Finding ratio of masses of two blocks

[RoboNerd]

Homework Statement

1. Two crates are stacked on top of each other on a horizontal floor. The coefficient of static friction between Box A and Box B is 0.6. The coefficient of static friction between Box A and the floor is 0.2. A force of 100 N is applied parallel to the floor on Box B. The boxes move together along the floor. The ratio of the mass of Box A to Box B is possibly
   

1. 1) 5:1
2. 2) 5:2
3. 3) 4:1
4. 4) 3:1
5. 5) 1:2

Homework Equations

none so far

The Attempt at a Solution

Hi people. I was trying to figure out how to solve this problem, but I do not even have any hints of an idea so far. Could anyone please help me gets started?
Thanks in advance!

 

[CWatters]

For the boxes to move friction between A and B must be greater than between A and the floor.

Write equations for both and set up that inequality. Rearrange.

 

[RoboNerd]

For the boxes to move friction between A and B must be greater than between A and the floor.

Why is this so?

 

[CWatters]

Think about how the applied force is transmitted to the lower box.

Drawing a FBD for the lower box may help.

 

[CWatters]

You posted one or two similar questions and they all depend on you understanding the forces acting on the lower box.