# E=mc^2 units

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

Dale
Mentor
2021 Award
What is the SI unit of energy?

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

Pengwuino
Gold Member
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.

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??

jtbell
Mentor
E=(43kg)(299792458m/s)^2
E=3.86464727 × 10^18

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?

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

Oh alright so the unit would be Joule.

Thanks all,
Al

Last edited:
Khashishi