Brakes, static equilibrium problem is my Free Body Diagram correct?

[Femme_physics]

Homework Statement

Calculate minimal force P of the brakes during the stopping of the wheel.

[PLAIN]http://img823.imageshack.us/img823/6017/problemzy.jpg


Solution is P=34.3 [N]

Homework Equations

(Written on scan above)

The Attempt at a Solution

[PLAIN]http://img7.imageshack.us/img7/5720/problemtry.jpg

My great confusion is what is the purpose of the radius and what is the purpose of distance a? Are my equations and diagram even correct? I'm not getting the written result...

 

[tiny-tim]

Hi Dory!

...is my Free Body Diagram correct?

hmm … this is a very good example why it is very important to draw a good free body diagram …

in this case you must draw two diagrams … one for the lever, and one for the wheel

the relevance of the distance "a" is that when you take moments about the usual point for the lever, the friction force (unusally) will not have zero moment

start again ​

 

[Femme_physics]

Ah...got it... So I completely forgot the force Fb in my FBD! That's why I need that 0.05! Thanks :D

 

[Femme_physics]

There it is...just for the record

 

[tiny-tim]

There it is...just for the record

Mazeltov!