# Homework Help: Relativity - Mass/Energy/Speed

1. Oct 15, 2011

### ZedCar

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. 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

2. Oct 15, 2011

### ehild

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

ehild

3. Oct 15, 2011

### ZedCar

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

4. Oct 15, 2011

### ehild

Yes.

ehild

5. Oct 15, 2011

### ZedCar

Thanks very much ehild.

6. Oct 15, 2011

### ehild

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=1.12

ehild