Help with energy-mass equivalence

[univeruser]

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

E=mc²

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?)

So;

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.

Thanks.

 

[Doc Al]

Mass is w*g, and g is 9.81 for so long you're on Earth.

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.

 

[mikeph]

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.

 

[univeruser]

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=[(\frac{98.1}{9.81}) 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 VII^th standard.

 

[Doc Al]

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.)

 

[S.Vasojevic]

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?

 

[univeruser]

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?

 

[Doc Al]

mc² = 98.1*(3 x 10⁸)² = 8.8 x 10¹⁸ J.