What is Disk: Definition and 816 Discussions

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  1. B

    Heat dissipated by a steel disk

    Hi everyone This is a quick Q but I don't understand why I got it wrong This is what I have done Could anyone let me know where I went wrong? Thanks for any help!
  2. yyfeng

    Simple Electric Field due to a Charged Disk

    My attempt at a solution is shown in attached file "work for #10.png". I used Desmos Scientific online calculator to obtain my final answer.
  3. madafo3435

    Electric field due to a flat hollow disk

    I would like if my procedure is correct ... Due to the symmetry of the problem, I only worry about the vertical coordinate of the field, so I will work with the magnitude of the field, and I will treat the problem in polar coordinates. ##E= \int_{R_1} ^ {R_2} \int_{0} ^ {\pi} \frac {\sigma...
  4. Another

    I Problem about uniform thin disk

    Why do they only think of kinetic energy of motion? Why don't they think of both kinetic of motion and kinetic of rolling energy? So. i think ## L = \frac{1}{2}mv^2 + \frac{1}{2}I \omega^2## ## L = \frac{1}{2}m(r \omega)^2 + \frac{1}{2}(\frac{1}{2}mr^2) \omega^2## ## L = \frac{3}{4} mr^2...
  5. sbrothy

    B Popular physics - The Nebra disk

    Popular Physics. "Evidences of the earliest Supernova observation in the Nebra disk R.G. Pizzone" ---- https://arxiv.org/abs/2005.07411I know this isn't exactly breaking news and I cannot comment of the quality or validity of the research, but I've found that due to the brevity of the paper...
  6. burian

    How can I find the velocity of a point on a disk rotating in a disc?

    From a freebody analysis I got, $$ \vec{r} \times \vec{F} = |r| |F| \sin( 90 - \theta) = (R-r) mg \cos \theta$$ and, this is equal to $$ I \alpha_1$$ where the alpha_1 is the angular acceleration of center of mass of small circle around big one, $$ I \alpha = (R-r) mg \cos \theta$$ Now...
  7. John100861

    David Morin classical mechanics Problem 2.6: Disk held up by a massless string

    The first part is easy, we have 2T= Mg T= 0.5 Mg Now for the second part where I'm having trouble understanding Morin's solution: I take the normal force on a small circle arc to be N, we know that the y component of the normal force must be balance with Mg for the whole disk, therefore Ny =...
  8. L

    Magnetic field of a rotating disk with a non-uniform volume charge

    -------------------------------------------------------------------------------------------------------------------------------------------------- This was a problem introduced during my classical electrodynamics course. I am not 100% sure, but I think I've solved up to problems (a) and (b) as...
  9. Ssnow

    I Schrodinger equation on the complex disk

    Hi to all member of the Physics Forums. I have this question: it is possible consider the analogue of the Schrodinger equation on the plane with configuration space ##(x,p)\in\mathbb{R}^4## on the complex disk ##\mathbb{D}=\{z\in\mathbb{C}: |z|<1\}##? Ssnow
  10. LCSphysicist

    A block and a disk connected by a rope

    Basically, there is a gravitational field g and a friction force acting on the cylinder, which does not slip in the plane. The question is, the acceleration of m'. I solve this question basically by this: " m*g*dh*sina + m'*g*dh = m*v1*dv1 + m'*v2*dv2 + I*w1*dw1 " {1} w*r = v1' + v2' So The...
  11. LCSphysicist

    Find the period of a pendulum consisting of a disk of mass M

    I did the first step pretty well, and the answers match, but i don't understand why in the second case the period changes.
  12. T

    I A cold, massive, rotating disk galaxy 1.5 billion years after the BB

    Neeleman, M., Prochaska, J.X., Kanekar, N. et al. A cold, massive, rotating disk galaxy 1.5 billion years after the Big Bang. Nature 581, 269–272 (2020). https://doi.org/10.1038/s41586-020-2276-y Abstract Massive disk galaxies like the Milky Way are expected to form at late times in traditional...
  13. LCSphysicist

