# Conservation of relavistic energy

Problem statement, equations, and work done:

A particle called a Kaon is moving at 0.8c through a detector when it decays into two pions.

Kaon particle: mass = 493.7 MeV/c^2
Pion+: mass = 139.6 MeV/c^2
Pion0: mass = 135.0 MeV/c^2

1) Apply conservation of momentum/energy to determine the momentum of each pion in the kaon frame of reference (where K is at rest).Use the fact that the momentum of the original kaon in its own frame is zero and don't forget to use the relativistic momentum energy conservation equation. Leave the momentum in MeV/c units.

Since the momentum of the kaon is 0 because it is at rest in its own frame, the magnitudes of the momentum of the pions should be equal.

$E_k = m_k c^2 = 493.7 \frac{MeV}{c^2} = (m_{\pi +} + m_{\pi 0}) c^2 + (p_{\pi +} + p_{\pi 0})c$

$493.7 \frac{MeV}{c} = (139.6 + 135.0) \frac{MeV}{c} + 2|p|$

$219.1 \frac{MeV}{c} = 2|p|$

$p = 109.55 \frac{MeV}{c}$

Solving for velocities:

$p{\pi +} = 109.55 \frac{MeV}{c} = \gamma_{\pi +} m_{\pi +} v_{\pi +} = \gamma_{\pi +} 139.6 \frac{MeV}{c^2} v_{\pi +}$

From this:

$v_{\pi +} = .61733 (c), and thus : \gamma_{\pi +} = 1.27113$

And:

$v_{\pi 0} = .63012 (c), and thus: \gamma_{\pi 0} = 1.28783$

From this there is no Conservation of energy:

$E_k = E_{\pi +} + E_{\pi 0} = 493.7 \frac{MeV}{c^2} = \gamma_{\pi +} m_{\pi +} + \gamma_{\pi 0} m_{\pi 0} = doesn't = 493.7!$

(I left units off)

2)Now transform the momentum-energy vectors for each pion and the kaon to the detector frame. Assume that the pion+ is moving in the direction the Kaon was moving in the detector frame.

3) Check to see that the momentum-energy conservation law is followed in the detector frame.

4) If the pions are moving off perpendicular to the Kaon's motion in the detector frame, calculate the momentum energy vectors for the pions in the detector frame and again check to see that momentum-energy is conserved.

Related Introductory Physics Homework Help News on Phys.org
DEvens
Gold Member
$E_k = m_k c^2 = 493.7 \frac{MeV}{c^2} = (m_{\pi +} + m_{\pi 0}) c^2 + (p_{\pi +} + p_{\pi 0})c$
What's that? What is the relativistic formula for kinetic energy?

PeroK
Homework Helper
Gold Member
Problem statement, equations, and work done:

A particle called a Kaon is moving at 0.8c through a detector when it decays into two pions.

Kaon particle: mass = 493.7 MeV/c^2
Pion+: mass = 139.6 MeV/c^2
Pion0: mass = 135.0 MeV/c^2

1) Apply conservation of momentum/energy to determine the momentum of each pion in the kaon frame of reference (where K is at rest).Use the fact that the momentum of the original kaon in its own frame is zero and don't forget to use the relativistic momentum energy conservation equation. Leave the momentum in MeV/c units.

Since the momentum of the kaon is 0 because it is at rest in its own frame, the magnitudes of the momentum of the pions should be equal.
I would do this for a general decay of a particle of mass M into two particles of masses m_1 and m_2, then plug the numbers into the final equation. You're getting bogged down in numbers. At the level you're at, you should be working more algebraically and not plugging in awkward values at the first opportunity!

That is E = mc^2 + pc which is total energy = rest energy + KE energy which equals the combined energy totals of the lions. The first part is how my teacher should us, but I can't bring it around to the conservation of energy

DEvens
Gold Member
That is E = mc^2 + pc which is total energy = rest energy + KE energy which equals the combined energy totals of the lions. The first part is how my teacher should us, but I can't bring it around to the conservation of energy
That's because kinetic energy is not equal to pc for a particle with mass.

jtbell
Mentor
I would do this for a general decay of a particle of mass M into two particles of masses m_1 and m_2, then plug the numbers into the final equation. You're getting bogged down in numbers. At the level you're at, you should be working more algebraically and not plugging in awkward values at the first opportunity!
I second this recommendation.

I also recommend that you always keep m together with c2 as (mc2), and p together with c as (pc). Then the only unit you need to use is MeV. That is, for the kaon, don't think m = 493.7 MeV/c2, but rather (mc2) = 493.7 MeV.

Finally, you should never need to calculate any velocities in a problem like this, unless the problem statement explicitly asks for it. Use conservation of energy, conservation of momentum, and the general relationship between energy, mass and momentum: E2 = (pc)2 + (mc2)2.

That's because kinetic energy is not equal to pc for a particle with mass.
I wonder what my fantastic teacher was doing then.

I would do this for a general decay of a particle of mass M into two particles of masses m_1 and m_2, then plug the numbers into the final equation. You're getting bogged down in numbers. At the level you're at, you should be working more algebraically and not plugging in awkward values at the first opportunity!
I only was trying to calculate the velocities to show it does not fit into conservation of energy.

Can anybody point me in the right direction, I'm only trying to understand this all.

PeroK
Homework Helper
Gold Member
I wonder what my fantastic teacher was doing then.

I only was trying to calculate the velocities to show it does not fit into conservation of energy.

