What is Disk: Definition and 816 Discussions

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  1. S

    Rotation: Speed of an object as it slips off a rotating disk

    Homework Statement A 75g mass sits 75cm from the center of a rotating platform undergoing a uniform angular acceleration of 0.125rad/s . The coefficient of static friction between the mass and the platform is 0.250. What is the speed of the mass when is slides off? A. 0.889 m/s B. 1.26...
  2. H

    Charge density of a disk with radius a in cylindrical coordinates

    To write the uniform charge density of a disk with radius a in cylindrical coordinates, If we do this form: \rho (x)=\frac{A\delta(z)\Theta (a-\rho)}{\rho} (A is constant that sholud be determined and \theta is step function), we get A=\frac{Q}{2\pi a} and so: \rho (x)=\frac{\frac{Q}{2\pi...
  3. Rugile

    Chain and spinning disk problem

    Homework Statement A chain is wrapped around a disk of radius R. The tension of chain is T. What is the coefficient of friction, if when the disk is spinning at angular velocity ω, the chain slips down? See image attached. Homework Equations II Newton law a_{centripetal} =...
  4. P

    2013 edits to the sci.physics.faq Rotating Rigid Disk in Relativity

    2013 edits to the sci.physics.faq "Rotating Rigid Disk in Relativity" I was just reading the sci.physics.faq http://math.ucr.edu/home/baez/physics/Relativity/SR/rigid_disk.html on the rotating disk. I was really dissapointed by the recent (2013) editorial comments. I felt that they totally...
  5. N

    The rotating disk of Albert Einstein

    Hello To demonstrate that we can deduce from special relativity the existence of an observer who finds that his three-dimensional space is not euclidean (a conclusion that is false in my opinion), in subsection "Behaviour of Clocks and Measuring Rods on a Rotating Body of Reference" of his...
  6. C

    Induced Charge on a Conducting Disk

    Homework Statement A large, thin plastic disk with radius R = 1.1 meter carries a uniformly distributed charge of Q = -7e–5 C. A circular piece of aluminum foil is placed d = 3 mm from the disk, parallel to the disk. The foil has a radius of r = 6 cm and a thickness t = 1 millimeter. a. Find...
  7. C

    Minimum distance between a disk in 3d space and a point above the disk

    Hi all, How can I calculate the minimum distance between the perimeter of a disk in 3d space and a point above the disk? (the point can be inside or outside the area above the disk) I've been trying to work this out for a while, but I'm getting no where. For example, a point at (1,1,1)...
  8. C

    How to calculate the solid angle subtended by an off axis disk

    Hi, It's surprising how little information is available on this topic, considering it seems like such a fundamental problem. The only tutorial I have found is http://www.umich.edu/~ners312/Course%20Library/SolidAngleOfADiskOffAxis.pdf, and my university does not have access to the other papers...
  9. E

    Interval ds^2 on Rotating Disk - Explained

    What is ds^2 on a rotating disk?
  10. R

    Induced potential difference for disk spinning in magnetic field

    Homework Statement A solid conducting disk of radius a rotates about its symmetry axis with angular speed ω rads/s. If there is a uniform magnetic field \mathbf{B} perpendicular to the disk derive an expression for the potential difference induced between the centre of the disk and its rim...
  11. S

    Electric Potential on z-axis due to circular disk

    Homework Statement Hey guys, I am a little bit confused how my textbook arrived at the answer it did for this particular question. The disk is in the xy-plane with radius b and uniform charge density. They go V=\frac{\rho_S}{4πε}\int_0 ^{2π}\int_0...
  12. E

    Mass: Accretion Disk vs. Black Hole

    I asked a question recently about orbiting black holes. Thanks for the answers. So if I'm correct in my thinking, long before the event horizons of orbiting black holes become close to each other, the two accretion disks get mightily disrupted and much of the mass of the two disks would fall...
  13. Y

    Question on Heat problem in a disk

    Homework Statement This is a question in the book to solve Heat Problem \frac{\partial \;u}{\partial\; t}=\frac{\partial^2 u}{\partial\; r^2}+\frac{1}{r}\frac{\partial\; u}{\partial\;r}+\frac{1}{r^2}\frac{\partial^2 u}{\partial \theta^2} With 0<r<1, 0<\theta<2\pi, t>0. And...
  14. zoobyshoe

    50 ft. Rotating Ice Disk Forms in River

    http://www.theverge.com/2013/11/28/5154240/north-dakota-river-ice-circle-is-50-foot-wide
  15. Avi Nandi

    How Do You Relate the Accelerations in a Disk and Pulley System?

