Pulley Problem: Finding Tension in Suspended Cord with Two Masses

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Homework Help Overview

The problem involves a pulley system with two different masses (1.2 kg and 3.2 kg) attached to either side of a cord. The objective is to determine the tension in the cord after the masses are released but before one of them hits the ground, while ignoring the mass of the pulley and the cord itself.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the forces acting on the masses and the pulley, considering free body diagrams and the relationship between tension and acceleration. Some express uncertainty about how to start the problem, while others suggest using equations of motion to analyze the forces involved.

Discussion Status

Some participants have made progress in calculating the tension in the cord for the masses, while others are exploring the implications of the pulley being massless. There is an ongoing dialogue about the forces acting on the pulley and how they relate to the tensions in the cords attached to the masses.

Contextual Notes

Participants are working under the assumption that the pulley is massless and that the system is in a state where one mass has not yet hit the ground. There is a focus on understanding the dynamics of the system rather than arriving at a final answer.

iheartpink
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a pulley is suspended by a cord with two masses attached. one mass is 1.2 kg. the second mass is 3.2 kg. the two masses are attached on different sides.

the question: determine the tension in this cord after the masses are released and before one hits the ground. ignore the mass of the pulley and cords.


thank you very much in advance!
 
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iheartpink said:
a pulley is suspended by a cord with two masses attached. one mass is 1.2 kg. the second mass is 3.2 kg. the two masses are attached on different sides.

the question: determine the tension in this cord after the masses are released and before one hits the ground. ignore the mass of the pulley and cords.


thank you very much in advance!

How did you approach the problem?
 
i am not sure [at all] how to start with the problem...
 
iheartpink said:
i am not sure [at all] how to start with the problem...

Hint: The tension above both masses are the same. The acceleration of the lighter mass upward = the acceleration of the heavier mass downward.

Do a freebody diagram of each of the masses. What are the forces acting on each mass... use:

[tex]\Sigma\vec{F} = mass*acceleration[/tex]

for each mass...
 
i have solved for the first tension. and tension is 17.1 N
i am clueless has to how to solve the tension before one mass hits the ground
 
iheartpink said:
i have solved for the first tension. and tension is 17.1 N
i am clueless has to how to solve the tension before one mass hits the ground

I think all they meant for that was while the masses are still suspended... before they hit the ground and change the system... 17.1N is the right answer for the rope holding up each of the masses... but what is the tension in the cord holding up the pulley.

You're almost there. The pulley is massless, so the force on it is ma = 0a = 0. You can also say... the pulley is fixed, so its acceleration is 0 and ma = m0 = 0. Either way, the force on the pulley is 0.

Use that to calculate the tension in the cord holding the pulley. You've done most of the work, already, this is just the last part.

Use F = ma for the pulley to calculate the tension in the cord above...
 
Last edited:
i'm confused. if the force is zero, then is the tension the same as it is for the other rope?
 
iheartpink said:
i'm confused. if the force is zero, then is the tension the same as it is for the other rope?

17.1N is the tension in the rope above mass 1. 17.1N is also the tension in the rope above mass 2.

You need to find the tension in the cord above the pulley. So the forces acting on the pulley are the tension above, and the two 17.1N below. net force needs to be 0 on the pulley.
 
it just clicked in my head. thank you!
 
  • #10
iheartpink said:
it just clicked in my head. thank you!

cool. just to make sure, what did you get as your final answer?
 

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