Solving system of equations to find angle and energy of Au atom and α

[amr55533]

Homework Statement

An alpha particle traveling with a kinetic energy of 5.5 MeV and a rest-mass of 3727.8 MeV/c^2 strikes a gold atom with a rest-mass of 183,476 MeV/c^2.

-The gold atom is initially at rest
-The alpha particle deflects perpendicular to the horizontal in the after state

Homework Equations

Conservation of Energy:

Ebefore=Eafter


Conservation of momentum:

Pbefore=Pafter


E^2=P^2c^2+(mc^2)^2

The Attempt at a Solution

Total Energy Before Collison:

Ebefore=E(α)+E(Au)=3727.8MeV+5.5MeV+183476MeV=187,209MeV


Momentum before (in x-direction):

E^2=p^2c^2+(mc^2)^2

==> (3727.8+5.5)^2=P^2c^2+(3727.8)^2

==> P=202.6 MeV/c


AFTER STATE:

Conservation of momentum:

in x: P(Au)cosθ=202.6MeV/c (1)

in y: P(Au)sinθ-P(α)=0 (2)


Conservation of Energy:

E(Au)=√(P(Au)^2+183476^2)
E(α)=√(P(α)^2+3727.8^2)

==> √(P(Au)^2+183476^2)+√(P(α)^2+3727.8^2)=187,209 MeV (3)



Solve the system of equations to find θ, P(Au), and P(α):

Solve (3) for P(α):

P(α)=√(1.36998E9-P(Au)^2)


Plug back into Eq. (2):

P(Au)sinθ=√(1.36998E9-P(Au)^2)


Solve (1) for θ:

θ=arccos(202.6/P(Au))


Plug into (2) and solve for Au:

P(Au)sin[arccos(202.6/P(Au))]=√(1.36998E9-P(Au)^2)

==> P(Au)=26,172.7 MeV/c


Solve for θ:

26,172.7cosθ=202.6

==> θ=89.56°


Solve for P(α):

26,172.7sin(89.56°)=P(α)

==> P(α)=26,171.9 MeV/c



However, when I plug these numbers back into equation (3) the solution doesn't come out right. I have been looking over this for hours and can't figure out where my error might be.

Thanks!

 

[Simon Bridge]

Possibly algebra problems?
Go through each step carefully - without going too deep, the following stood out:

E(Au)=√(P(Au)^2+183476^2)
E(α)=√(P(α)^2+3727.8^2)

Are you missing a c^2 in there? Typo?

Solve (3) for P(α):
P(α)=√(1.36998E9-P(Au)^2)

#3 was:

√(P(Au)^2+183476^2)+√(P(α)^2+3727.8^2)=187,20 9 MeV (3)

... is there a c^2 missing there too?
How did you get from this to your conclusion

P(Au)sin[arccos(202.6/P(Au))]=√(1.36998E9-P(Au)^2)

==> P(Au)=26,172.7 MeV/c

Have I read that right:
$p_{Au}\sin(\arccos(202.6/p_{Au}))=\sqrt{1.36998\times 10^9 - p_{Au}} \\ \Rightarrow p_{Au}=26172.7\text{MeV/c}$
... how did you manage to ull out the $p_{Au}$ ?

... I'm not saying these are the mistakes - only that this is what stands out.

 

[amr55533]

I looked back over each step carefully. I must have made my mistake when solving for Pα in Equation 3. I ended up writing a script on MATLAB that solved for the three unknowns and got the answers that way. Thanks for the tips!

 

[Simon Bridge]

That's where I think it first went pear shaped too ... the c^2 thing you can get away with by adopting units so that c=1. Many people do that implicitly.

A good discipline is to do all the algebra in the symbols, leaving the numbers to last.
Worst case is, you'll have a clear path to backtrack and troubleshoot - in this case, a lot was hidden when you combined numbers so I couldn't tell what you'd done.

Still... no worries aye ;)