    Homework Statement A disk of mass M and radius R unwinds from a tape wrapped around it. The tape passes over a frictionless pulley and mass m is suspended from the other end. Assume that the disk drops vertically. a. relate the accelerations of mass m and disk ,a and A, respectively to...
  16. T

    Ehrenfest paradox debate, how does a pinning disk look like?

    I was in a debate with a guy about "how does a spinning disk may look like in relativity." His point was that the disk will be flat as always. But the radius will be curved, i couldn't actually imagine it. He gave me a picture too http://casa.colorado.edu/~ajsh/sr/cartbig_gif.html So i...
  17. J

    How to Calculate Moment of Inertia for a Disk Using Calculus?

    I know that the moment of inertia of a disk about the axis perpendicular to the plane of the disk is I=1/2 MR^2 and by the perpendicular axis theorem the moment of inertia about the other axis is I=1/4 MR^2 I want to get the 2nd result with calculus. I can't get the right answer...
  18. Z

    Physics: Torque on a Disk problem - Multiple forces

    Homework Statement A uniform disk with mass m = 9.27 kg and radius R = 1.42 m lies in the x-y plane and centered at the origin. Three forces act in the +y-direction on the disk: 1) a force 344 N at the edge of the disk on the +x-axis, 2) a force 344 N at the edge of the disk on the –y-axis...
  19. P

    Troubleshooting Windows Installation: Converting Dynamic Disk to Basic Type

    I have partitioned my hd into 3 drives. I installed Windows 8.1 on C drive and have used the OS until now. I realize I need windows 7. So I shink D Drive to create another partition - E drive where I would like to install the OS. But I gwt an error message stating that I the partition contains...
  20. D

    Electric field a distance z above flat circular disk.

    Homework Statement Find the electric field a distance above the center of a flat circular disk of radius R, which carries a uniform surface charge σ. Homework Equations The Attempt at a Solution Basically, I want to solve this usually trivial problem without using symmetry arguments and the...
  21. Y

    Recovering Win 8 Computer Without System Disk

    I got my new computer with Win 8 and the Mcafee and another program keep popping up. I went on the control panel and deleted a few of the programs. Now it won't connect to the internet anymore because there is no wireless connection. I might have deleted that accidentally. How do I recover...
  22. S

    Electric Field above a Quarter Disk

    1. Problem Statement. Find the Electrostatic Field at a distance z above the xy-plane and along the z-axis due to a constant surface charge σ confined to the region a < √(x2+y2) < b, 0 < α < pi/2. In other words, find the Electric Field a distance z above a quarter disk occupying the...
  23. T

    Why Does Integration Differ Between Electric Fields of a Disk and a Line Charge?

    Homework Statement I am suppose to find an expression for the electric field of a ring. Homework Equations E =\frac{Kq}{r^2}The Attempt at a Solution I calculated my results and I reached up to this: \frac{Kx\Delta q}{(R^2 + x^2)}^{3/2} where R = radius, x = distance, K = constant, q =...
  24. P

    Rotating Disk Spinoff: Is 3D Timelike Congruence Born Rigid?

    Let me apologize in advance for not reading the entire rotating disk thread. I think that the following question is closely related, but if it was answered in that thread, I didn't spot it. Let us consider the following timelike congruences, which maps congruence parameters t,r,theta and z...
  25. P

    What is the Angular Momentum of a Spinning Disk Subject to Applied Forces?

