# Reason for the obvious movement of a block

1. May 2, 2015

### mooncrater

1. The problem statement, all variables and given/known data
Consider the given system. It is obvious that the weight of the block will lead the system to move in the leftward direction. But the reason for that is not clear to me.
2. Relevant equations
Using the FBD of the block(B):
$mg-T=ma$ where $a$ is assumed its downward acceleration.
$N=mA$ where $A$ is the rightward acceleration as the normal force from the bigger block A is in the right ward direction. (Which is the problem)
$T+N=M\alpha$ where I assume that $\alpha$ is the acceleration of block A in the leftward direction (as both T and N are in the left direction for A)
3. The attempt at a solution
So there is nothing pulling B towards left so why will go in the left direction ¿

2. May 2, 2015

### CWatters

What happens if the pulley and block A move to the left and B doesn't?

Edit: Seems to me you are over thinking the problem. It would probably be reasonable to assume that the mass of A >>B so that the acceleration to the left is small and the string remains vertical.

3. May 2, 2015

### mooncrater

No... we can't assume that since in the question (from where I have asked this part) it's given that mass of A =40kg and mass of B is 20kg.

4. May 2, 2015

### Staff: Mentor

What if block B were to lose contact with block A. What could then possibly make block B accelerate to the left?

Chet

5. May 2, 2015

### mooncrater

Hmmm.....
If we assume that A loses contact with B and goes towards left a small distance $x$, then the thread will form a small angle $\theta$ with the vertical because of which tension will have a leftward component along with the vertical one for B. Due to which "I think" the block B will move left. Is that what you want to say?

6. May 2, 2015

### Staff: Mentor

Yes. That's what I wanted you to say.

Chet

7. May 2, 2015

Me too.

8. May 4, 2015

### aardwolf.sg

Assuming there is no friction between block A and the surface it rest on, it will move.

9. May 4, 2015

### nasu

Isn't the net force on the pulley pushing left even without contact?

10. May 4, 2015

### mooncrater

Yes. Then what?

11. May 4, 2015