<p>Okay, I have two blocks, block a and block b. Block b weighs 10 N and sits on top of block a, which is 20 N. Block a is sitting on a table where the coefficient of static friction is .4. The coefficient of static friction between blocks a and b is 0.5. The two blocks are connected to a massless pulley with massless rope/cord, so that the tension force from the string is to the left on both blocks. There's also a rope hooked up to block a (on the right side) that someone is pulling. I need to know what maximum force I can apply to block a before the block system begins to slide. </p>
<p>The answer is ~22N; I'm getting 17. I have no idea where I'm going wrong.<br>
Here's what I did:
My freebody diagram for block a was 30N down, Normal force of 30N up to cancel that, friction of table surface/block and tension to the left, and force applied to the right.<br>
For block b, I had 10N down, Normal force of 10N up, tension to the left, and friction of block a/block b surface to the right. </p>
<p>Please tell me what I'm missing, thanks!</p>