Find Min Force to Keep 50kg Block in Place

[rainingurl]

Homework Statement

a 50kg block of wood is pressed against a smooth wall by a force perpendicular to the wall. if the coefficient of static function is .13, what is the minimum force that is necessary to keep the block of wood in place.

Homework Equations

Fg= mg
Fn = ma
Ff= µ Fn = µma
Fa = ma

The Attempt at a Solution

Ff+Fn-fa = 0 since we want the block to be still.
µma +ma = fa
.13*50kg*a + 50kg*a = fa
[(.13*50)+50]a=ma
56.5a= 50a
this is wrong.

 

[jIyajbe]

Actually, what you want is for the frictional force (which points upward, in this case) to be equal in magnitude to the weight of the block of wood (which points downward). So, your equation Ff+Fn-fa = 0 is not correct; it should be just Ff - Fg = 0. You then need to use the relationship between the normal force and the frictional force.

HTH,

jIyajbe