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HELP A rolling without slipping problem

  1. Nov 26, 2008 #1
    HELP!!!A rolling without slipping problem

    1. The problem statement, all variables and given/known data
    A solid cylinder of radius R and mass M has a string wrapped around it and is placed on its side on a horizontal surface. The free end of the string is pulled horizontally with a force F as shown in the figure. As the string unwraps, the cylinder rolls along the surface without slipping.
    a) show that the acceleration of the center of mass is given by a_cm=4F/3M
    b) what is the magnitude and direction of the frictional force acting on the cylinder?
    c) what is the acceleration of the free end of the string?

    diagram:
    http://tinypic.com/view.php?pic=2qsw4kj&s=4

    3. The attempt at a solution
    V_cc=0 and it is the velocity at reference frame of center to center
    a) accelertaion of CM:
    A_tot=sqrt((atan)2+(ac)2)
    =sqrt((R*alpha)2)2+(R*(Vcc)2)
    because vcc=0 therefore A_tot=R*alpha
    4F/3M=4(M*alpha)/3M=4/3*alpha and R=4/3 in this case

    b) the frictional force opposite to the applied force, and it is equal to the applied force, which is equal to M*alpha
    c) the acceleration of the free end of the string would be same as the acceleration of the cylinder, which is R*alpha

    I am not sure whether I did the problem correctly or not, can anyone help me with it? thank you very much.
     
  2. jcsd
  3. Nov 26, 2008 #2

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    I can't follow your solution. Why not start with the basics? Draw a free body diagram and apply Newton's 2nd law to both translation and rotation.
     
  4. Nov 26, 2008 #3
    Re: HELP!!!A rolling without slipping problem

    i tried but how can I related the force to CM?
    the angular velocity for CM is omega and alpha is the angular acceleration around the cylinder
    What I know now is mass of the cylinder. Even though I also know the acceleration but it is not for center of mass.
     
  5. Nov 26, 2008 #4

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    Using Newton's 2nd law for translational motion: Fnet = macm, where acm is the acceleration of the center of mass.
     
  6. Nov 26, 2008 #5
    Re: HELP!!!A rolling without slipping problem

    Do you mean that acm=F/m, which I could use to should for part a. Sorry but where did 4/3 come from? is it part of the acceleration?
     
  7. Nov 26, 2008 #6

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    Yes, but the force you need here is the net force, not just the applied force "F". That's why I wrote Fnet.
    What individual forces act on the cylinder?
    The 4/3 will appear when you solve for the acceleration. As I said earlier, you'll need to apply Newton's 2nd law twice: for translational motion and for rotational motion.
     
  8. Nov 26, 2008 #7
    Re: HELP!!!A rolling without slipping problem

    Yes, but the force you need here is the net force, not just the applied force "F". That's why I wrote Fnet.
    What individual forces act on the cylinder?

    since it is a rotational motion there is [tex]\alpha[/tex] for this equation,
    The 4/3 will appear when you solve for the acceleration. As I said earlier, you'll need to apply Newton's 2nd law twice: for translational motion and for rotational motion.[/QUOTE]


    net force=F-Ffr=F-[tex]\mu[/tex]*FN=macm
    the above is the translational motion, sorry but how can i get rid of [tex]\mu[/tex]

    net force=F-Ffr=F-[tex]\mu[/tex]*FN=m*[tex]\alpha[/tex]
    there is a new variable appears in this equation,[tex]\alpha[/tex], which we can write it as r/a

    i think what i can do next after i know how to get rid of [tex]\mu[/tex] is set the two equation equal to each other and solve then we will be able to get 3F/4M
    is it right?
     
  9. Nov 26, 2008 #8

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    You don't need to use μ, just leave the friction force as Ffr. In what direction does the friction force act?

