# Homework Help: Need help on friction problem

1. Sep 25, 2007

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

I 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?

2. Sep 26, 2007

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

3. Sep 26, 2007

### 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

4. Sep 26, 2007

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?

5. Sep 27, 2007

### TVP45

How much mass are you pushing?

6. Sep 27, 2007

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

7. Sep 27, 2007

### rocomath

we're not here to please you, be patient

8. Sep 27, 2007

### TVP45

With what are you pushing the 4kg?

9. Sep 27, 2007

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.

10. Sep 27, 2007

### TVP45

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

11. Sep 27, 2007

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