# Spherical Explosion (Easy?)

1. Sep 28, 2008

### Khlaar

Ok, some of you guys will probably find this easy because I've only just started Physics to a very slightly higher level, but whatever. Basically the question is as follows (I'm just remembering this off the top of my head, won't use the proper figures, it's doing it that matters):

A supernova explodes, sending out 500 neutrinos in a spherical explosion. There is a planet 1000km away of radius 50km. How many neutrinos pass through the planet?

IIRC I did this by doing 4πr² on the 1000km, giving the "surface area" of the neutrino explosion. Then doing πr² on the planet's 50km you get it's area. Then divide the area of the planet by the surface area of the explosion, and multiplying that decimal by the number of neutrinos. Was this correct? (I suspect not..)

2. Sep 28, 2008

### LowlyPion

Welcome to PF.

You are only interested in the cross sectional area of the planet as a percentage of the surface area of the sphere measured from the emission.

3. Sep 28, 2008

### Khlaar

Ok, I'm pretty sure I did that - did I?

4. Sep 28, 2008

### LowlyPion

Only your hair dresser knows for sure that you meant cross-section and not surface area of the planet.

If you used πr² for that, then you should be ok.

5. Sep 29, 2008

### Khlaar

No... "Then doing πr² on the planet's 50km you get it's area." surface area would require a completely different formula

6. Sep 29, 2008

### LowlyPion

Neutrinos pass through like it's a shadow disk of area on the surface of the event sphere.

7. Sep 29, 2008

### Khlaar

There's been some miscommunication somewhere here

Then I asked whether I did do it, to which you replied something about a hairdryer..

Then I asked, for a third time, whether I did it, and now you're talking about neutrinos.

I appreciate your offer, but you're not helping at all.

8. Sep 29, 2008

### LowlyPion

Fair enough. I'll just bow out then.

9. Oct 1, 2008

### Khlaar

There's no need to bow out, just you've made 4 posts so far and you've not helped whatsoever.

Can anyone help? Did I do it correct?