Finding momentum of a photon/ Finding the mass of a object

[LionLieOn]

Homework Statement

So me and my friend were comparing our homework and we noticed that although we had the same answers, we both had different ways of answer them.

So I was just wondering who's work, my friend or mine, is the most correct?Here's the questions.

1. Calculate the momentum of a 140 eV photon.
2. A certain microscopic object has a wavelength of 8.4 X 10^-14 m and a speed of 1.2 X 10⁵ m/s. What is the mass of the object?

Homework Equations

The Attempt at a Solution

Answers for the first question.My work:

140 eV Convert into joules:
E=(1.60 X 10^-19)(140)
E= 2.24 X 10^-17 J

Finding momentum:

p=E/c

p=(2.24 X 10^-17) / ( 3.0 X 10⁸ )

p= 7.466666667 X 10^-26
Rounded,
p= 7.5 X 10^-26 kgm/s

-------------------------------------------------

My friends work:140 eV Convert into joules:
E=(1.60 X 10^-19)(140)
E= 2.24 X 10^-17 J

First find the wavelength:

E=hc/ (Lambda symbol)

(Lambda symbol)= hc/E

(Lambda symbol)= 8.9 X 10^-9 m

Now find momentum:

p=h/ (Lambda symbol)

p=(6.63 X 10^-34) / (8.9 X 10^-9 )

p= 7.5 X 10 _-26 kgm/s
----------------------------------------------------------------------------------
Second question.

My work:

(Lambda symbol) = h/mv

m=h/ (Lambda symbol)v

m= (6.63 X 10^-34)/ (8.4 X 10^-14) (1.2 X 10⁵)

m= 6.577380952 X 10^-26)
Rounded,
m= 6.6 X 10^-26) kg
---------------------------------------------------------
my friends work:

Finding momentum of the object.

p= h/ (Lambda Symbol)

p= (6.63 X 10^-34) / (8.4 X 10^-14)

p= 7.89 X 10^-21 kgm/s

Now find the mass of the object

p=mv

m=p/v

m= 7.89 X 10^-21 / (1.2 X 10⁵)

m= 6.577 X 10^-26

Rounded,
m= 6.6 X 10^-26 kg

 

[PeroK]

For 1) I would say the photon has a momentum of $140 eV/c$.

 

[LionLieOn]

For 1) I would say the photon has a momentum of $140 eV/c$.

Ohh did I get the unit wrong for the momentum of a photon?

 

[PeroK]

Ohh did I get the unit wrong for the momentum of a photon?

If you measure energy in $eV$ it would seem appropriate to measure momentum in $eV/c$.

 

[LionLieOn]

If you measure energy in $eV$ it would seem appropriate to measure momentum in $eV/c$.

Hmmm what if I converted 140eV into joules? Would I still use eV/c or put it as kgm/s ?

 

[TJGilb]

Hmmm what if I converted 140eV into joules? Would I still use eV/c or put it as kgm/s ?

Your units aren't wrong per se, it's just strange that you would choose to convert to Joules instead of staying in eV which would be much easier in this case.

 

[PeroK]

Hmmm what if I converted 140eV into joules? Would I still use eV/c or put it as kgm/s ?

If you want the momentum in SI units just convert energy to Joules and divide by $c$. No need for any rigmarole.

 

[TJGilb]

As to which approach is more correct, both are perfectly fine. You'll find that problems in physics tend to have a great number of viable approaches. It's just a matter of which makes the most sense to you.

 

[LionLieOn]

If you want the momentum in SI units just convert energy to Joules and divide by $c$. No need for any rigmarole.

As to which approach is more correct, both are perfectly fine. You'll find that problems in physics tend to have a great number of viable approaches. It's just a matter of which makes the most sense to you.

Thank you guys very much!