# Relativistic particle problem.

An unstable particle at rest breaks up into two fragments of unequal mass. The rest mass of the lighter particle is 2.5 x 10-28 kg, and that of the heavier fragment is 1.67 x 10-27 kg. If the lighter fragment has a speed of 0.983c after the breakup, what is the speed of the heavier fragment?

What is the idea here? Where does the energy come from, from an external source, or from the mass in the particles? (note: lower case m corresponds to the mass of the light particle and upper case M corresponds to the mass of the heavy particle)

v1: speed of the lighter particle
v2: speed of the heavier particle

We know that the total relativistic energy is:

E = KE + moc2

KE = 1/2mv12, m is the relativistic mass

So if relativistic energy of the light particle is conserved we get this equation.

mc2 = 1/2mv2 + moc2

We can also conserve the relativistic mass of both particles:

(m + M)c2 = 1/2mv12 + 1/2Mv22 + (mo + Mo)c2

Is this the correct way to setup the problem?

I tried solving for v2 without any luck. So I hope there is an easier way--the correct way.

Thanks

Last edited:

Related Introductory Physics Homework Help News on Phys.org
Tom Mattson
Staff Emeritus
Gold Member
Originally posted by frankR
What is the idea here?
The idea is to apply conservation of energy and momentum to a relativistic problem.

Where does the energy come from, from an external source, or from the mass in the particles?
It comes from the mass of the inital body. When you are done solving the problem, you can verify that the combined masses of the fragments is less than the mass of the whole. Rest mass energy was converted to kinetic energy.

(note: lower case m corresponds to the mass of the light particle and upper case M corresponds to the mass of the heavy particle)
That's a bad choice, in my opinion. I would use capital M for the mass of the body before disintegration, and mL, mH for the masses of the light and heavy fragments, respectively. Descriptive variable names can help you track which quantity is which throughout the problem.

v1: speed of the lighter particle
v2: speed of the heavier particle

We know that the total relativistic energy is:

E = KE + moc2
OK

KE = 1/2mv12, m is the relativistic mass
No, that's nonrelativistic KE. Look up relativistic KE in your book. It is:

KE=(&gamma;-1)mc2

and if you add that to the rest mass energy to get the total energy of a free particle, you get:

E=mc2+KE=mc2+(&gamma;-1)mc2
E=&gamma;mc2

This should all be in your book.

So if relativistic energy of the light particle is conserved we get this equation.

mc2 = 1/2mv2 + moc2
No, even if you were right about KE you wouldn't get that. The energy of the light particle by itself is not conserved.

We can also conserve the relativistic mass of both particles:

(m + M)c2 = 1/2mv12 + 1/2Mv22 + (mo + Mo)c2
No, on the left you have the total mass equal to (m+M), which is not true.

I am going to use the notation I suggested.

The total energy prior to disintegration is:

Ei=Mc2

The total energy after disintegration is:

Ef=&gamma;LmLc2+&gamma;HmHc2

Let Ei=Ef, and you have the equation for conservation of energy. Since you have two unknowns (M and vH), you need another equation. Luckily, conservation of momentum saves the day.

Total momentum before disintegration:

pi=0

Total momentum after disintegration:

pf=&gamma;LmLvL+&gamma;HmHvH

Let pi=pf, and you have the equation for conservation of momentum.

edit: fixed variable subscripts

Last edited:
I got vH = .3265*c

However the correct answer should be .285*c

I founs these equations:

From: pL = pH I got:

VH = (YL/YH)*(mLo/mHo)*VL

From Ei = Ef:

I found:

YH = (mLo + mHo - YL*mLo)/mHo

I then substited YH in VH = (YL/YH)*(mLo/mHo)*VL and calculated my answer.

What went wrong?

Thanks.

Tom Mattson
Staff Emeritus
Gold Member
Originally posted by frankR
From Ei = Ef:

I found:

YH = (mLo + mHo - YL*mLo)/mHo
How did you eliminate the mass M of the initial particle?

Originally posted by Tom
How did you eliminate the mass M of the initial particle?
Uhhhhh... By making it go to zero.

Nevertheless you can solve it strickly with ph=pl.

Tom Mattson
Staff Emeritus