Understanding Tension in a Block and Tackle System

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SUMMARY

The discussion centers on the mechanics of a block and tackle system, specifically addressing the tension forces involved when a boy lifts himself using this system. The combined weight of the boy and the beam is 120 lb, requiring an analysis of the forces at play. The correct number of tension forces is established as five, with four tensions acting at the lower pulley block and one from the boy's exertion on the rope. This clarification resolves the confusion regarding the distribution of tension in the system.

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  • Understanding of basic physics concepts, particularly forces and tension.
  • Familiarity with block and tackle systems and their mechanical advantage.
  • Knowledge of free body diagrams (FBD) for analyzing forces.
  • Basic algebra for calculating forces and tensions.
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  • Study the principles of mechanical advantage in pulley systems.
  • Learn how to draw and interpret free body diagrams (FBD) in physics.
  • Explore the calculations involved in tension forces in multi-pulley systems.
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Physics students, engineers, and anyone interested in understanding the mechanics of lifting systems and tension forces in block and tackle setups.

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Homework Statement



The boy is lifting himself using the block and tackle shown. If the weight of the block and tackle is negligible, and the combined weight of the boy and the beam he is sitting on is 120 lb, what force does he have to exert on the rope to raise himself at a constant rate? (Neglect the deviation of the ropes from the vertical.)

I drew a FBD for the bottom of the block with the weight going down and 4 tensions pulling up, each with a force of 30lb, but the book has 5 tensions. Which one is correct?

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Do you dispute that the rope the boy is holding is also at tension T? Do you dispute that all five tensions act on the boy+beam system?
It might be clearer to you if you consider the lower pulley block and the boy+beam separately. What is the tension in the rope connecting them?
 
haruspex said:
Do you dispute that the rope the boy is holding is also at tension T? Do you dispute that all five tensions act on the boy+beam system?
It might be clearer to you if you consider the lower pulley block and the boy+beam separately. What is the tension in the rope connecting them?

oh, so the tension occurs 5 times b/c 4 times at the lower block, but then also from the boy like the picture because the whole thing is one system, and the block is only part of it.

thanks for the explanation to clear it up!
 

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