How do I calculate the minimum force needed to prevent sliding due to friction?

[silvashadow]

Homework Statement

The drawing shows a large cube (m=25kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (m=4kg) is in contact with the fron surface of the large cube and will slide downward unless P is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that P can have in order to keep the small cube from sliding downwardsI know that the weight of the small cube is 39.2N. I also know that Fn is in the same direction as P. Past that, I'm confused. I have a equation: F=u*Fn, which is friction is equal to coef. of friction times normal force.

Can someone explain to me what to do next?

 

[silvashadow]

11 views, 12 hours and no help... guess you guys can help me or even tell me what else I need to put down...

 

[TVP45]

Hints

On the small block, mass m, the downward force will be due to gravity mg.
The upward force will be due to friction and a normal force from the acceleration uma.
In order for the block not to fall uma=mg where u = 0.71
Once you have solved for a, ask yourself how much force it takes to give that much a to the whole system.
Tom

 

[silvashadow]

Ok. I have W of 4kg = 39.2N. In order for the block to not fall, friction has to be 39.2N.

mg=uma
4*9.8=.71*ma
39.2=.71*ma
ma=Fn=55.2N.

Where do I go from there? Can you at least tell me what m is supposed to be: 4kg or 25kg object?

 

[TVP45]

How much mass are you pushing?

 

[silvashadow]

I am pushing 4kg to the right. Is that correct?

 

[rocomath]

11 views, 12 hours and no help... guess you guys can help me or even tell me what else I need to put down...

we're not here to please you, be patient

 

[TVP45]

With what are you pushing the 4kg?

 

[silvashadow]

You guys have been great. I've gotten help from my teacher and some fellow classmates. Guess this place just isn't good for those who seek help.

 

[TVP45]

Personally, I already know how to do the problem. I ask questions to try to guide you. Do you now understand the problem?

 

[silvashadow]

friction of 4kg = weight of 4kg
friction = uFn
Ff=mg=uma
mg=uma
g=ua
9.8=.71a
a=13.8

P=ma=(mass of big + mass of small)*13.8=400.2N