Problem over Newton's Laws and Friction

1. Sep 23, 2008

rphung

1. The problem statement, all variables and given/known data

2. Relevant equations

f=ma
Force of frickion = u x normal force

3. The attempt at a solution

I think the answer is choice 5.
Force of friction is 3mgu. So if the applied force is twice that amount then it will equal to 6mgu.

6mgu-3mgu=ma
m's cancel so 3gu=a

can someone please confirm?

The question i dont understand is the second part

i have no idea what to do

2. Sep 23, 2008

LowlyPion

Your first part looks right.

For the second, isolate m3. What are the forces acting on it? What is its contribution to frictional force. How much is it accelerating?

3. Sep 23, 2008

rphung

force of contact of 2 on 3, gravity, normal, and frictional force.
frictional force=umg
acceleration= 3ug from above.
so choice 5?

~edit~
i submitted my answer for the first part and it was wrong

yeah i was wrong i forgot to add up the masses on the ma part. I got it

Last edited: Sep 23, 2008