63. The wavelength associated with the unknown particle is λ

p

= h/p

p

= h/(m

p

v

p

), where p

p

is its momen-

tum, m

p

is its mass, and v

p

is its speed. The classical relationship p

p

= m

p

v

p

was used. Similarly, the

wavelength associated with the electron is λ

e

= h/(m

e

v

e

), where m

e

is its mass and v

e

is its speed. The

ratio of the wavelengths is λ

p

/λ

e

= (m

e

v

e

)/(m

p

v

p

), so

m

p

=

v

e

λ

e

v

p

λ

p

m

e

=

9.109

× 10

−31

kg

3(1.813

× 10

−4

)

= 1.675

× 10

−27

kg .

According to Appendix B, this is the mass of a neutron.