    Solid disk rolling down an inclined plane -- Some conceptual questions

    I was always a little confused about the rolling down of a body, let's say, a sphere. It's know that to body rotate, from the rest, in a referential frame on the ground [inertial], is necessary a friction, that will just act like a medium that transforms kinetic energy of translation into...
  14. Leo Liu

    The inelastic collision between a disk and a rotating platform

    A disk is dropped on a platform rotating at a constant angular speed ##\omega_i## as shown below. The question asks whether the final kinetic energy of the platform is conserved. I understand the angular momentum is always conserved provided that the net torque is 0, so I wrote the following...
  15. Serhatakguc0

    Dynamics homework -- Motion of a stick attached to a rolling disk

    Hi everyone, I need help for this homework. I'm a mechatronics engineering student and i want to solve this question but no matter how hard I try, I can't solve the question. Sorry for my bad english...The disk connected to the AB stick with a length of 2 meters is rolled as shown in the figure...
  16. archaic

    Releasing a hollow cylinder on a rotating disk

    There is no net external torque since the cylinder is slipping (no friction), so the angular momentum should be conserved. $$L_f=\frac 12MR^2\omega_i=\frac 12\times3.8\times0.52^2\times50\times\frac{2\pi\times0.52}{60}$$
  17. J

    Rotational Inertia: Hoop vs Disk

    I know that a hoop should have a higher rotational inertia than a solid disk because its mass is distributed further from the axis of rotation. What I don't understand is how a disk of the same mass and radius can have a higher rotational inertia. If the objects roll freely their axes of...
  18. C

    Potential vector (A) of a disk with a surface current

    Hi, I've been stuck for a long time with this exercise. I am not able to calculate the potential vector, since I do not know very well how to pose the itegral, or how to decompose the disk to facilitate the resolution of the problem. I know that because the potential vector must be parallel to...
  19. M

    Kinetic energy of a disk that is rolling and not slipping

    Let ##\Theta## be the angle, following the movement of the center of the disk. In order to find the kinetic energy, we brake the movement of the disk into 2: The translation of the center of mass, and the rotation of the disk around it. So, the kinetic energy will be given by: $$T= \frac 1 2...
  20. F

    Can I use a new SSD Hard Disk to store data without formatting?

    Can I use a brand new removed SSD HardDisk to store data without doing any thing at the beginning time(e.g without formating or other things)?I will use SSD to store ebooks(PDF,Djvu,epub,Audio files). Can I open books when I attach the removed SSD to laptop?
  21. B

    Confusing Derivation of torque acting on disk question

    Not sure how they obtained an answer of (2meukenetic *Mg/R^2)*r^2 dr = dt When they say disk I am assuming it's not ring shaped. So since F * Dr= DT where I guess F acting on a small point on the disc is constant from the table top. And Dr is the variable distance from a random small point from...
  22. A

    How Does Torque Affect Angular Acceleration in a Circular Disk?

    moment of inertia= [(1/2)(1.1kg)(0.96)^2+ (1.1kg) (0.75*0.96)^2]= 1.08 kg*m^2 θ=9.8 degrees= 0.17 rad torque= (mass*gravity) * radius * sin(theta) radius= 0.17rad * 0.96m = 0.16m torque= (1.1kg*9.8m/s^2) * 0.16m * sin(0.17rad) = 0.29 N*m torque = inertia * angular acceleration 0.29N*m=...
  23. M

    A Eigenfunctions of an Airy Disk

    Does anyone know if the eigenfunctions of the Airy disk function (or Bessel function) \frac{J_1(x)}{x} has a closed form?
  24. zizzle

    Minimum initial speed to spin a particle around a disk (with gravity)

    For this problem, since the weight force on the "particle" (child) is not always aligned with the tangential circular path of the disks, I couldn't think of a way to use rotational kinematics equations. As such, I tried to solve the problem using work principles (namely, that the change in...
  25. CrosisBH

    I Why does this needle get an E field while this disk gets a D field?

    So I'm reviewing old lectures to prepare for an exam soon. This is about polarization. E fields, D Fields, etc. My professor labeled this diagram like so. The figures my professor drew are cavities in a dielectric if you can't read her handwriting. However, I can't seem to figure out why the...
  26. A

    What is the correct moment of inertia for a spinning disk?

