# Homework Help: Help with force problem

1. Nov 28, 2013

### anon1

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

A mass "b" is placed on top of a larger mass that is 2.4 times the mass of the smaller one as shown. The coefficient of friction between the larger block and the surface is 0.35. A horizontal force acts on the larger block.

a) What force would be necessary to maintain an acceleration of 4.0 m/s2? (in terms of "b")
b) What would be the minimum coefficient of static friction necessary to keep the smaller block from slipping?

2. Relevant equations

a = Fnet / m
Ff = µFN
Fnetx = Fa + Ff
Fnety = Fg + FN

3. The attempt at a solution

Fg = ma
Fg = 3.4b (9.81)
Fg= 33.354b N [down]

Fnety = Fg + FN
0 = 33.354 N [down] + FN
FN = 33.354b N [up]

Ff = µFN
Ff = (0.35)(33.354b N)
Ff = 11.6738b N

Fnetx= Fa + 11.6738b N

I don't know how to get Fnetx from here, and I don't know if this is right so far

4.0 m/s2= Fnet / 3.4b

Also I don't understand part b)
Thanks for the help

2. Nov 29, 2013

### haruspex

No, Fnetx = Fa. You want to find Fapp, where Fnetx = ƩF = Fapp + Ff.
The smaller mass is to accelerate at 4m/s2. What force is necessary to achieve that? Where is that force coming from?

Btw, I recommend working entirely symbolically, only plugging in numbers when you get to the final equation. It makes it easier to find errors, and much easier for others to follow.