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geerky42

  • one year ago

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

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

  4. shubhamsrg
    • one year ago
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    Product of mr will decide that

  5. geerky42
    • one year ago
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    Why does it work?

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

  7. geerky42
    • one year ago
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    I'm confused...

  8. shubhamsrg
    • one year ago
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    Also, am assuming its a massless pulley , right ?

  9. geerky42
    • one year 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
    • one year ago
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    Yes pulleys has mass.

  11. geerky42
    • one year ago
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    Here's the problem.

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  12. shubhamsrg
    • one year ago
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    Be right back.

  13. geerky42
    • one year 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
    • one year ago
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    If mass goes upward, T goes first, otherwise, m·g goes first.

  15. geerky42
    • one year 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
    • one year ago
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    Okay am back

  17. shubhamsrg
    • one year ago
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    Well simply write eqn of rotation.

  18. geerky42
    • one year 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
    • one year 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
    • one year 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
    • one year 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
    • one year ago
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    Ugh, I hope I make right guess first, lol.

  23. geerky42
    • one year ago
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    What is ve?

  24. shubhamsrg
    • one year ago
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    negatiVE

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

  26. geerky42
    • one year ago
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    Problem asks me to "take clockwise direction as positive."

  27. geerky42
    • one year ago
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    Wait, angular acceleration cannot be negative, right?

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

  29. shubhamsrg
    • one year ago
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    The linear acceleration of both masses will not be equal

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

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

  32. shubhamsrg
    • one year ago
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    I am getting confused. @Vincent-Lyon.Fr

  33. geerky42
    • one year ago
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    It seems like there is too much unknown variables.

  34. geerky42
    • one year ago
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    lol, me too...

  35. geerky42
    • one year ago
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    @hartnn @JamesJ @saifoo.khan Can you help me?

  36. geerky42
    • one year ago
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    @zepdrix @AccessDenied

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

  38. rajathsbhat
    • one year 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
    • one year 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
    • one year ago
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    It makes sense. Thanks, everybody!

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