# Alpha particle momentum

## Homework Statement

An alpha particle (a helium nucleus, containing 2 protons and 2 neutrons) starts out with kinetic energy of 10 MeV (10*10^6 eV), and heads in the +x direction straight toward a gold nucleus (containing 79 protons and 118 neutrons). The particles are initially far apart, and the gold nucleus is initially at rest. Answer the following questions about the collision.
What is the initial momentum of the alpha particle? (You may assume its speed is small compared to the speed of light).

## Homework Equations

1 ev=1.69e-19 J= 1.69e-19 kg*m2/s2
K=.5*m*v2
p=m*v
mproton=mneutron=1.7e-27 kg

## The Attempt at a Solution

I solved for velocity of the particle then multiplied it by its mass to find momentum.
10*1.69e-19=.5*m*v2
3.38e-18=8.8e-27*v2
497058823.5=v2
22294.81607=v

P=mv
P=8.8e-27*22294.81607
P=1.96194e-22

## Answers and Replies

um, yeh. It seems correct.

i thought so too... maybe the online program has the wrong answer.

lightgrav
Homework Helper
10 Mega-eV ...

10 Mega-eV ...

what about it?

lightgrav
Homework Helper
the solution you posted started with 10 eV , not 10 MeV ... Mega = 1 000 000 .

the solution you posted started with 10 eV , not 10 MeV ... Mega = 1 000 000 .

I would have never caught that. Thank you. The process seems correct to you?

lightgrav
Homework Helper
the PROCESS looks okay, but where did you get m = 8.8e-27 kg ?

by the way, p^2 /2m is USUALLY more useful than ½mv^2 ... especially in collisions !

the PROCESS looks okay, but where did you get m = 8.8e-27 kg ?

by the way, p^2 /2m is USUALLY more useful than ½mv^2 ... especially in collisions !

I used numbers in the back of our book that our homework is based off of

lightgrav
Homework Helper
proton mass is 1.673 e-27kg ... only keeping 2 digits leads to severe roundoff error.

But I was asking "what computation did you use" which yielded that as the answer.
... it is a "leading" question

proton mass is 1.673 e-27kg ... only keeping 2 digits leads to severe roundoff error.

But I was asking "what computation did you use" which yielded that as the answer.
... it is a "leading" question

This homework which is based off of our book only uses 2 significant digits for mass of proton and neutron.
total mass = 4*1.7e-27
= 6.8e-27

I apologize for the typo earlier as well.

Last edited: