How to Calculate Relativistic Momentum Using M, m, and c?

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SUMMARY

The discussion focuses on calculating the relativistic momentum of a particle with mass m resulting from the disintegration of a particle with mass M at rest. Participants emphasize the importance of using conservation laws, specifically conservation of mass-energy and momentum, to derive the momentum of the mass m particle. Key equations include the conservation of mass-energy equation, \( Mc^2 = \frac{mc^2}{\sqrt{1 - \frac{v^2}{c^2}}} + pc \), and the momentum conservation equation, \( 0 = \frac{mv}{\sqrt{1 - \frac{v^2}{c^2}}} + p \). The discussion concludes that the momentum can indeed be calculated using these equations by eliminating the velocity variable v.

PREREQUISITES
  • Understanding of special relativity concepts, particularly relativistic momentum.
  • Familiarity with conservation laws in physics, specifically conservation of mass-energy and momentum.
  • Knowledge of the relationship between energy, momentum, and mass, expressed in the equation \( E^2 = p^2c^2 + m^2c^4 \).
  • Ability to manipulate algebraic equations involving square roots and fractions.
NEXT STEPS
  • Study the derivation and implications of the equation \( E^2 = p^2c^2 + m^2c^4 \).
  • Learn about conservation of 4-momentum in special relativity.
  • Explore examples of relativistic momentum calculations in particle physics.
  • Investigate the role of massless particles in relativistic equations.
USEFUL FOR

Students of physics, particularly those studying special relativity, as well as educators and anyone interested in understanding the principles of relativistic momentum and energy conservation in particle interactions.

Saptarshi Sarkar
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Homework Statement
A particle X of mass M at rest disintegrates into a particle of mass m and another massless particle. Calculate the momentum of m.
Relevant Equations
What I figured:

From conservation of mass-energy
##Mc^2 = \frac {mc^2} {\sqrt {1 - \frac {v^2} {c^2}}}+ pc##

From conservation of momentum
##0 = \frac {mv} {\sqrt {1 - \frac {v^2} {c^2}}} + p##
The answer is required to be in terms of M,m and c only. But, I am not able to calculate the momentum of the m mass particle using the above two. Can anyone help me by telling me what I am missing?
 
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Saptarshi Sarkar said:
Homework Statement:: A particle X of mass M at rest disintegrates into a particle of mass m and another massless particle. Calculate the momentum of m.
Homework Equations:: What I figured:

From conservation of mass-energy
##Mc^2 = \frac {mc^2} {\sqrt {1 - \frac {v^2} {c^2}}}+ pc##

From conservation of momentum
##0 = \frac {mv} {\sqrt {1 - \frac {v^2} {c^2}}} + p##

The answer is required to be in terms of M,m and c only. But, I am not able to calculate the momentum of the m mass particle using the above two. Can anyone help me by telling me what I am missing?

Try working with the quantities ##E## and ##p## for all particles involved.
 
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Saptarshi Sarkar said:
The answer is required to be in terms of M,m and c only. But, I am not able to calculate the momentum of the m mass particle using the above two. Can anyone help me by telling me what I am missing?

You are missing that you *can* calculate the momentum of m using those equations. Two equations, two unknowns.
 
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DEvens said:
You are missing that you *can* calculate the momentum of m using those equations. Two equations, two unknowns.

From the second equation I found the momentum to be equal to -p, but when I try to get the value to -p using the equation for conservation of mass-energy, I am not able to eliminate the v.
 
Saptarshi Sarkar said:
From the second equation I found the momentum to be equal to -p, but when I try to get the value to -p using the equation for conservation of mass-energy, I am not able to eliminate the v.
Then don't have the ##v## in your equations in the first place. ##v## and ##\gamma## just get in the way.
 
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Saptarshi Sarkar said:
Can anyone help me by telling me what I am missing?

Probably the most important equation in SR is $$E^2 = p^2c^2 + m^2c^4$$
Which holds for both massive and massless particles.
 
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For a more systematic approach,try working with conservation of 4 momentum and using the fact that(c=1):

$$P.P=m^2 $$
$$P= P_1 + P_2$$
 
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