Is My Assumption About Rope Tension Correct?

[stuc]

Yes, it's wrong. (That would be the right answer if the angle were 45 degrees.)
You have established that the weight is 10sin(60) N. Draw a free body diagram, exactly like the original diagram, but with only one rope.
So the rope is still at 60 degrees to the horizontal.
Let the tension in the rope be T. You know the weight, and hence the mass. The tension and the weight are the only forces acting, agreed?
Which way will the mass accelerate?
What can you therefore say about the forces orthogonal to that direction?

thank you very much for your guide..i will try your idea. i hope you have time to check it

 

[stuc]

thank you very much for your guide..i will try your idea. i hope you have time to check it

Yes, it's wrong. (That would be the right answer if the angle were 45 degrees.)
You have established that the weight is 10sin(60) N. Draw a free body diagram, exactly like the original diagram, but with only one rope.
So the rope is still at 60 degrees to the horizontal.
Let the tension in the rope be T. You know the weight, and hence the mass. The tension and the weight are the only forces acting, agreed?
Which way will the mass accelerate?
What can you therefore say about the forces orthogonal to that direction?

dear haruspex ... is it like this?

 

[haruspex]

View attachment 92448

dear haruspex ... is it like this?

The free body diagram is right, but you need to think about which way the acceleration must be. In your force analysis, you have summed the vertical components and assumed the total to be zero. In other words, you have supposed the acceleration to be horizontal. Clearly it will not be. Which way will it be?