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geerky42

  • 2 years ago

In this system, how can you determine which direction it will rotate?

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  1. geerky42
    • 2 years ago
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  2. geerky42
    • 2 years ago
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    Like, if \(m_1\) is larger than \(m_2\), how can you tell which way it will rotate?

  3. geerky42
    • 2 years ago
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    Sorry for keeping asking you for help, but can you answer this question, @shubhamsrg ?

  4. shubhamsrg
    • 2 years ago
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    Product of mr will decide that

  5. geerky42
    • 2 years ago
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    Why does it work?

  6. shubhamsrg
    • 2 years ago
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    Clockwise torque = (m2)g(r2) ANti clockwise torque = (m1)g(r1) Greater torque will prevail

  7. geerky42
    • 2 years ago
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    I'm confused...

  8. shubhamsrg
    • 2 years ago
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    Also, am assuming its a massless pulley , right ?

  9. geerky42
    • 2 years ago
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    Well, the problem I'm currently working on, it doesn't say anything about the mass of pulley... but it does say that the combined moment of inertia of the two wheels is 2.7 kg·m². So pulley or wheels has mass?

  10. shubhamsrg
    • 2 years ago
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    Yes pulleys has mass.

  11. geerky42
    • 2 years ago
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    Here's the problem.

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  12. shubhamsrg
    • 2 years ago
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    Be right back.

  13. geerky42
    • 2 years ago
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    The reason I ask this question is because I want to set up a sum of force for both two masses correctly and I don't know which to go first; mg or T (tension)

  14. geerky42
    • 2 years ago
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    If mass goes upward, T goes first, otherwise, m·g goes first.

  15. geerky42
    • 2 years ago
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    My attempt is to set up some equations then do some substitution and isolate and find \(\alpha\) in term of variables that already have given value. I don't know if this can work.

  16. shubhamsrg
    • 2 years ago
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    Okay am back

  17. shubhamsrg
    • 2 years ago
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    Well simply write eqn of rotation.

  18. geerky42
    • 2 years ago
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    \(\sum \tau = T_1R_1 - T_2 R_2 = I\alpha\) OR \(\sum \tau = T_2R_2 - T_1 R_1 = I\alpha\) It depends which direction it rotates... I also need to find two tensions...

  19. geerky42
    • 2 years ago
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    \[\sum F_1 = m_1g - T_1 = m_1 a\] OR \[\sum F_1 = T_1 - m_1g = m_1 a\] \[\sum F_2 = m_2g - T_2 = m_2 a\] OR \[\sum F_2 = T_2 - m_2g = m_2 a\] That's why I need to determine which direction it rotates...

  20. geerky42
    • 2 years ago
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    Any ideas? "Clockwise torque = (m2)g(r2) ANti clockwise torque = (m1)g(r1) Greater torque will prevail" This works only if pulley is massless, right?

  21. shubhamsrg
    • 2 years ago
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    You can take any direction to be +ve. If your assumption would be wrong, it'll automatically come to be -ve

  22. geerky42
    • 2 years ago
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    Ugh, I hope I make right guess first, lol.

  23. geerky42
    • 2 years ago
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    What is ve?

  24. shubhamsrg
    • 2 years ago
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    negatiVE

  25. geerky42
    • 2 years ago
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    What is angular acceleration is negative? How can I know if my assumption is right or wrong?

  26. geerky42
    • 2 years ago
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    Problem asks me to "take clockwise direction as positive."

  27. geerky42
    • 2 years ago
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    Wait, angular acceleration cannot be negative, right?

  28. shubhamsrg
    • 2 years ago
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    It is a vector, it can be negavtive, -ve will just denote the direction.

  29. shubhamsrg
    • 2 years ago
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    The linear acceleration of both masses will not be equal

  30. shubhamsrg
    • 2 years ago
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    m1g - T1 = m1 a1 T2 - m2g = m2 a2 T1 r1 - T2 r2 = I alpha

  31. geerky42
    • 2 years ago
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    Is it possible to find angular acceleration in this way?

  32. shubhamsrg
    • 2 years ago
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    I am getting confused. @Vincent-Lyon.Fr

  33. geerky42
    • 2 years ago
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    It seems like there is too much unknown variables.

  34. geerky42
    • 2 years ago
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    lol, me too...

  35. geerky42
    • 2 years ago
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    @hartnn @JamesJ @saifoo.khan Can you help me?

  36. geerky42
    • 2 years ago
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    @zepdrix @AccessDenied

  37. geerky42
    • 2 years ago
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    Apparently I'm not going to get any help for long time.

  38. rajathsbhat
    • 2 years ago
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    You don't have to know the direction of rotation of the pulley before you solve the equations. You just take one of the directions as positive and the other one as negative. This choice is completely arbitrary (But since the given problem asks you to take the clockwise direction as positive, you just go ahead with that). After you solve the problem, if the end up with positive angular acceleration, it's in the direction that you chose (In this case, clockwise). If it turns out to be negative, it's in the opposite direction (In this case, counter-clockwise).

  39. Vincent-Lyon.Fr
    • 2 years ago
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    Quote : "Clockwise torque = (m2)g(r2) ANti clockwise torque = (m1)g(r1) Greater torque will prevail" This works only if pulley is massless, right? No, this works whatever the moment of inertia of the pulley. It is a good idea to anticipate the direction of motion this way.

  40. geerky42
    • 2 years ago
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    It makes sense. Thanks, everybody!

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