# E=mc^2 units

1. Mar 17, 2012

### Allojubrious

When solving with E=mc^2 what units should I use in the answer??
For example: (if I have an object with a mass of 43kg)
E=mc^2
E=(43kg)(299792458m/s)^2
E=3.86464727 × 10^18
Now here's my problem, do I include units of J/kg (joules per kilogram) or eV (electron volts)??
If someone could help me it would be very helpful.

Thanks,
Al

2. Mar 17, 2012

### Staff: Mentor

What is the SI unit of energy?

3. Mar 17, 2012

### alexg

Joules are kg*m2/s2. Since the units you chose for mass and c are kg, m and s, Joules are what you get.

4. Mar 17, 2012

### Pengwuino

Electron volts would require the mass to be given in ${{eV}\over{c^2}}$, which is common in high energy physics or when you're talking about atomic scale stuff. For example, the rest mass of a proton is $938 \times 10^8 {{eV}\over{c^2}}$. By itself, eV is not an SI unit.

5. Mar 18, 2012

### Allojubrious

Oh ok so the unit would be J/kg, no?
Or would it just be J, because I read that it might be J/kg, so which one??

6. Mar 18, 2012

### Staff: Mentor

1. What do you get with you multiply/divide the units together as indicated by your calculation? (Don't do any conversions!)

2. What is a joule (J) in terms of kg, m, and s?

7. Mar 18, 2012

### technician

The SI unit of energy is Joule.
eV is a convenient unit of energy in some areas of physics. The relationship between eV and Joule is
1eV = 1.6 x 10^-19 J

8. Mar 18, 2012

### Allojubrious

Oh alright so the unit would be Joule.

Thanks all,
Al

Last edited: Mar 18, 2012
9. Mar 19, 2012

### Khashishi

Either is correct, since joules and electron volts are both units of energy. If you are working with a macroscopic sized system, you probably want to use joules. If you are working with a microscopic system (i.e. single particles) then electron volts are probably more convenient.

10. Mar 19, 2012

### Allojubrious

Ah ok now that explanation makes a lot of sense thanks.

Al