# Finding KE when given de Broglie wavelength

[SOLVED] Finding KE when given de Broglie wavelength

## Homework Statement

What is the kinetic energy, in MeV, of a proton with a de Broglie wavelength of 10fm?
1 MeV = 10^6 eV
1 fm = 10^-15m
1 eV = 1.602 x 10^-19 J
h = 6.63 x 10^-34 Js
wavelength = 10 x 10^-15 m = 10^-5 nm
Mass proton = 1.673 x 10^-27 kg

## Homework Equations

wavelength = h/mv
KE = (1/2)(m)(v^2)

## The Attempt at a Solution

I feel like I did this right, but I still got the incorrect answer and I can't seem to notice what I did wrong.

v = (6.63 x 10^-34) / (10^-5)(1.673 x 10^-27) = .03962 m/s
K = .5(1.673 x 10^-27)(.03862^2) = 1.316 x 10^-30 J = 8.2 x 10^-12 eV
= 8.2 x 10^-18 MeV

Hootenanny
Staff Emeritus
Gold Member
Welcome to PF,
v = (6.63 x 10^-34) / (10^-5)(1.673 x 10^-27) = .03962 m/s
K = .5(1.673 x 10^-27)(.03862^2) = 1.316 x 10^-30 J = 8.2 x 10^-12 eV
= 8.2 x 10^-18 MeV
Shouldn't that be 10*10-15? Welcome to PF,

Thanks.

Shouldn't that be 10*10-15? I tried that the first time I did the problem, and realised 10 x 10^-15 is in meters. Wouldn't I need the wavelength to be in nm? So it would be 1 x 10^-5 nm.

Last edited:
Hootenanny
Staff Emeritus