Coefficient of friction on wood problem

[airken]

Homework Statement

the coefficient of starting friction for wood on wood is 0.55. What is the force of friction of a wood block of mass 3.5 kg being pulled on a wood floor?

Homework Equations

i think i use the formula F=mMA where F is the force needed to move a solid from rest, m=coefficient of friction, M=mass of object and A=acceleration.

The Attempt at a Solution

so... I'm confused as to what to use for acceleration. I think it's gravity, but not sure. F=.55(3.5)(9.8)=18.865N
is this correct?

my confusion gets worse as the next question is a boulder of mass 45 kg is pushed on a surface with a coefficient of sliding friction of 0.85. What force has to be applied to produce an acceleration of 0.20 m/s2?

i'm confused because it seems that the acceleration is in different directions in each question. am i using incorrect equations? incorrect accelerations? or both?

thanks in advance for the help

 

[NobodySpecial]

The friction is how much the weight causes the block to stick.
So if you have a Cf of 0.55 then the force 'sticking' the wood down is 0.55 * the force downward (=weight)
So you work out the weight downward then the force needed to pull it along (or actually the force resiting your pulling it along) is 0.55 * this weigt