Solving Some Friction Thing Homework

[kchau]

Homework Statement

http://img152.imageshack.us/img152/4671/img3328cj5.jpg

Homework Equations

The Attempt at a Solution

i Found part a is 2.7
and b i think is 12.0N?

 

[Doc Al]

Show how you solved part b.

(Hint: Just because b comes before c doesn't mean you have to solve them in that order.)

 

[PhanthomJay]

Draw FBD's and tell us why you think the answer is 12N for part b. (Note that the acceleration of each block (and both blocks together) must be the same if there is no slippage.)

 

[kchau]

well I am not sure how to solve it, i just used logic.
basically the way i see it, there is always 4.4kg resting on top of the slab, and no matter where you push, there will always be 12.0N applied to both blocks at the same time, mainly becuase the bottom of the slab is frictionless.

 

[kchau]

anyone?

 

[PhanthomJay]

well I am not sure how to solve it, i just used logic.
basically the way i see it, there is always 4.4kg resting on top of the slab, and no matter where you push, there will always be 12.0N applied to both blocks at the same time, mainly becuase the bottom of the slab is frictionless.

Your logic is flawed. Draw a FBD of the top block and calculate its acceleration. The max friction force that can act on the top block without slippage is 12N. The 12N friction force is the only force acting on the top block. Once you get the acceleration, solve for F with a FBD of the system.

 

[kchau]

this problem just doestn make sense to me, and none of the equations that i have found in teh textbook, or have used so far make sense.

 

[Doc Al]

PhanthomJay gave you a precise presciption for solving this problem. The block just begins to slide when the force on it exceeds 12 N. Figure out what its maximum acceleration must be as a result of that force. (Use Newton's 2nd law.) In order for "block + slab" to have that acceleration, what force must be exerted on the slab? Note that the force on the slab is the only external force acting on "block + slab". (Again, use Newton's 2nd law.)