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Don't Understand Tension in this Instance

  1. Nov 20, 2014 #1
    Hello everyone!

    I was doing a homework problem tonight, and a concept came up that I didn't quite understand. Don't worry, this is not a question asking for homework help -- I've already done the research and figured out how to solve for the solution. What I don't understand is why it's true.

    For the sake of simplicity, I will be copying the homework problem here to clarify the circumstances.

    To hoist himself into a tree, a 72.0 kg man ties one end of a nylon rope around his waist and throws the other end over a branch of the tree. He then pulls downward on the free end of the role with a force of 358 N. Neglect any friction between the rope and the branch, and determine the man's upward acceleration.

    I've already researched a few other sources, so I know how the answer is derived, but I don't understand why it's true. In this instance, the man's exertion of 358 N creates a tension of 358. His weight, W = mg, is 705.6 N in this instance. Because tension acts on both ends of the rope, apparently, he rises because the tension is doubled, which would exceed 705.6 N and allow him to accelerate upwards.

    According to the sources I've looked into, one end of the tension acts on his body, while the other acts on his hands. However, I have a hard time imagining a pulling force on his hands. When this action is done in real life, I don't feel as if my hands are being pulled upwards. Rather, my hands stay in the same location on the rope, and are possibly even lowered from their previous position while my body rises.

    I suppose because the hands are a "part of the body" then the pulling force would transfer over to the body, but in that instance there would still be a pulling force on the hands. How does this force manifest? A picture/diagram would probably be very helpful in explaining this! ;)

    Thanks in advance!
     
  2. jcsd
  3. Nov 20, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    If you are pulling down on the rope, the rope must be pulling you up. Just because your hands lower, doesn't mean that an upward force isn't being exerted on your body. (The force is transmitted through your hand and arm to your torso.)
     
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