# Help: Physics Homework

#### Astronomer186

A radioactive nucelus (cobalt - 60, has 27 p+'s and 33 e-'s) emits a photon (gamma ray) of wavelength 2.7 x 10^-13 m. Assuming the nucleus was intially at rest, what si its recoil velocity upon emitting this photon?

I have already figured out the momentum using this information. When I looked at the solution guide of how to find the mass, I am not sure why they use:

m = 60(1.67 x 10^-27 kg)?

to figure out the mass? I mean, I can understand the number 60, because that's the atomic mass, but why multiply it by the mass of a proton? Cobalt has two charges: 2+ and 3+. The most common one is 2+, so why multiply it by the mass of a proton, rather than an alpha particle (which has a 2+ charge)?

If someone can explain this, that would be greatly appreciated, because I am so confused. Thank you.

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#### Tide

Homework Helper
You're talking about the mass of a cobalt nucleus and not the charge. Cobalt 60 has 60 nucleons and there is little difference between the masses of protons and neutrons.

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