    Homework Statement A solid disc of mass m = 5.0 kg and radius r = 0.45 m is spinning clockwise about the axis passing through the center of the disc. At t = 0, three forces are applied to the disc as shown. Part 1. What is the magnitude of the net torque acting on the disc about its...
  26. D

    Disk radial change with temperature rise

    Hi guys, Heard an interview question from a friend, the guy asked him what happens to a disk with internal radius r and external R such as the following: When heated. He told me the answer is that the internal radius gets smaller and the external gets bigger. Can anyone help me understand why...
  27. B

    Electric field from a charged disk

    This is rather strange and has been bugging me, from chapter 22 (Electric fields) of Resnick & Halliday: So suppose you have a charged disk of radius ##R## (the disk has no thickness and charge is uniformly spread on the surface of the disk). If you are at a distance ##z## away from the...
  28. T

    Exploring Dark Matter: The Gravitational Field Strength Within a Thin Disk

    I'm researching dark matter and how it affects galaxy rotation curves, I came up with the problem below. Imagine a very thin, flat disk which has uniform mass per unit area. What is the gravitational field strength (g) within the disk itself? How does g vary with respect to r, the...
  29. B

    Electric Field Due To A Ring and Disk

    Homework Statement Assume a uniformly charged ring of radius R and charge Q produces an electric field Ering at a point P on its axis, at a distance x away from the center of the ring. Now the same charge Q is spread uniformly over a circular area the ring encloses, forming a flat disk of...
  30. Avi Nandi

    Time period of oscillation of a physical pendulum and spinning disk

    Homework Statement Find the period of a pendulum consisting of a disk of mass M and radius R fixed to the end of the rod of length l and mass m. How does the period change if the disk is mounted to the rod by a friction less bearing so that it is perfectly free to spin? The centre of the...
  31. J

    Disk Washer Method: Vol of Solid Generated by y=1/sqrt(x)

    Homework Statement Find the volume of the solid generated by revolving the region bounded by y=1/sqrt(x) and y=0 for 1≤x≤2 about the line y=-1 Homework Equations Disk Method? ∏ * ∫[f(x)]^2 dx The Attempt at a Solution A and B (lower and upper limits will be 1 and 2)? ∏ *...
  32. G

    Faraday's law on rotating disk

    Homework Statement A metal disk is rotating with constant angular velocity in a constant magnetic field perpendicular to it. Use Faraday's law to fint the the induced voltage difference between the two points on the wire. The attempt at a solution So to use Faraday's Law, I need to...
  33. A

    Disjoint dyadic squares in unit disk

    Homework Statement Given ε > 0, show that there is a collection of disjoint dyadic squares in the unit disk that has a total area which exceeds π - ε. Homework Equations Define a dyadic interval as an interval of the form [a, b] such that a = p/2k and b = (p + 1)/2k, p and k are integers. A...
  34. L

    Direction of Axle Force on Pivoted Disk

    Hello everyone, I came across this problem from my Physics textbook. I do not understand why the direction of the axle force is pointing downward. I think the upper part of the disk is exerting a downward force on the pivot (as it is pulled down by the Earth's gravity), and according...
  35. E

    Rotating disk/ sphere and moving mass along it

    Coriolis effect - In a non-friction system, f I roll something along the surface of the planet from on of the poles to the equator, it will appear to move to the west, it will essentially stay behind the planets rotation and actually rotate it in the opposite direction. Now, if we add friction...
  36. U

    Airy disk and resolution in confocal microscopy

    Hi all, I have a question about airy patterns and how these determine resolution in images. So to increase resolution the airy pattern should be made smaller. In microscopy this is achieved by using higher numerical aperture lenses and shorter wavelengths. I have also read that the more...
  37. H

    Angular momentum in ElectroMagnetic fields(Feynman's Disk Paradox)

    In Griffiths book, "Introduction to Electrodynamics" example 8.4 he calculates the angular momentum density for a set up that is a version of Feynman disk paradox. His answer for the angular momentum points in the z direction. But if we you assume that the r vector has component in the s...
  38. N

    Solving Solid Disk Rolling Up Incline: Find Height h

    Homework Statement A solid disk of mass m and radius R rolls without slipping with a velocity v. Assuming it doesn't slip, how far vertically will it roll up an incline? Homework Equations I=0.5mr2 E=0.5Iω2 KE=0.5mv2 PE=mgh The Attempt at a Solution I'm thinking that we need to...
  39. R

