Relativity - Mass/Energy/Speed

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Homework Help Overview

The discussion revolves around a problem in relativity involving a spaceship's rest mass, kinetic energy, and total energy calculations. The original poster presents a scenario where the kinetic energy is a fraction of the rest energy, prompting calculations for total energy and speed.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the relationship between rest mass, kinetic energy, and total energy. There are attempts to clarify the correct usage of terms and equations, particularly regarding rest mass versus rest energy. Some participants suggest corrections to the calculations presented by the original poster.

Discussion Status

The discussion is active, with participants providing corrections and alternative approaches to the calculations. There is a collaborative effort to refine the understanding of the equations involved, though no consensus on the final calculations has been reached.

Contextual Notes

Participants are navigating through the definitions and relationships in relativistic physics, questioning assumptions about energy forms and their calculations. The original poster's calculations are under scrutiny, and there are indications of differing interpretations of the problem setup.

ZedCar
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Homework Statement



A spaceship of rest mass 7.5 x 10^3 Kg has a kinetic energy that is 0.12 times it's rest energy.

a) Calculate it's total energy
b) Calculate it's speed


Homework Equations





The Attempt at a Solution


a)
E = E0 + K
E = mc^2 + K
E = 7.5 x 10^3 x (3 x10^8)^2 + 0.12(7.5 x 10^3)
E = 6.75 x 10^20 J


b)
E = γmc^2
substituting above values in and solving for v, I get v = 0
 
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ZedCar said:
a)
E = E0 + K
E = mc^2 + K
E = 7.5 x 10^3 x (3 x10^8)^2 + 0.12(7.5 x 10^3)

Correct the red part. It is the rest mass now, not the rest energy.


ehild
 
ehild said:
Correct the red part. It is the rest mass now, not the rest energy.


ehild

So should it be...

E = 7.5 x 10^3 x (3 x10^8)^2 + 0.12(7.5 x 10^3 x (3 x10^8)^2)
E = 7.56 x 10^20 J
 
Yes.

ehild
 
Thanks very much ehild.:smile:
 
You can save calculations if you not multiply with c^2 at the beginning:

E= γmc^2 = mc^2+0.12mc^2

γmc^2=mc^2(1.12)

y=1.12ehild
 

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