Analyzing Forces in a Platform and Pulley System

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SUMMARY

The discussion focuses on analyzing the forces in a platform and pulley system involving a girl (mass m) standing on a platform (mass M) with a pulley (mass u). The tension in the rope and the normal force between the girl and the platform are critical to understanding the system's dynamics. The equation T - F = ma is identified as a starting point for solving the problem, emphasizing the need for a net upward force for the platform to accelerate. Clarifications are provided regarding the conditions under which the platform can move upwards, specifically that the rope's attachment to the ceiling allows for this upward acceleration.

PREREQUISITES
  • Understanding of Newton's Second Law of Motion
  • Basic knowledge of tension in ropes and forces in pulley systems
  • Familiarity with free body diagrams
  • Concept of net force and acceleration
NEXT STEPS
  • Study the application of Newton's Second Law in multi-body systems
  • Learn how to draw and analyze free body diagrams for complex systems
  • Explore the mechanics of pulleys and their impact on force distribution
  • Investigate the effects of different mass distributions on acceleration in pulley systems
USEFUL FOR

Students studying physics, educators teaching mechanics, and anyone interested in understanding the dynamics of forces in pulley systems.

Paola Cavazos
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A girl stands on a platform and pulley system. The masses of the platform, girl and pulley are M, m and u respectively. The rope is mass-less. Let the girl pull up on the rope so that she accelerates a upwards. What is the tension of the rope? What is the normal force between the person and the platform? WHat is the tension connecting the pulley and the platform? I think that I should start from T - F = ma, but I really am kind of lost. I hope someone can help me find the accurate solution.

I uploaded an image of the exercise. Please help :)
 

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Paola Cavazos said:
I think that I should start from T - F = ma
You should explain your thinking when asking for help. How did you get this equation? What is it saying?
 
I have a small question Nathanael, I'm a bit confused with the problem too. Will the platform even move?? I'm sorry if this is a stupid question!
 
Suraj M said:
I have a small question Nathanael, I'm a bit confused with the problem too. Will the platform even move?? I'm sorry if this is a stupid question!
It's not a stupid question. I can see how one would think the platform may not move, because the harder you pull up on the rope, the harder your feet push down on the platform. But the upwards force on the platform from pulling is greater than the increased force downward from pushing with your feet.
For the platform to move upwards (or more specifically, for it to accelerate upwards) there has to a net upwards force acting on the system. If the other end of the rope was attached to the platform, and the person was pulling up on it, then clearly it could not move upwards. But since the end of the rope is attached to the ceiling, an upwards external force acts on the platform (and person) allowing upwards acceleration.
P.S.
I like your avatar.
 
I'm sorry but i still don't get how the upward force would be more than the downward. Could you explain?
 
Suraj M said:
I'm sorry but i still don't get how the upward force would be more than the downward. Could you explain?
I purposely left the specifics of that detail out so as not to ruin it for the OP :) I will explain it to you in a private message.
 

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