Physics Classical Mechanics

posted by .

MIT
A uniform disc of mass m and radius r is acted upon by three forces. find the magnitude of angular acceleration and the sense of rotation. Three different cases. force F, 3F, 2F. need help, not doing so good.

  • Physics Classical Mechanics -

    This makes no sense.

  • Physics Classical Mechanics -

    remember that the sum of the torques is equal to I*alpha

    so sigma(t) = r1xF1 + r2xF2 +... +rnxFn = I*alpha

    where I is the moment of inertia through the axis of rotation. look for I in a table, easily found on google. notice the similarity to the application of Newtons second law.

    set up your problem like that and then just solve for alpha.

    remember to set signs for positive and negative torques, i like sticking to a positive CCW, so that when all the analysis is done, the direction of rotation resolves with the sign automatically.

    since the moment of inertia depends on the axis of rotation, part b requires the parallel axis theorem to get the moment of inertia through the new axis of rotation, although some tables include the moment of inertia you're looking for already calculated for you.

    watch out for the fact that if a force's line of action intersects with the axis of rotation, the force will not cause a torque because r=0 so rxF=0. Remember this paragraph while setting up part B

    once you've found your new moment of inertia. the same idea that worked for part A will help you solve for alpha in part B.

    part C is a bit more laborious but if you solved A and B correctly, it's just more of the same. as opposed to part B, all three forces provide a torque.

    all three forces are applied tangentially to their Rs in all three cases so theres no need for the cross product in any part of this problem.

    so, algorithmically:
    1.- find out the pertinent moment of inertia
    2.- set up your problem as following:
    sigma(t)= I*alpha = (r1 x F1) + (r2 x F2) +... + (rn x Fn)
    3.- get your alpha algebraically.

    if the algebra messes you up, google wolfram alpha

    Good luck

  • Physics Classical Mechanics -

    a) alpha= 2F/mr ccw
    b) alpha= 2F/mr cw
    c) alpha= 8F/3mr ccw

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. Physics

    A student puts on a Dresden Dolls disc in her cd player which spins at 1800 revolutions per minute. A compact disc has a mass of about 0.016 kg and a radius of 6.0 cm. a) What is the period of rotation in seconds?
  2. Physics

    A student puts on a Dresden Dolls disc in her cd player which spins at 1800 revolutions per minute. A compact disc has a mass of about 0.016 kg and a radius of 6.0 cm. a) What is the period of rotation in seconds?
  3. Mechanics _ physics

    A uniform bar 1m long is pivoted at the centre has a mass of 3kg. It is acted upon by a conquer which cause to possess an angular acceleration of 6rad/sec^2; calculate a. The moment of inertia of the bar b. The torque on the bar
  4. Physics Classical Mechanics

    A turntable is a uniform disc of mass m and radius R. The turntable is initially spinning clockwise when looked down on from above at a constant frequency f0. The motor is turned off at t=0 and the turntable slows to a stop in time …
  5. Physics Classical Mechanics

    A gyroscope consists of a uniform disc of mass radius M= 2 kg and radius R= 0.2 m . The disc spins with an angular speed ω= 300 rad⋅s-1 as shown in the figure below. The gyroscope precesses, with its axle at an angle 30∘ …
  6. PHYSICS (HELP)

    A gyroscope consists of a uniform disc of mass radius M= 1 kg and radius R= 0.1 m . The disc spins with an angular speed ω= 300 rad⋅s-1 as shown in the figure below. The gyroscope precesses, with its axle at an angle 30∘ …
  7. physics (Urgent)!!!!!!!!!!!!!!!!!!!!!!

    A gyroscope consists of a uniform disc of mass radius M= 1 kg and radius R= 0.3 m . The disc spins with an angular speed ω= 300 rad⋅s-1 as shown in the figure below. The gyroscope precesses, with its axle at an angle 30∘ …
  8. Physics HELP!!

    A gyroscope consists of a uniform disc of mass radius M= 1 kg and radius R= 0.3 m . The disc spins with an angular speed ω= 200 rad⋅s-1 as shown in the figure below. The gyroscope precesses, with its axle at an angle 30∘ …
  9. Physics Classical Mechanics

    Three smooth balls of iron of mass m and radius R are placed inside a tube of diameter 3R (see Figure). Find the magnitude of the forces (A, B, C and D) exerted by the sides of the container on each ball. Write your answers in terms …
  10. mit

    1 point possible) A uniform disc of radius R and mass m1 is rotating with an angular speed ωi about an axis passing through its center and perpendicular to the disc's plane. A small box of mass m2, initially at the center of the …

More Similar Questions