## Doubts

infinityzero
Posts: 4
Joined: Sun Jun 27, 2010 8:15 am

### Doubts

What could be the possible answer ?
Acco to me it should be option (B). But i am not sure about it....

CarlBrannen
Posts: 381
Joined: Mon May 24, 2010 11:34 pm

### Re: Doubts

Here's another vote for (b).

If it were an infinite mass atom then it would be (a). With a finite mass, some of the energy of the photon will become kinetic energy of the energized atom. This means you need a little extra energy hence (b).

kroner
Posts: 218
Joined: Fri Sep 25, 2009 1:58 am

### Re: Doubts

To get the answer exactly, you can use the relation $E^2 = (pc)^2 + (m_0c^2)^2,$ and the fact that the momentum of the atom after the collision is the same as the momentum of the photon, to get $(Mc^2 + fh)^2 = (fh)^2 + ((M + \Delta)c^2)^2,$ and then solve for f.

WhoaNonstop
Posts: 853
Joined: Mon Sep 21, 2009 1:31 am

### Re: Doubts

kroner wrote:To get the answer exactly, you can use the relation $E^2 = (pc)^2 + (m_0c^2)^2,$ and the fact that the momentum of the atom after the collision is the same as the momentum of the photon, to get $(Mc^2 + fh)^2 = (fh)^2 + ((M + \Delta)c^2)^2,$ and then solve for f.

Solving for f looks too haaaaaaaaard.

-Riley

HappyQuark
Posts: 762
Joined: Thu Apr 16, 2009 2:08 am

### Re: Doubts

WhoaNonstop wrote:
kroner wrote:To get the answer exactly, you can use the relation $E^2 = (pc)^2 + (m_0c^2)^2,$ and the fact that the momentum of the atom after the collision is the same as the momentum of the photon, to get $(Mc^2 + fh)^2 = (fh)^2 + ((M + \Delta)c^2)^2,$ and then solve for f.

Solving for f looks too haaaaaaaaard.

-Riley

That's why Mathematica has the solve function. Just the other day I came across some really difficult algebra and had to have Mathematica break it down for me. How in the hell am I supposed to know how to solve x+2=4?

infinityzero
Posts: 4
Joined: Sun Jun 27, 2010 8:15 am

### Re: Doubts

grae313
Posts: 2297
Joined: Tue May 29, 2007 8:46 pm

### Re: Doubts

Mind sharing where these problems are from?

infinityzero
Posts: 4
Joined: Sun Jun 27, 2010 8:15 am

### Re: Doubts

Well these problems are from last year's entrance exam for PhD(physics) program at TIFR,india...