    Since torque= |force||radius|sin(theta) : î = |71E6||5000m|sin(30)= 177.5E9 N*m ĵ = |0||5000m|sin(30) = 0 k̂ = |-158E6||5000m|sin(30)= 395 N*m
  27. J

    Magnetoresistance and the Corbino disk

    My answers so far are: a) J=I/(2*pi*r*t) b) u=J/ne=I/(2*pi*r*t*n*e) c) F=IB/(2*pi*r*t*n), in theta direction (polar coordinates) d) This is where I am stuck. I understand the example for motional emf with a rod moving through a magnetic field but I'm not sure how to apply it to this scenario...
  28. K

    Rotating and tilting charged disk induces a voltage inside a ring

    As I`` m learning for an upcoming exam I found an electrodynamics problem I struggle with. In the first task I need to calculate the magnetic dipole moment of a uniformly charged,thin disk with the Radius R and a total charge Q which rotates with a angular speed omega round its symmetry axis...
  29. S

    Radius of disk suspended on its edge

    I can calculate the angular speed but I don't know how to calculate the radius. Is there certain formula to calculate it? Thanks
  30. Yalanhar

    Electric Field due a charged disk

    Homework Statement: uniformly charged disk, radius r, with surface charge density ##\sigma## . I want to find the electric field along the axis through the centre of the disk at a h distance Homework Equations: ##dE=\frac {kdq}{r^2}## My Solution: ##dE=\frac {kdq}{r^2}## in this case r=s...
  31. badger1999

    Rotating Disk and Hanging Mass Calcululations

    I used the above equations to solve for tension, torque, inertia, and angular acceleration. Are the formulas I used correct for the given system? How can I calculate Inertia from the trendline equation, I'm drawing a blank.
  32. P

    A Dark Matter Accretion Disk for Black Hole (Neutron Star)

    Hello! Dark Matter (if exists)(if it is in form of particles - collisionless) could also be attracted by Black Holes (or Neutron Stars) and form accretion disk. I would like to discuss possible properties of such hypothetical object. DM particles could be trapped and settled in around...
  33. S

    I How to apply the disk/washer and shell methods

    In Calculus II, we're learning about solids of revolutions and computing their volumes. I'm unsure when to apply the appropriate methods and how to make the correct partitions. Please tell me if my reasoning is correct: The disk/washer method is applied when your partitions are perpendicular...
  34. Zack K

    Potential difference in a 2 disk system (Capacitor)

    Homework Statement A capacitor consists of two large metal disks placed a distance ##s## apart. The radius of each disk is R ## (R \gg s)## and the thickness of each disk is ##t##. The disk on the left has a net charge of ##+Q## and the disk on the right has a net charge of ##-Q##. Calculate...
  35. S

    B Solid angle by placing one disk on a cylinder

    Hello, I am trying to find an analytical expression to determine the solid angle subtended by a disk source onto the face of the cylinder. I will appreciate if someone can provide me directions. I am aware how to calculate solid angle by a point source to cylinder's face ( omega =...
  36. jtbell

    Did a Massive Ice Disk Really Form in Maine's Presumpscot River?

    A giant ice disk, almost perfectly circular, has formed in the Presumpscot River in Westbrook, Maine: https://www.npr.org/2019/01/16/685816019/a-massive-moving-ice-disk-takes-center-stage-mesmerizing-maine As it slowly rotated, contact with one shore smoothed away the irregularities at its edge.
  37. JD_PM

    Conservation of Energy and Angular Momentum in a Rotating Train-Disk System

    Homework Statement [/B] A train stands in the middle of a rotating disk with an initial angular velocity of $\omega_i$. The mass of the train is m and the moment of inertia of the train-disk is I. At one point the train departs on a straight track to a distance R from the disk's centre. (R...
  38. J

    I Why hasn't the Oort cloud converged to a disk shape?

