Disk Definition and 819 Threads

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  1. J

    Faraday's Disk Dynamo: why is there an emf?

    Consider a rotating disk on a uniform magnetic field. Faraday's law states that \epsilon = - \frac{d\phi}{dt} In this situation, \vec B is constant and the area of the disk is constant. Hence, the magnetic flux is constant and there should be no emf. What am I missing?
  2. A

    Which Way Will the Disk Rotate With Two Hanging Masses?

    Homework Statement A circular disk has two masses acting on it, m1=10g, m2=50g. The forces caused by these masses are 4.5cm (F1) and 6.8cm (F2) away from the axis of rotation (center). The angles are 130 degrees (F1, and on outer side away from center) and 98 degrees (F2, also on outer edge...
  3. D

    Induced Current Density On Disk Due to Changing Magnetic Field

    Homework Statement A very thin conducting disc of radius a and conductivity \sigma lies in the x-y plane with the origin at its center. A spatially uniform induction is present and given by B=B_0 cos\(\omega t \)\hat{z}. Find the induced current density \vec{J} in the disc. Homework...
  4. T

    Magnetic field above rotating disk

    Homework Statement What is the magnetic field a distance z above a rotating disk (ang.velocity w) with surface charge density inversely proportional to the square of the distance from the centre of the disk. Homework Equations K = v*surface charge density B = (constant)*integral of...
  5. N

    Give a formula the open unit disk

    Homework Statement In the open unit disk D = {(x,y)|x2 + y2 < 1} give a forumla for: (a) \frac{\partial \sum ^{inf}_{n=0} (xy) ^{n}}{\partial x} (b) \frac{\partial (e^{x+y} \int ^{y}_{x} e^{-t} dt)}{\partial y} Homework Equations The Attempt at a Solution...
  6. N

    How Can I Simulate an Accretion Disk Using the Naviar-Strokes Equations?

    I've recently been messing around with the ol' Naviar-Strokes equations, creating a fluid simulator with the help of Mr. Jos Stam. I added vorticity confinement, which made for some really beautiful visual effects. I noticed these appeared similar to an accretion disk. I'd like to model an...
  7. S

    Supernova energy reached by detector disk 1560 light years away

    Homework Statement A super nova releases 1.9E45 J of energy over a 10 day period. It is 1560 ly from earth. A detector, facing the star, is a disk of radius 7cm. How much energy reaches the detector? (The detector is in orbit, so it can face the star for the entire 10-day period) Homework...
  8. B

    Calculating Resistance of a Graphite Disk

    Resistance=resisivity(of a certain material)Xlength/cross-sectional area resisivity is expressed in ohm-cm I have a graphite material that is 6ohm-cm the material is .032" thick I will cut the graphite material into a 3' disk I will place an electrode in the center of the disk and have...
  9. B

    Conservation of momentum, spinning disk

    Assumptions: 2D problem only, no gravity, no friction, no external torque. Flat circular disk spinning at a constant angular velocity w. Disk is initially at (0,0), but is NOT restrained in translation. Disk has some finite mass. A small ball (finite mass) is tethered to a string which...
  10. F

    Number of revolutions a disk has made after a given time

    [b]1. A computer hard disk starts from rest, then speeds up with angular acceleration of 218 rad/s^2 until it reaches its final angular speed of 6660rpm. How many revolutions has the disk made 10.1s after it starts up? [b]3. The Attempt at a Solution . I have no idea how to find the...
  11. J

    Method of disk (in respect to y)

    Homework Statement Revolve the region under the curve y =√x on the interval [0, 4] about the x-axis and find the volume of the resulting solid of revolution. YET, NOW, I want to do in respect to y Homework Equations integral formula The Attempt at a Solution I did dx and I...
  12. C

    Calculating Net Torque of Uniform Disk

    The circular blade on a radial arm saw is turning at 265 rad/s at the instant the motor is turned off. In 12.0 s the speed of the blade is reduced to 85 rad/s. Assume the blade to be a uniform solid disk of radius 0.140 m and mass 0.400 kg. Find the net torque applied to the blade. Relevant...
  13. P

    Volume of Rotated Shapes: Finding the Volume using the Disk Method

    Homework Statement Use the functions: y=x+1, y=0, x=0, x=2 Find the volume when rotated around y=3 Find the volume when rotated around y=-1 Homework Equations PI\int R2dx or perhaps PI\int R2 - r2dx The Attempt at a Solution i don't think that this is a particularly hard question, but I...
  14. C

    Spinning disk approaching c, what will happen?

