find the tension in the string

Question

Refer to the attached diagram. In that diagram, a 20-kg orange block is connected by a string to a 5-kg yellow block. The string slides over a frictionless pulley. Assume that the string is horizontal after the orange block and vertical to the yellow block.  

Answer 1

If the situation is such that the orange block is not moving due to friction with the surface it's on, then it sounds like the yellow block is just hanging from the pulley by the string. This is causing the tension on the string between the pulley and the orange block.

Forces basically just change direction across pulleys in situations like this. So the only force is coming from gravity pulling on the yellow block hanging. We can compute this force using Newton's 2nd Law F=ma, which can be expressed here as weight, W=mg, where g is the usual 9.81m/s^2.

So W = 5.0 * 9.81 Newtons.

T-W=0

The weight force vector points down and the T vector up along the string.

Since the weight is the only force, that force IS the tension on the string because they must sum to zero.

*If the orange block starts sliding the situation is different.