# What is Disk: Definition and 816 Discussions

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1. ### A small disk and a bigger half disk

I need to find ##v_1## and I know what are the initial conditions: ##\theta(0)=\pi## and ##\dot{\theta}(0)=0##. Then what is ##v_1## and how to find it? Thanks!
2. ### Potential and Electric Field near a Charged CD Disk

Hi! I am a very lost physics student here. I got a) but I have no idea how. The formula I used was from an online source and it was: I think I need a contextual explanation of this formula before I attempt b). My understanding of electric potential is that it is NOT potential energy, but...
3. ### Why Won't a Disk Rotate on a Frictionless Bearing?

I was able to solve first part I.e. time period of the system when bearing has friction I am unable to figure it out why disk will not rotate when it is mounted to frictionless bearing ? I know that due to absence of friction disk cannot rotate but then Mg is also there which can rotate the...
4. ### How can I calculate the cumulative mass of a disk using disk mass density?

I want to find the cumulative mass m(r) of a mass disk. I have the mass density in terms of r, it is an exponential function: ρ(r)=ρ0*e^(-r/h) A double integral in polar coordinates should do, but im not sure about the solution I get.
5. ### Density of a patch of an accretion disk

In the frame of the patch ##-(1/\rho) \nabla p = - \nabla \phi##, and putting ##\nabla p = (\partial p/\partial \rho) \nabla \rho = c_s^2 \nabla \rho## and taking the ##z## component gives\begin{align*} -\frac{c_s^2}{\rho} \frac{\partial \rho}{\partial z} = -c_s^2...
6. ### A Forces on rotating disk object

Forces on rotating disk object Hi. Is it convenient to ask following question. Suppose we have solid circular object and 5 different moments like in the picture:In moment 1 we apply force (downwars direction) so as to start rotating the object around center of the mass (green dot) , Only...
7. ### Circular Motion: A coin on a rotating disk

I believe I've solved this problem, however, I got through it pretty quickly and since it's the last problem on the assignment, I feel that I may have had an oversight. For part a, I got: fs=md(α^2)(t^2) and for part b, I got: ω=Sqrt((µs*g)/d) Could someone confirm my answers? I've attached a...
8. ### Wrong B-Field Result: What Should I Have Done?

I integrated B within the limits of a (from 0 to 0.007) teh result was 3.64E-10 T and it was wrong. the correcto one would be 5.8 E-4 T and it is a major diference (aprox 1 million times ) Waht shoud I have done? Regards
9. ### Electric Field due to a disk of radius R in the xy-plane

Hello, This question, which I found in various electricitiy and magnetism books (e.g. Introduction to electrodynamics grif.). There are many variations of this question, I am mainly interested in the following setup of it: -Suppose there is a charged disk of radius R lying in the xy-plane, and...
10. ### Analyzing the Force of Two Disk Magnets on a Metal Plate

The situation is as follows. We have two disk magnets. One is fixed on the ground, table, or surface and has the north pole facing up. Then we have a metal plate fixed on the vertical axis rod or something similar such that it can't move up or down, but only rotate horizontally with as less...
11. ### Why is electric field at the center of a charged disk not zero?

The electric field strength at the center of a uniformly charged disk should be zero according to symmetry of concentric rings about the center, where each ring is contributing to the electric field at the center of the disk. For a thin ring of uniform charge distribution the formula is ##E =...
12. ### Describing Rolling Constraint for Rolling Disk With No Slipping

Let ##R=\sqrt{x^{2} + y^{2}}##. Then \begin{align}v_{tangential}&=\frac{dR}{dt} \nonumber\\ &=\frac{dR}{dx}\frac{dy}{dt} + \frac{dR}{dy}\frac{dy}{dt} \nonumber\\ &=\frac{x}{R}\frac{dx}{dt} + \frac{y}{R}\frac{dy}{dt} \nonumber\\ &= cos\phi \frac{dx}{dt} + sin\phi \frac{dy}{dt}.\nonumber...
13. ### Finding Area of Ring Segment to Find Electric Field of Disk

Hi! For this problem, Why is the area of each ring segment dA equal to (2π)(r)(dr)? However, according to google the area of a ring segment (Annulus) is, Many thanks!
14. ### B Disk with holes falling on water

Just yesterday evening I saw a video of a few minutes where metal disks with holes in them were let fall into a basin of water.The water went up in twisted columns and made the most astounding and beautiful patterns. I thought I would show it to other people who might be interested - but I have...
15. ### Why can't I treat the disk as a point mass?

Since the question made no indication of the disk rotating about its center, I just straight up assumed that the disk did not rotate about its center, and instead treated it as a point mass. However, to my surprise my calculations did not bear me any fruit. Below is my first attempt at the...
16. ### Torques exerted on a vertical disk by multiple forces

This was how the solution was arrived in the text, Net torque = F block x d block x sin ϴ0 + F rod x d rod x sin ϴ0 - T R sin 90 0 = 2mg x 2R x sin ϴ0 + m x R x sinϴ0 - T R T = 5 mg sinϴ0 I'm wondering do we have to resolve the forces for rod and block in to...
17. ### How Does the Wedge's Movement Change with the Disk's Motion Upward?

