# Homework Help: Cosmology help

1. Feb 6, 2008

### Sag. A*

1. The problem statement, all variables and given/known data
Suppose that in sherwood forest, the average radius of a tree is equal to R= 1m and the average number of trees per unit area sigma = 0.005 m^-2 . If Robin Hood shoots an arrow in a random direction , how far , on average, will it travel before it strikes a tree?

2. Relevant equations
Since in the problem robin is shooting in a random direction, im assuming that the area in question is circular. and the area of a circle is A = pi X r^2.

3. The attempt at a solution
Im not sure how to tie in the area with the distance is question. Ive tried integrating over a circle ( sin, cos) but I dont know if I should solve for the radius some how. Or if i should find a max distance and a min distance somehow and average them. Also the radius of the trees thing is throwing me off. ......... Any insight and help would be great
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution