# Physics Problem: Two blocks stacked

• November 14th 2011, 03:09 PM
over9000
Physics Problem: Two blocks stacked
There are two blocks, one on top of the other. The top block has a mass of 5kg and the bottom block has a mass of 12kg. The bottom block rests on a frictionless plain, but the coefficient of friction between the two blocks is 0.6. What minimum force applied on the bottom block is required to get the top block to move relative to the bottom block?

I don't know why I'm having so much trouble with this problem. I thought that the top block might as well be on a surface in which the coefficient of friction was 0.6 and the answer would just be solving for F_static in F_static = 0.6 (5)(9.8) but the professor told me that was wrong and he didn't give me an intuition as to why my thinking was wrong.
• November 14th 2011, 03:47 PM
emakarov
Re: Physics Problem: Two blocks stacked
Consider the maximum acceleration of the bottom block where the top block does not slide. Then if you push the bottom block with this acceleration, both blocks move as a unit. Find the force to do this. If you increase the force a little, the acceleration of the bottom block would increase and the top block would start to slide.
• November 15th 2011, 11:02 AM
over9000
Re: Physics Problem: Two blocks stacked
$F_top_y & = F_n - F_w$ test
• November 18th 2011, 01:25 AM
over9000
Re: Physics Problem: Two blocks stacked
I'm sorry about that test earlier. I should have used the "preview post" option to see how that tex was going to come out. Anyway, I just wanted to complete the problem since I now know how and I want to practice using LaTEX.

$\sum F_{top_x} = F_{friction}$
$\sum F_{bottom_x} = F_{applied} - F_{friction}$
$F_{friction} = F_{normal} * {\mu}_{friction}$
$F_{friction} = 29.4N$
$m_{top}*a = F_{friction}$
$a = 5.88 m/s^2$

$m_{bottom}*5.88 = F_{applied} - 29.4N$
$F_{applied} = 99.96N$

Hm, would have been nice to align everything. I'm not sure how to do that yet.