1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Solving rotational motion without torque

  1. Aug 24, 2015 #1
    1. The problem statement, all variables and given/known data
    A potter’s wheel—a thick stone disk of radius 0.500 m
    and mass 100 kg—is freely rotating at 50.0 rev/min.
    The potter can stop the wheel in 6.00 s by pressing a
    wet rag against the rim and exerting a radially inward
    force of 70.0 N. Find the effective coefficient of kinetic
    friction between wheel and rag

    2. Relevant equations


    3. The attempt at a solution
    Actually, I could get the answer using angular acceleration and torque, but I could do the same thing with Newtonian Laws and I want to know why.

    Tangential velocity of the wheel:
    50 rev/ min = 2.618 m/s

    u = 2.618 m/s, v = 0, a = ?, t = 6
    v = u + at
    0 = 2.618 + 6a
    a = 0.436 m/s^2

    Deceleration force
    F = ma = (100)(0.436) = 43.6N

    The friction coefficient
    43.6 = (coefficient)(70)
    coefficient = 0.623, which is exactly the double of the correct answer.

    Thanks!
     
  2. jcsd
  3. Aug 24, 2015 #2

    andrevdh

    User Avatar
    Homework Helper

    not all of the mass is moving at the calculated deceleration (or speed)
     
  4. Aug 24, 2015 #3
    Why and how is that? The whole wheel is spinning.
     
  5. Aug 25, 2015 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It's all moving at the same angular speed, but you have taken it all to be moving at the same linear speed. What equation relates the two?
     
  6. Aug 25, 2015 #5
    v = rw?

    Do you mean I can only use angular velocity to calculate angular motion and newtonian laws to calculate linear motion?
     
  7. Aug 25, 2015 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, that's the right equation. All parts of the disc have the same ##\omega##, but they do not all have the same r.
     
  8. Aug 25, 2015 #7
    So I eventually need to convert the tangetial velocity back to angular velocity. Ok I get it now

    Thanks!
     
  9. Aug 25, 2015 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I'm not quite sure what you mean by that.
    Anyway, it would be more usual to apply standard theory regarding moments of inertia. You are aware of that?
     
  10. Aug 25, 2015 #9
    Yes, I know. But when you calculate circular motion, it is ok not to use angular velocity or angular acceleration, so I was wondering if I can do the same thing in rotational motion. You know what I saying?
     
  11. Aug 25, 2015 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Ok, but it will turn out to be equivalent to reinventing the concept of moment of inertia. How will you now calculate the KE of the disc?
     
  12. Aug 25, 2015 #11
    Moment of inertia = 1/2MR^2 = 0.5(100)(05)^2 = 12.5 kgm^2
    KE = 1/2(12.5)(2.618 / 0.5)^2 = 171 J
     
  13. Aug 25, 2015 #12

    andrevdh

    User Avatar
    Homework Helper

    I get that 50 rpm is 5.24 rad/s?
     
  14. Aug 25, 2015 #13

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Which is indeed about 2.618/0.5.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Solving rotational motion without torque
Loading...