    Rotating disk dropped onto another rotating disk

    Homework Statement A wheel, mounted on a vertical shaft of negligible rotational inertia, is rotating at 500 rpm (CCW from above). Part a) asks to find the new angular velocity if an identical wheel is dropped onto the shaft. I got this part right. Part b) is: Now suppose the dropped wheel...
  40. N

    Find the Angular Acceleration with and Without the disk inertia

    Homework Statement Determine the angular acceleration of the uniform disk if (a) the rotational inertia of the disk is ignored and (b) the inertia of the disk is considered. The system is released from rest, the cord does not slip on the disk, and bearing friction at O may be neglected. The...
  41. K

    Use spherical cord to compute area of a disk

    Homework Statement Use spherical cords to compute area of a disk, that's center at x,y=0, and z=4, having a radius of 3 Homework Equations I set up a triple ∫∫∫ (r^2* sin(theta)), running from phi =0 to 2pi, theta=0 to arcsin(3/5), r=5sin(theta) to 5. The Attempt at a Solution It doesn't...
  42. R

    Why is there current induced on Faraday's disk?

    Because according to Faraday's law there should be a change in the flux to generate electric potential for a current to take place. But in that case no flux change happens. This can be reasonably explained by Lorentz force(charge separation due to radial Lorentz force) but can we still apply...
  43. B

    Disk on a Motion: Find the Coordinates of Point A at t= 4piR/v

    Homework Statement Adisk of radius R centred at the origin is placed on a frictionless x-y plane with origin as the centre of the disk.At t=0, the disk is given initial motion such that the point A(R,0) has velocity v towards +ve x-axis and ω=v/2R. Find the coordinates of Point A at t=...
  44. A

    Mass of disk given mass per unit

    Homework Statement I need to find the mass of a disk with a hole, given inner and outer diameters of 0.10m and 0.30m, respectively. The disk has a mass per unit of 20 \frac{kg}{m^2}. The Attempt at a Solution Since I needed mass units, I thought that I could multiply the mass per unit...
  45. C

    Moment of Inertia of Compound Disk

    Homework Statement A compound disk of outside diameter 152 cm is made up of a uniform solid disk of radius 41.0 cm and area density 3.30 g/cm2 surrounded by a concentric ring of inner radius 41.0 cm , outer radius 76.0 cm , and area density 2.10 g/cm2 . Find the moment of inertia of this...
  46. E

    Equation of motion, rolling disk

    I'm trying to get the position vector of the offset mass relative to the inertial frame of reference. Do I account for the fact that the disk has rolled a little bit with respect to the inertial frame of reference? With that, I have r,vector = d n1 + R n2 + R/2 er = R*theta n1 + R n2 + R/2 er...
  47. N

    Motion of a ball along a groove on a rotating disk

    Will a ball placed tightly (radius of ball=width of groove) in a groove(length of grove along radius) on a rotating disk have any motion along the groove. The frictional force is zero.
  48. C

    A rotating disk with two attached masses that slide without friction

    Homework Statement A disk rotates with angular velocity w. Two masses, Ma and Mb, slide without friction in a groove passing through the cnter of the disk. They are connected by a light string of length L, and are initially held in position by a catch, with mass Ma at distance Ra from the...
  49. twoski

    Find the Volume of a Rotating Region: Washer & Disk Method

    Homework Statement Using disks or washers, find the volume of the solid obtained by rotating the region bounded by the curves y = sec(x), y = 1, x = 1, x = -1 on the x-axis. The Attempt at a Solution This should be ridiculously easy but apparently my answer is wrong?! To calculate the...
  50. A

    Using the V-K Thm to find fundamental grp of sphere union disk in R3

    Hi, I am trying to get my head around the Van Kampen Theorem, and how this could be applied to find the fundamental group of X = the union of the unit sphere S2 in R3 and the unit disk in x-y plane? I was thinking of splitting the sphere into 3 regions - two spherical caps each having open...
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