# modern physics - relativity

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A photon of energy E collides with a stationary particle of rest mass m0 and is absorbed by it .

(A) what is the velocity of resulting composite particle ?

(B) What is the mass of the resulting composite particle ?

use conservasion of energy / momentum please show working

• modern physics - relativity -

A) You can find the velocity by cmputing the total energy and momentum. From:

E = gamma m c^2

and

P = gamma m v

it follows that

p/E = v/c^2

The total energy is:

E + m0 c^2

The total momentum is the momentum of the photon, which is E/c. The total energy and momentum are conserved, so these are the energy and momentum of the particle after the photon has been absorbed. The velocity of that particle is thus:

v = c^2 momentum/energy =

c^2 E/c 1/(E + m0 c^2 ) =

E c/(E + m0 c^2)

The mass follows from the invariant:

E^2 - p^2 c^2

Fora system with eenrgy E and momentum p, this quantity has the same valyue in all frames. If you evaluate this in the rest frame then p = 0 and E = m c^2, so it is E^2 = m^2 c^4. The fact tha the quatity is invariant, means that it has this value in any frame. This means that given the enrgy and momentum of a system, you can compute the mass:

m^2 = E^2/c^4 - p^2/c^2

In this case, you find:

m^2 = (E + m0 c^2)^2/c^4 - E^2/c^4 =

2 m0 E/c^2 + m0^2

So, the mass is:

m = m0 sqrt[1 + 2 E/(m0 c^2)]

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