    1. a clever engineer solves the high velocity exploding flywheel problem (structure fails) and any other serial problems. 2. therefore a bench top a. torus, b. disk, c. cylinder spinning about its axis with a tip speed approaching c 3. what will happen to space time and gravity or anything...
  15. T

    Stargazing Could a very large telescope see a star as a disk?

    Hi When I was a kid, the largest telescope was Mt Palomar, a little over 5 meter diameter mirror. In spite of its large size, I always read that stars appeared as points of light through the scope if it were used visually, i.e. stars are too small and too distant to appear as disks. However...
  16. B

    Determine period for off center disk

    Homework Statement A disk of mass m and radius a is suspended horizontally by a rod bored through the disk at a location d distance from the center. When in equilibrium the center of the disk is directly below the rod. Obtain an approximate expression for the period, ignore disapative...
  17. B

    Is the Polarity of the Induced Voltage in Faraday's Disk Correct?

    Homework Statement The problem can be found http://whites.sdsmt.edu/classes/ee382/homework/382Homework3.pdf" . It is the first one. Homework Equations V_{emf} = \oint_L (\vec{\boldsymbol u} \times \vec{\boldsymbol B}) \cdot d\vec{\boldsymbol l} The Attempt at a Solution...
  18. N

    Calculating Net Torque for Disk in Diagram D

    Homework Statement http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys211/oldexams/exam3/sp08/fig10.gif What's the net torques about the pivot, represented by the dot (•), in the diagram D. The solid arrows represent the magnitude and direction of the forces...
  19. M

    How Does Charge Distribution Affect Electric Field Calculation on a Disk?

    Homework Statement A charge of -5.00 nC is spread uniformly over the surface of one face of a nonconducting disk of radius 1.15 cm. Find the magnitude of the electric field this disk produces at a point P on the axis of the disk a distance of 3.00 cm from its center.Homework Equations Surface...
  20. Hootenanny

    Solve Unperturbed Field on Rectangle w/ Small Disk

    Homework Statement The overall question is to construct an asymptotic approximation for a harmonic field on a rectangle with a small disk. However, I'm having difficulty finding the unperturbed field. Perhaps I've been staring at it for too long, but I can't seem to find a solution. The...
  21. D

    Electric Field of a Charged Disk

    I'm having a brain freeze and have a hard time understanding why the area of the ring is 2*pi*a*da. Can someone explain why it is the circumference times da? Link to derivation: www.phys.uri.edu/~gerhard/PHY204/tsl36.pdf
  22. K

    Tension producing torque on a disk

    When a string is wrapped around a disk and pulled taut to rotate the disk, such as in a pulley, how can tension be the force producing the torque? Tension would only act upon the string next to the disk. I suppose there would be the normal force of the disk on the string, but that wouldn't...
  23. F

    Null geodesics of light from a black hole accretion disk

    Sorry I don't know latex so this may look a little messy. Homework Statement I'm trying to solve the equation for null geodesics of light traveling from a rotating black hole accretion disk to an observer at r = infinity. The point of emission for each photon is given by co-ordinates r, phi...
  24. G

    What is the Angular Frequency of Small Oscillations on a Nonuniform Disk?

    Well, I had a couple problems on my final I was hoping to go over- hope nobody minds. Here's the second. Homework Statement A nonuniform disk of radius R and mass m has the center of mass at a distance A from the geometrical center. Its moment of inertia about the axis passing through the...
  25. V

    How Do Accretion Disks Around Black Holes Form and Function?

