# Need help with a question about momentum

• stayfocused
In summary, the momentum of a proton can be equated to the energy of a photon by using the equation P=E/C, but it is important to use consistent units, such as SI units for both energy and speed. Once the units are consistent, the velocity of the proton can be solved for using the mass of the proton.

## Homework Statement

How fast would a proton have to travel to have the same momentum as a 6.0-MeV photon

P= E / C

## The Attempt at a Solution

P= E / C
= 6.0-MeV / 3.00x108m/s

stayfocused said:

## Homework Statement

How fast would a proton have to travel to have the same momentum as a 6.0-MeV photon

P= E / C

## The Attempt at a Solution

P= E / C
= 6.0-MeV / 3.00x108m/s
Welcome to Physics Forums.

You need to be careful with your units here, your mixing natural units and S.I. units. You need to pick a system and stick to it.

stayfocused said:

## Homework Statement

How fast would a proton have to travel to have the same momentum as a 6.0-MeV photon

P= E / C

## The Attempt at a Solution

P= E / C
= 6.0-MeV / 3.00x108m/s

What's the problem here? Have you even put in any thought to it?

Yes P = E/c but if you're going to use that you need to make sure your E and c are in SI - that is - m/s and J

Once you find that you can equate that with the momentum of a proton - mv. You can look up the mass of the proton, and solve for v

That was my problem, I hadn't realized that i had the different units. Thank you!