is xxx", when you have directly quoted the amount. What other information do you want? This is like asking "how much is $1.50?".

    For your second question I think you are getting very confused over units: mass is not measured in Newtons, force is. Mass is measured in kilograms, and is the measure of the amount of matter in an object. To clear things up here is a simple E = mc^2 example,

    1kg of mass is converted into pure energy. How much is produced?

    E = mc^2 = 1kg * (3*10^8 m/s)^2 = 9*10^16 kg m^2 / s^2.

    Note that the units, kg m^2 / s^2 are identical to the Joule, J. There is no mention of forces in this question, and no need to express anything in Newtons.
  5. Nov 3, 2009 #4
    All said and done, here's how I've understood it. That means...

    My weight is 98.1 kilos. The g, of course, is 9.81. So...

    E=[([tex]\frac{98.1}{9.81}[/tex]) x 89,875,517,900,000,000] j
    E=890,875,517,900,000,000 joules

    Doesn't seem to correct to me. After all, I'm just in the VIIth standard.
  6. Nov 3, 2009 #5

    Doc Al

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    Staff: Mentor

    It's not correct. 98.1 kg is your mass, not your weight, so no need to divide by 9.81. (In everyday usage, it's also your "weight". But in physics weight is a force, measured in Newtons, given by mg.)
    Last edited: Nov 3, 2009
  7. Nov 3, 2009 #6
    Try to distinguish weight and mass. Mass is property of matter. In a empty space you would be weightless, but not massless. Weight is measure of how match force is acting on a object in gravitational field. Only on Earth for 1 kg of mass is customary to say that it has weight of 1 kg. In order to avoid confusion it is best that weight is expressed in Newtons. And Doc Al meant that mass is w/g if you express weight in Newtons. Anyway what are you trying to calculate?
  8. Nov 26, 2009 #7
    All right. I got it. 98.1 kg is my mass. My weight is measured in the unit of kilogram weight. Like for example, 1 kilogram of mass equals 1 kilogram weight.

    But is the equation correct? 890,875,517,900,000,000 joules?
  9. Nov 26, 2009 #8

    Doc Al

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    Staff: Mentor

    mc² = 98.1*(3 x 108)² = 8.8 x 1018 J.
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