    How is it formed? How do particles move on it?
  26. N

    Reaction force on disk by pivot

    Homework Statement A uniform disk of mass M and radius R is pivoted about a point on its edge. The disk is released from rest when its center of mass is at the same height as the pivot. What is the reaction force exerted on the disk by the pivot the initial instant the disk is released? The...
  27. T

    Uniformly-charged disk electric field at ANY point (i.e., off-axis)?

    Homework Statement I need to find the equation for the electric field intensity \vec{E}(\vec{R}) of a uniformly-charged disk lying in the xy-plane with radius \rho_{0} and surface charge density \rho_{s}' at any point. Homework Equations I know that a charged disk doesn't have the symmetry...
  28. D

    Moment of Inertia of a Disk and mass

    Homework Statement Figure (a) shows a disk that can rotate about an axis at a radial distance h from the center of the disk. Figure (b) gives the rotational inertia I of the disk about the axis as a function of that distance h, from the center out to the edge of the disk. The scale on the I...
  29. P

    Formula for total KE of disk rolling across table 8 m/s?

    Homework Statement A solid disk (mass = 5 kg, R=0.9 m) is rolling across a table with a translational speed of 8 m/s. What is the total KE of the disk? Homework Equations K = the sum of (1/2 x mass initial x velocity initial^2) (in my textbook, there are other derivations of this formula...
  30. N

    Rolling motion of an unbalanced disk

    Homework Statement The mass center G of a 5kg wheel of radius R = 300 mm is located at a distance r = 100 mm from its geometric center C. The centroidal radius of gyration is k = 150 mm. As the wheel rolls without sliding, its angular velocity varies and it is observed that it is = 8 rad/s in...
  31. J

    Why Does Penrose Modify the Hyperbolic Distance Formula with Lambert's Constant?

    For better or worse, I've begun to work Penrose's book Road to Reality. In the second chapter, one of his exercises (supposedly simple) has entirely eluded me. Penrose presents the conformal Poincaré disk and gives the equation for hyperbolic distance between two points A and B inside the...
  32. B

    Question about charged disk and current loop similarity

    I have a question regarding the similarity between the Biot-Savart law and the equation that gives the magnetic field inside a current loop, and the similar version of Coloumb's law that gives the electric field due to a charged ring. In the equation for a charged ring at...
  33. T

    Radiative transfer equation: Intensity from a disk.

    Hello to everyone who reads, and thank you very much. 1. Problem: The actual problem is this: A thin disk with thickness 2H and radius R (H<<R), has an absorption co. "alpha" and emision co. j. Calculate the intensity I("miu", "tau"), where "miu" = cos("theta"), and "theta" is the angle...
  34. D

    What is the Angular Momentum of a Rotating Disk?

    I need some help with this problem. Any help would be greatly appreciated. It is due tonight. http://session.masteringphysics.com/problemAsset/1035644/7/knight_Figure_13_47.jpg Use the link above to solve the problem. Part 1) What is the magnitude of the angular momentum of the 3.1 kg...
  35. C

    Volume under a paraboloid and above a disk

    Ok its a simple question really... say that I have to find the volume (using polar coordinates) of the solid under the paraboloid z=x^2+y^2 and above the disk x^2+y^2≤9. My approach would be to find the z value of where the cylinder and paraboloid intersect. Then find the volume of the...
  36. M

    Disk acceleration physics homework

    Homework Statement A force of 2.2N acts tangentially to the rim of a solid 52kg disk, with a radius of 32cm. How long does it take to accelerate the disk, rotating about an axis though it's center and perpendicular to it's plane, from rest to 210 rev/min? Homework Equations Idisk=.5mr^2...
  37. J

    Mass being dropped on spinning disk problem NEED HELP

    Mass being dropped on spinning disk problem NEED HELP! 1. A disk has a radius of 1.25m and a mass of 10kg. It lies flat on a smooth table and is spinning with an initial velocity of 10 rad/s about a pin at its center. A 5kg mass is dropped on it at a distance of 0.50m from its center. Find...
  38. H

    What is the Friction Force on a Spinning Disk?

