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# Physics E=mc^2 watch

1. The present mass of the Sun is 2.0 × 10^30 kg. The Sun emits radiation at an average rate of 3.8 × 10^26 J s–1. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass.

From markscheme: "E = mc2
energy = 2.0*10^30 *10^-6 *(3.0108 )2"

why 10^-6? How is that a change in mass? because the equation the markscheme uses is change in energy = change in mass * speed ^2
2. (Original post by MrToodles4)
The present mass of the Sun is 2.0 × 10^30 kg. The Sun emits radiation at an average rate of 3.8 × 10^26 J s–1. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass.

From markscheme: "E = mc2
energy = 2.0*10^30 *10^-6 *(3.0108 )2"

why 10^-6?

EDIT: wait nvm, its for a millionth of the mass isn't it..
You have to find the time and not the energy.
3. (Original post by MrToodles4)
The present mass of the Sun is 2.0 × 10^30 kg. The Sun emits radiation at an average rate of 3.8 × 10^26 J s–1. Calculate the time in years for the mass of the Sun to decrease by one millionth of its present mass.

From markscheme: "E = mc2
energy = 2.0*10^30 *10^-6 *(3.0108 )2"

why 10^-6? How is that a change in mass? because the equation the markscheme uses is change in energy = change in mass * speed ^2
2.0*10^30 *10^ is one million of the mass.
4. (Original post by WiSi)
You have to find the time and not the energy.
Yeah but I thought delta m is the change in mass. I can find the time afterwards by doing p=e/t so e/p = t
5. (Original post by WiSi)
2.0*10^30 *10^ is one million of the mass.
Is that considered 'change in mass then'? i thought it would be the mass to decrease to get to one millionth of the mass
6. (Original post by MrToodles4)
Yeah but I thought delta m is the change in mass. I can find the time afterwards by doing p=e/t so e/p = t
7. (Original post by WiSi)
Basically normally change in mass would be final mass - initial mass right? So in this case it it right to assume that the DECREASE of one millionth IS the change in mass?
8. (Original post by MrToodles4)
Basically normally change in mass would be final mass - initial mass right? So in this case it it right to assume that the DECREASE of one millionth IS the change in mass?
It should be its actual mass minus one million of his mass.
9. (Original post by WiSi)
It should be its actual mass minus one million of his mass.
The markscheme does not do that though...

3b: http://pmt.physicsandmathstutor.com/...y%204%20MS.pdf

Question 3B: http://pmt.physicsandmathstutor.com/...y%204%20QP.pdf
10. (Original post by MrToodles4)
The markscheme does not do that though...
The question states that the mass decreases BY one millionth of its mass.

so delta m = 1x10^-6 times the mass

It does not decrease TO one millionth of the mass
11. I understand the solution.
So, the Sun have a mass of 2.0 × 10^30 kg.
Your problem ask time sun needs to decrease by one millionth of its present mass.
So you are going to calculate the Energy the Sun lose, while losing one millionth of its present mass.
For this reason you have to use the value of the mass the sun lose, and not the mass that remains in the Sun.

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