Why Does Calculating Tension in Multiple Blocks Require Different Forces?

[arianemati]

Homework Statement

Newtons 1st law, constant velocity

Relevant Equations

Sum F = 0

If I want to find the tension in the rope that connects block A and B. I can apply Newtons first law.

Sum of all forces in opposite direction equals 0.

The forces that work on block A is Friction (Ra) and the force on the rope that pulls the block towards the right (T) So we get T-Ra = 0 and therefore T=Ra. We don't look at the force T2 or mcg, or the friction working on block B.

So my question is. If we on block A only use the first force to the right and the first force to the left. (T-Ra) to find the tension in the rope connecting A and B . When we find the tension in the rope (T2) connecting B and C. Why do we have to calculate all forces that work to the left (T2 = P+Rb+Ra) instead of T2=P+Rb ?

 

[BvU]

You forgot the problem statement. What is the exercise ?

 

[arianemati]

You forgot the problem statement. What is the exercise ?

Nevermind. I think I got it :)