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Torque and Moment of Inertia

  1. Apr 14, 2013 #1
    1. The problem statement, all variables and given/known data

    A drink can is placed on a rough-surfaced, stationary conveyor belt of a supermarket checkout. The can is modelled as a cylinder consisting of two equal discs, each of mass m1 and radius R, that are the top and bottom, and a curved surface of mass m2, radius R and length h. The thickness of the metal is so small that the can may be modelled as a thin cylinder with a thin disc at each end, of constant surface density. The liquid contents of the can are modelled as a solid cylinder of radius R, length h and mass M.
    The conveyor belt is switched on with acceleration f i, where f is a constant, such that the can rolls backwards relative to the direction of motion of the belt, without slipping. The axis of the can remains at right angles to the direction i of the acceleration of the belt. The time that elapses from the instant when the can begins to move is denoted by t, θ is the angle through which the can rotates in this time, and x, taken as positive in the direction of i, is the distance moved by the centre of mass of the can due to the rotation through angle θ.
    (a) Use the rolling condition to find an expression for x in terms of R and θ. Hence show that the acceleration of the centre of mass of the can is (f −R¨ θ)i.
    (b) Draw a force diagram for the can, expressing each force in vector form.
    (c) By applying Newton’s second law, find the equation of motion for the centre of mass of the can.
    (d) Write down the torque, relative to the centre of mass of the can, associated with each force acting on the can. Hence use the torque law to determine the equation of relative rotational motion of the can in terms of R, the moment of inertia I of the can about its axis of symmetry, and the magnitude of the frictional force.
    (e) Show that the moment of inertia of the can and contents about the axis of symmetry is I = (1/2M +m1 +m2)R2.
    (f) Hence show that the acceleration of the centre of mass of the full can is
    (M + 2m1 + 2m2)/(3M + 6m1 + 4m2)f i.

    2. Relevant equations

    Rolling condition = R theta = x
    I = Moment of inertia = 1/2MR^2
    second law:F = ma (where m is the total mass of the system)
    m = (M + 2m1 + m2)
    v = dx/dt
    w = -d^2x/dt^2
    a = f- Rtheta

    3. The attempt at a solution

    a) When the can rolls anticlockwise, the distance rolled is -x. Using the rolling condition, we have -x= -R theta. The position vector of the centre of mass is Rj-xi. Therefore the velocity is -x' and the acceleration is -x'', since R is a constant. Therefore the acceleration of the centre of mass of the can is (f-R theta'') as required.
    b) The force diagram contains N upwards, W downwards and F in the positive i direction.
    c) Newton's second law gives F= ma. This gives F= (2m1+m2)(f-R theta), which is the equation of motion of the can (without contents).
    d) Since W and N act through the centre of mass, they do not apply any torque. The torque is given by
    Torque= -Rj x (2m1+m2)(f-R theta'') i= -R (2m1+m2)(f-R theta) k.
    The torque law is torque^rel= Iw'= I theta''.
    The moment of inertia of the can without contents is I=(m1+m2)R^2.
    So torque^rel= (m1+m2)R^2 theta'' k.
    Equating the relative torque and the torque gives f= m1 R theta''/ (2ma+m2)
    e) The moment of inertia of the solid cylinder is 1/2 MR^2, and of the can (from part d) is (m1+m2)R^2. Hence the moment of inertia is (1/2 M+ m1+m2) R^2, as required.
    f) Now this is where I'm stuck! I don't have a clue where to start and am unsure if I should be using my friction value found in part d)? Any pointers greatly appreciated hehe...

    Thanks a lot, Hannah
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 14, 2013 #2
    Don't worry, I've sorted it now and know where I was going wrong, but thanks in advance for any help people were going to give!!!
  4. May 8, 2013 #3

    Hi! I'm facing problems understanding this question,
    my question is exactly the same as yours, except that it has 2 more sub-questions:
    The contents of the can are now drained, leaving an empty can. Ignore the
    absence of the ring-pull. The process is now repeated.
    (g) Write down the magnitude of the acceleration of the centre of mass of
    the empty can.
    (h) L is the distance that the centre of mass of the can has to travel along
    the belt before reaching the checkout assistant. Determine
    mathematically whether the full or empty can covers the distance L in
    the lesser time.
    Judging by what is demanded by these 2 sub-questions I would assume that sub-questions (b),(c) and (d) are considering the can and its contents.

    But what makes me MORE CONFUSED, is in (a) and (f) it has different acceleration of the C.O.M, which means what you're doing makes sense, THEN, question (g) asked about the acceleration of the empty can again??... pls help, and also it'll be nice if u can show me your solution! Thanks!
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