Tension Direction in FBDs & Tension HW

[uradnky]

Homework Statement

While working through problems I always have trouble determining the direction that tension acts in on objects in the system.

In system B.) block A has a velocity of 4m/s to the right, with mu(k)=0.2, causing the mass B to fall, in class tension was assumed to be in the direction of A's motion.

But in A.) the system starts at rest, and A slides down the frictionless inclined plane moving masses B and C. In this case the tension opposes the motion of A, I just can't visualize this.


Is there some rule of thumb for this that I'm just not realising?

If someone could give me a brief explanation on how tension acts on each object in these systems I would appreciate it.

 

[cepheid]

It doesn't matter how A is moving. In both cases, you'll notice that the tension force is in the same direction: AWAY from block A. There's no other direction it could be in that would correspond to the rope being taut, right?

You can't push on a rope.

 

[uradnky]

Hah that makes things easy. I guess I was thinking back to statics where if I was assuming a member to be in tension, but my answer came out negative then I had assumed wrong and it was in compression.

Thanks a lot.