    The second equation you need is Newton's 2nd law for rotation: [tex]\tau = I \alpha[/tex]
     
  10. Nov 26, 2008 #9
    Re: HELP!!!A rolling without slipping problem

    [tex]\tau = I \alpha[/tex]
    I can write I as 1/2mR2 and [tex]\alpha[/tex] as R/acm
    therefore, [tex]\tau[/tex]=(1/2mR2)(R/acm)

    or there is another way to describe [tex]\tau = I \alpha[/tex], which is [tex]\tau[/tex]=radius*force, which is equal to radius*macm
     
  11. Nov 26, 2008 #10

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    Careful. [tex]\alpha = a/R \ne R/a[/tex]

    I suggest that you find the net torque due to the forces acting on the cylinder.
     
  12. Nov 26, 2008 #11
    Re: HELP!!!A rolling without slipping problem

    do you mean use the latter equation to solve(torque=radius*F)?
    and by the way, I think that I have inserted my answers for the previous question into the qutation just now...
    about the frictional force, it is equal and opposite to the applied force.

    If I use the first equation I wrote for torque then torque=(1/2mR2) (a/R)=1/2ma*R=1/2F*R
    How do we connect this equation with F-Ffr=macm?
     
  13. Nov 26, 2008 #12

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    What torque is exerted by each force acting on the cylinder? What's the net torque?
    No. If friction were equal and opposite to the applied force, then the cylinder would not accelerate.

    This is OK up until the last step. Note that F is just one of the forces acting on the cylinder.
    You still need to find the net torque.
     
  14. Nov 26, 2008 #13
    Re: HELP!!!A rolling without slipping problem

    If there is just one force applied on the system then isn't there just one torque? which means that the net torque is equal the only torque, which is r*F? Sorry, I am a bit confused...because in order to have net torque you need to have more than one force

    Or do should I also include the frictional force?
    then the net torque would be r*Fsin[tex]\theta[/tex]+r*Ffrsin[tex]\theta[/tex]
    Sorry again...how do we find the angle for sin?
     
  15. Nov 26, 2008 #14

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    Of course you should include the friction force. (For both translation and rotation.)
    First things first: Which way do the forces act?* Draw the forces on your diagram. Since both forces are horizontal and tangent to the cylinder, the only angles needed are 90 degrees.

    *You still haven't figured out which way the friction acts. Note that friction always opposes slipping between surfaces.
     
  16. Nov 26, 2008 #15
    Re: HELP!!!A rolling without slipping problem

    the free body diagram I drew
    http://tinypic.com/view.php?pic=i4h2ed&s=4

    As you said the friction is indeed opposes to the slipping direction

    Or am I wrong on drawing free body diagram?
     
  17. Nov 26, 2008 #16

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    You have the direction of the friction wrong. Friction opposes slipping between the bottom of the cylinder and the floor. If there were no friction, the cylinder would spin clockwise thus the bottom of the cylinder would tend to slip to the left against the floor. Friction on the cylinder opposes that slipping, thus it acts to the right.
     
  18. Nov 26, 2008 #17
    Re: HELP!!!A rolling without slipping problem

    OH!!!that's why
    since the direction of Ffr and applied force are the same
    then the translational motion: F+Ffr=macm
    the rotational motion:r(F+Ffr)=1/2maR2
    F+Ffr=1/2maR
    then how do we relate these two motions together?
    sub translational motion into the rotational motion: r(macm)=1/2maR2
    or set these two equations equal to each other
    1/2maR=macm
    but then we still can't relate it to 4F/3M
     
  19. Nov 26, 2008 #18

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    Good.
    Redo this one. Does the torque due to each force act in the same direction? (And be careful with the Rs.)

    Once you get the two equations correct, you'll have two equations and two unknowns (Ffr and acm). You can solve them together.
     
  20. Nov 26, 2008 #19
    Re: HELP!!!A rolling without slipping problem

    Sorry I don't really get what you mean by torque acting on the different direction. If Ffr has the same direction as F and the angles for sine are equal then why do they have different direction
    Just a bit ahead of myself, if there are two unknowns then do we use the sub one equation into another or do we set both equations equal to each other?
     
  21. Nov 26, 2008 #20

    Doc Al

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    Re: HELP!!!A rolling without slipping problem

    One acts at the top of the cylinder, creating a clockwise torque; the other acts at the bottom, creating a counter-clockwise torque.
    Combine them to eliminate one of the variables. Substituting one into the other would work.
     
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