- #1

- 217

- 1

- Thread starter johann1301
- Start date

- #1

- 217

- 1

- #2

UltrafastPED

Science Advisor

Gold Member

- 1,912

- 216

Momentum is a vector quantity!

- #3

- 217

- 1

- #4

jtbell

Mentor

- 15,640

- 3,688

In general, your situation has no solution because it is over-specified: too much initial information, with only two unknown quantities.

[added later: see my next post below]

All of the momentum vectors lie in a plane, therefore we need to use only two components of momentum, e.g. x and y. We have three equations for conservation of x-momentum, y-momentum, and energy. In order to guarantee a solution, we need three unknown quantities.

[added later: see my next post below]

All of the momentum vectors lie in a plane, therefore we need to use only two components of momentum, e.g. x and y. We have three equations for conservation of x-momentum, y-momentum, and energy. In order to guarantee a solution, we need three unknown quantities.

Last edited:

- #5

bobie

Gold Member

- 720

- 2

If we know Es then we know the direction of ps and from this we can get the direction of e,

isn't it so?

- #6

jtbell

Mentor

- 15,640

- 3,688

He already knows (is given) the momentum of the scattered photon, therefore he already knows its energy.If we know the momentum of e we know the Ke of e, and then we you know, by difference, the energy Es of the scattered photon ps.

I just now noticed that he didn't say that he knows the momentum (or energy) of the incoming photon. If indeed he doesn't know that, then that is his third unknown quantity, and he can solve the momentum and energy conservation equations to find:

- the momentum (and energy) of the incoming photon
- the direction (angle) of the outgoing photon
- the direction (angle) of the outgoing electron

- #7

bobie

Gold Member

- 720

- 2

and after,and the electrons momentum, is it possible to know the direction of the electron and the photon after the collision?

It's complementary, jitbell, I missed the second bit and you the first.I just now noticed that he didn't say that he knows the momentum (or energy) of the incoming photon

The information he has is more than enough

- #8

jtbell

Mentor

- 15,640

- 3,688

If you give three "random" values for the momenta, in general they won't work because energy won't be conserved.