    Homework Statement A 3.10 kg, 31.0-cm-diameter disk in the figure is spinning at 300 rpm, how much force must a brake apply that is perpendicular to the rim of the disk to bring the disc to a halt in 2.4 s.Homework Equations t=i*alpha wf=0+alpha*t t=fr*sin(theta)The Attempt at a Solution...
  39. M

    Uniform roating disk - find inertia

    A uniform disk of mass Mdisk = 4 kg and radius R = 0.22 m has a small block of mass mblock = 2.1 kg on its rim. It rotates about an axis a distance d = 0.17 m from its center intersecting the disk along the radius on which the block is situated. a) What is the moment of inertia of the block...
  40. S

    Is the Surface S a Disk Parallel to the yOz Plane?

    Homework Statement Given the vectorfield \textbf{F}(r) = y^2\bold{i} + z^2\bold{j} + x^2\bold{k} and the surface S defined by a mapping (\rho,\phi) \in [0,1] \times [0,2\pi] \mapsto \bold{r}(\rho,\phi) = 4\bold{i} + (1+\rho cos(\phi))\bold{j} + (2+\rho sin(\phi))\bold{k}...
  41. M

    Rolling puck on a spinning disk

    Homework Statement A small puck is placed on a disk spinning with angular speed OMEGA around axis perpendicular to the disk and passing through its center. The coefficient of static friction between the disk and the puck is MU. At what distance(s) from the center of the disk will the puck...
  42. A

    Comparing Rotational and Linear Motion - block vs. disk vs. ring

    Comparing Rotational and Linear Motion -- block vs. disk vs. ring Homework Statement A disk and a ring, both of mass M and radius R, are placed atop an incline and allowed to roll down. A block, also of mass M, is placed atop the same frictionless incline and allowed to slide down. How do...
  43. S

    Moment of inertia of a disk by integration

    Homework Statement Show that the moment of inertia of a disk is 0.5 mr^2 . The Attempt at a Solution I = \int R^2 dm Using dm = \lambda dr such that m = \lambda r : = \int_{-r}^{r} R^2 \lambda dr = \frac { \lambda } {3} ( 2r^3 ) = \frac {2} {3} (\lambda r ) (r^2) = \frac {2}...
  44. E

    Disk with Hole Parallel Axis Thm problem

    Greetings everyone, been having some problems with this question, A uniform circular disk has radius 36 cm and mass 350 g and its center is at the origin. Then a circular hole of radius 7.2 cm is cut out of it. The center of the hole is a distance 10.8 cm from the center of the disk. Find the...
  45. R

    Need help analyzing the failure of a rotating disk

    Homework Statement hi, i'm working on a project for a materials class. we chose to analyze a spacer in a jet engine that catastrophically failed. The spacer failed initially due to rubbing between it and a vane case on the outer most radius of the spacer. We're not analyzing the vane case...
  46. B

    Is It Zero? Tangential and Centripetal Acceleration on a Rotating Disk

    rotating disk short question! Homework Statement If object is rotating on a disk with a constant angular speed, which of the following is zero? the object's tangential acceleration the object's centripetal acceleration both (a) and (b) are zero niether (a) or (b) are zero...
  47. Somefantastik

    Airy Disk Patterns: Quality Criteria

    What criteria is used to judge the quality of an airy disk pattern?
  48. T

    Is My Disk Method Calculation Correct for Finding Volume?

    Homework Statement Find the volume of the solid generated by revolving the shaded region about the given axis. Homework Equations I used the disk method to solve this V = int from a to b(pi * R(x)^2) The Attempt at a Solution V = int from 0 to 2( pi * (3Y/2)^2 dy) = 6pi
  49. I

    How Does a Charged Disk Influence a Point Charge?

    Homework Statement A disk is located at 0<r<1, z = 1. It has a uniform charge distribution of 200pC/m^2. A 30 \muC point charge is located at the origin. Determine the force acting on the point charge due to the electric dield produced by the disk. Homework Equations E =...
  50. B

    Moment of Inertia of half disk

    Hello, just found this forum didn't know it existed. hopefully someone can help with this. I am trying to figure out the mass moment of inertia for a half disk spining around the center axis. A full disk is I=1/2*M*R^2. Would the inertia for spining a half disk around the center axis just...
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