Can anybody point me in the right direction, I'm only trying to understand this all.
Your starting point is the three equations:

$E_1 + E_2 = Mc^2, \ \ E_1^2 = p^2c^2 + m_1^2c^4, \ \ E_2^2 = p^2c^2 + m_2^2c^4$

Can you explain these - at least to yourself?

There are now common ways to work with these sorts of equations: common algebraic tricks. In this case I recommend trying to get an equation for the energy of either pion. Once you have the energy, you can get the momentum from that.

DEvens
Gold Member
Can anybody point me in the right direction, I'm only trying to understand this all.
I just did.

Kinetic energy for a particle with mass is (gamma - 1) mc^2, total energy (mass plus kinetic) is gamma mc^2.
Momentum is gamma m v.

Alternatively, as suggested by jtbell, E^2 = (pc)^2 + (mc^2)^2
You have E = pc + mc^2, which is wrong.

Your very first equation is wrong. As I have now told you for the third time.

Last edited:
I just did.

Kinetic energy for a particle with mass is (gamma - 1) mc^2, total energy (mass plus kinetic) is gamma mc^2.
Momentum is gamma m v.

Alternatively, as suggested by jtbell, E^2 = (pc)^2 + (mc^2)^2
You have E = pc + mc^2, which is wrong.

Your very first equation is wrong. As I have now told you for the third time.
That equation is an approximation that our teacher said we could use in this situation.

Your starting point is the three equations:

$E_1 + E_2 = Mc^2, \ \ E_1^2 = p^2c^2 + m_1^2c^4, \ \ E_2^2 = p^2c^2 + m_2^2c^4$

Can you explain these - at least to yourself?

There are now common ways to work with these sorts of equations: common algebraic tricks. In this case I recommend trying to get an equation for the energy of either pion. Once you have the energy, you can get the momentum from that.
Yes I understand those, thank you. Isn't that 3 equations with 4 unknowns? Don't I need: p = p1 + p2 = 0?

PeroK
Homework Helper
Gold Member
Yes I understand those, thank you. Isn't that 3 equations with 4 unknowns? Don't I need: p = p1 + p2 = 0?
I've used p1 = p2 = p, as the momentum of the two pions must be equal.

DEvens
Gold Member
That equation is an approximation that our teacher said we could use in this situation.
Ok, I think I have discovered the fault.

I've used p1 = p2 = p, as the momentum of the two pions must be equal.
Oh I see that now. Thanks.

I just solved the system of equations using the approximation and got 109.55 for the momentum. I also got the conservation of energy with E1 = 249.15 and E2 = 244.55, so that works.

Now to convert E1 and E2 to the second frame, what equation do I use? If it involves gamma, then what gamma is it and how to calculate it.

Thanks guys.

PeroK
Homework Helper
Gold Member
I just solved the system of equations using the approximation and got 109.55 for the momentum. I also got the conservation of energy with E1 = 249.15 and E2 = 244.55, so that works.

Now to convert E1 and E2 to the second frame, what equation do I use? If it involves gamma, then what gamma is it and how to calculate it.

Thanks guys.
The mometum should be nearly double what you've calculated. The energy looks correct. If you have the energy and the mass of a particle, then you can calculate its momentum.

Well if E1 = m1 + p then p = E1 - m1 = 249.15 - 139.6 = 109.55.

Leaving the c and units out.

PeroK
Homework Helper
Gold Member
Well if E1 = m1 + p then p = E1 - m1 = 249.15 - 139.6 = 109.55.

Leaving the c and units out.
Energy isn't the sum of mass and momentum. It's $E^2 = p^2 + m^2$

Energy isn't the sum of mass and momentum. It's $E^2 = p^2 + m^2$
I see now. So was it OK to use that approximation to solve the system of equations? Even though I got a different p in that system. Would that throw the values of E off too?

PeroK
Homework Helper
Gold Member
I see now. So was it OK to use that approximation to solve the system of equations? Even though I got a different p in that system. Would that throw the values of E off too?
I haven't seen how you solved the equations. You seem to have got the right energies.

If you look at that approximation mathematically, it can only be valid where one of the terms is very small: where E is approx p or E is approx m. That's not the situation here.

Basically it was your 3 equations without the squares. <shrugs> when I get the time I'll do the system with the squares and see how it comes out.

Can you point me in the right direction to get the energy and momentum in the second frame?

jtbell
Mentor
2)Now transform the momentum-energy vectors for each pion and the kaon to the detector frame
What have you studied on the Lorentz transformation? The LT for momentum and energy is similar to the LT for position and time, because momentum and energy form a four-vector, like position and time form a four-vector.

Energy isn't the sum of mass and momentum. It's $E^2 = p^2 + m^2$
One thing with this equation is that the magnitude of the pion's momentum is not the same:

$p1 = \sqrt{E1^2 -m1^2} = \sqrt{249.15^2 - 139.6^2} = 206.37$
$p2 = \sqrt{E2^2-m2^2} = \sqrt{244.55^2 - 135.0^2} = 203.91$

Last edited:
Basically it was your 3 equations without the squares. <shrugs> when I get the time I'll do the system with the squares and see how it comes out.
I worked out the 3 equations using the squares and it works out to almost the same:

E1 = 245.57
E2 = 248.13

jtbell
Mentor
Your E1 and E2 are correct for the $\pi^0$ and $\pi^+$ respectively; but the 1 and 2 don't match up with the masses you used in post #22. Hopefully that's just a typo.

Yes I corrected that, but they still do not equal in magnitude?!