Total forces acting downward on the plane

[goldfish9776]

Homework Statement

I'm asked to find whether the object is moving or not
in this question , i was given the solution like this , i am wondering why the tension from 100kg to the roller(up) and the tension from point B to roller doesn't cancel each other?[/B]
Total force acting down the slope ignoring friction for the moment is

150 + 100*9.81*sin 20 - 784 = -299.3F_s = 0.2 *100 * 9.81 *cos(20) = 276.6N. Since the upward force is greater than this, the block is moving. Now we can use sliding friction in the sum of forces:

Total F = -299.3 + 0.2 x 100 x 9.81 x cos(20) = -299.3 + 184.4 = -114.9N. the minus sign says the sum of forces is upward, and the block slides up the ramp.

Homework Equations

The Attempt at a Solution

 

[goldfish9776]

 

[faradayscat]

You have to use static friction to find if the block is moving or not, you are using the kinetic friction!

For the tension, when you isolate one of the blocks, tension force acts on it. When you isolate the ENTIRE system, tension cancels itself because it does not contribute to the motion of the system as a whole.