    The Oort cloud is often drawn as a sphere. Everything in it orbits the sun in circles or ellipses. Since the shape is not a disk, collisions are expected. Why hasn't the Oort cloud converged to the more stable, collision-free, disk shape?
  39. WhiteWolf98

    Calculating Dimensions of Titanium Disk Under Load

    Homework Statement A titanium disk (with ##E=107 ~GPa##, Poisson's ratio, ##v=0.34##) precisely ##l_0=8~mm## thick by ##d_0=30~mm## diameter is used as a cover plate in a mechanical loading device. If a ##P=20~kN## load is applied to the disk, calculate the resulting dimensions, ##l_{01}## and...
  40. Dorian

    Solid Disk Pulley + 2 Mass system, v derivation using 2 methods

    Homework Statement [please see attached photo] Homework Equations [please see attached photo] The Attempt at a Solution [please see attached photo] The issue for me starts with (but probably doesn't end with) replicating the velocity equation using the Conservation of Energy equations. Is...
  41. jra12222

    B Force at which a cutting disk comes off from an angle grinder

    Hi all, This is not exactly a homework question, more like for my own safety So I have a project going on and I need to use an angle grinder without the safety guard which will be enclosed in a box (made from plastic most probably). So I decided to check if the plastic box would be able to...
  42. F

    Why Do Moments Balance in a Faraday Disk but Not the Forces?

    Hi to everybody! Please: can you help with this problem? A conductive disk with a radius a = 20 cm, with negligible moment of inertia, rotates around its horizontal axis. The disc region around the radius is inserted in a magnetic field B = 0.75 T perpendicular to the disk itself. A mass is...
  43. J

    Small oscillation frequency of rod and disk pendulum

    Homework Statement Consider a rod of length ##L## and mass ##M## attached on one end to the ceiling and on the other end to the edge of a disk of radius ##r## and mass ##m##. This system is slightly moved away from the vertical and let go. Let ##\theta## be the angle the pendulum makes with the...
  44. cookiemnstr510510

    Derivation of electric potential charged disk from ring

    Homework Statement a)recall that electric potential is a scalar quantity. For a circular ring of radius, R', carrying charge, Q, what is the electric potential at a height,y, above the center of the ring? b)Use your above answer to determine the electric potential at a height,y, above the...
  45. A

    Finite dual disk capacitor: estimating charge distribution

    Homework Statement Working through Purcell (among others) as fun applied math/math modeling refresher. But, I have struggled all week in establishing from first principles that the potential/field/distribution for a configuration of two capacitive disks of radius 1 and separation s along the...
  46. MPavsic

    Dynamic reaction versus Coriolis acceleration of the disk

    << Thread moved from the technical forums, so no Homework Help Template is shown >> I have mind boggling problem, trying to distinguish between Dynamic reaction versus Coriolis acceleration of thin hoop. Problem: A thin, hoop of mass m and radius r spins at the constant rate ω 1 about an axle...
  47. cookiemnstr510510

    Electric field above a disk that equals half the max electric field

    Homework Statement At what distance, a, along the axis of a uniformly charged disk of radius R is the axial electric field strength equal to one half its value at the surface of the disk at the center. Homework Equations electric field of disk (attached) The Attempt at a Solution First I...
  48. J

    Vector torque problem: Force applied to a disc

    Homework Statement A force of magnitude 50N is applied at the bottom point Q of a disk of radius 8m that is pinned at P (leftŸmost point) See attached picture (a) Find the angle between the force and the vector from P to Q. (b) Find the magnitude of the applied torque. Homework Equations...
  49. evinda

    MHB Solve Laplace equation on unit disk

    Hello! (Wave) I want to solve the Laplace equation on the unit disk, with boundary data $u(\theta)=\cos{\theta}$ on the unit circle $\{ r=1, 0 \leq \theta<2 \pi\}$. I also want to prove that little oscillations of the above boundary data give little oscillations of the corresponding solution of...
  50. phinds

    I Dark matter "halo" is a "flattened disk" ?

    I apologize for the fact that this information is 2nd hand but a very intelligent and reliable friend tells me that in “Dark Matter and the Dinosaurs” by Lisa Randall she hypothesizes a disk of dark matter, which she calls “Double-Disk Dark Matter (DDDM)” that is superimposed over the center of...
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