Block against a wall (friction problem)

[3ephemeralwnd]

Homework Statement

A 1.0 kg wood block is pressed against a vertical wood wall by a 12 N force (into the wall, at a 30 degrees angle up from the horizontal). if the block is initially at rest, will it move upward, move downward, or stay at rest?

The attempt at a solution

ive written out the equations for Fnet in the x and y directions;

Fnet(x) = Fn - Fpush(x) , and since Fnet(x) is 0, then Fn = Fpush (x)
so, Fn = Fpush(x) = 12cos30

Fnet(y) = Fstatic friction + Fpush (y) - Fg
= (mu)Fn + 12sin30 - 9.8
and subbing in Fn... i got,
= (mu)12cos30 + 12sin30 - 9.8

but now what? I can't seem to complete the problem without being given the coefficient of static friction.. any suggestions?

 

[tiny-tim]

hi 3ephemeralwnd!

(have a mu: µ and a degree: ° )

(mu)12cos30 + 12sin30 - 9.8

that's right!

(though you haven't actually explained why the friction acts up and not down )

strange question … i guess you now find the value of µ for which the solution changes

 

[3ephemeralwnd]

oh.. that's true! I guess I just assumed that the block was going to move downwards (therefore static friction is pulling upwards on the block). Why is the friction not downwards?

 

[tiny-tim]

oh.. that's true! I guess I just assumed that the block was going to move downwards (therefore static friction is pulling upwards on the block).

And yes it is, because 9.8 is greater than 12sin30° …

but you should still say so! ​