Maximum load on pine beam with safety factor of 5

[Neon32]

Homework Statement

A uniform pine beam, 3.6 m long and 9.5 cm x 14 cm in cross section, rests on two supports near its ends, as shown. The beam’s mass is 25 kg and two vertical roof supports rest on it, each one-third of the way from the ends. What maximum load force FL can each of the roof supports exert without shearing the pine beam at its supports? Use a safety factor of 5.0.

Homework Equations

The Attempt at a Solution

Answer sheet:
F= 1/5 x A x Maximum stress

I just don't understand the first step. Why did I multiply it by 1/5? And what does safety factor mean?

 

[David Lewis]

Safety factor is how many times stronger the member is than it needs to be.