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I've run into a bit of a problem while studying the concept of tension.

If a block which had a weight of 100N was to be hung from a rope which in turn was tied to say, the branch of a tall tree what would the tension in the rope be? According to all the text books it should be 100 N. This is where I get confused.

According to newtons third law if the block exerts a force of 100N downwards then the branch of the tree must exert a force of 100 N upwards. This would mean that the rope would be pulled upwards with 100 N as well as downwards with 100 N. In that case why isn't the tension 200 N?

thanks