So, when the mass reached the peak, its horizontal velocity will be the same as the wedge's. Using conservation of momentum : $$mu = 2mv$$ $$v = \frac u 2$$ With v is the final velocity for both objects. Now, what we need is the acceleration of the wedge, which we can find by using Newton's...
18. ### A spring, disk and pulley system

(a) By setting up a coordinate system with the x-axis pointing to the right and the y-axis pointing downward we have ##\begin{cases}-kx_{eq}+T_1+F_{s}=0\\ -RF_{s}+rT_1=0\\ r_p (T_2-T_1)=0\\ -T_2+mg=0\end{cases}\Rightarrow x_{eq}=\frac{mg}{k}\left(1+\frac{r}{R}\right)## which coincides with the...
19. ### Angular momentum of a disk about an axis parallel to center of mass axis

I am using the following formula to solve this problem. $$L_a= L_c + \text { (angular momentum of a particle at C of mass M)}$$ Because the point C is at rest relative to point A, so the second term in RHS of above equation is zero. Hence, the angular momentum about A is same as angular...
20. ### Moment of inertia of a disk about an axis not passing through its CoM

I have come up with two different approaches, but I'm not sure which one is correct since they give different answers. We use the following equation to get the total moment of inertia. ##I_o## = moment of inertia of disk about O axis + moment of inertia of road about O axis Approach 1...
21. ### B Time Dilation on Rotating Disk: Clocks on Disk Perspective

Obviously, a third observer who is at rest with respect to the disk will see that the clock on the outside has a much faster velocity than a clock on the interior of the disk, so clearly the outside clock will show that it has measured less time. But that's one question. What about looking at...
22. ### Find Free Software to Log Disk Usage & Monitor Processes

I'm after a piece of (preferably free) software, or even a built in Windows 11 Home tool, that will log disk usage throughout the day, and what processes and application are using the disk, when, and by how much. For far too long now, I notice my laptop get into a frenzy of being at 100% disk...
23. ### A Mass dropped onto rotating disk

Picture a flat disk of radius r with a radial vane. The disk is rotating at angular velocity w. Assume the vane is straight, starts at the center and ends at the perimeter of the disk. A very small round mass ( of m grams) is dropped onto the disk very near the center. The vane contacts it and...
24. ### I Can you solve a Disk, Washer, Shell method problem without drawing a graph?

Is there a way to solve a Disk,Washer,Shell method problem without actually creating a graph?
25. ### Any suggestions for an open source Windows 10 disk encryption app

The summary says it all, really, but in addition, I've done a lot of googling and am finding it hard to identify trustworthy options. LibreCrypt, DiskCryptor and VeraCrypt are candidates, but I have not used any of these tools so am wary of jumping in and finding that I have bricked my PC.
26. ### Disk with rod attached rotating about the center of the disk

1) Since the rod is uniform, with mass m and length l, it has a linear mass density of ##\lambda=\frac{m}{l}##, so ##I_{rod_O}=\int_{x=r}^{x=r+l}x^2 \lambda dx=\frac{\lambda}{3}[(r+l)^3-r^3]=\frac{\lambda r^3}{3}[(1+\frac{l}{r})^3-1]=\frac{1}{3}mr^2[3+\frac{3l}{r}+\frac{l^2}{r^2}].##...
27. ### Why is the angular momentum negative in a disk and stick collision?

figure 11.12 I need someone to explain why the angular momentum of the ball is ## L_{f} = -rm_{d}V_{df} + I\omega## rather than ## L_{f} = rm_{d}V_{df} + I\omega ##. How to distinguish the sign of the angular momentum?p.s. ##\Delta\vec{L}_{total} = \vec{L}_{f} - \vec{L}_{i} = (-rm_{d}v_{df} +...
28. ### Problem parameterising an off-center disk

Greeting (x-3/2)^2+y^2=1/4 x^2+y^2-3y+9/4=1/4 r^2-3*r*sinθ+2=0 Δ=9sinθ^2-1 r=[3rsinθ-sqrt(sinθ^2-1)]/2 is my approch correct? thank you!
29. ### Engineering Finding the force acting on a point charge with a disk around it

Question: Here's my attempt with the relevant equations: The correct answer is F = -99.4 az μN. Could someone please figure out my mistake?
30. ### Python Numerical integration over a disk with polar coordinates

In my job, I was given the task of calculating a force that operates an ultrasound transmitter on a receiver. The calculation is made by assuming that each point on the transmitter is a small transmitter and integration should be made on the surface of the transmitter. Since the transmitter is...
31. ### Calculate the electric field due to a charged disk (how to do the integration?)

I am interested in particular in the second integral, in the ##\hat{r}## direction. Here is my depiction of the problem: As far as I can tell, due to the symmetry of the problem, this integral should be zero. $$\int_0^R \frac{r^2}{(x^2+r^2)^{3/2}}dr\hat{r}$$ I don't believe I need to...
32. ### What is the final angular displacement of a disk hit by two masses?

1) By conservation of linear momentum: ##m_1 v_1-m_2v_2=(m+m_1+m_2)v_{cm}\Rightarrow v_{cm}=\frac{m_1}{m+m_1+m_2}v_1-\frac{m_2}{m+m_1+m_2}v_2=\frac{3}{8}\frac{m}{s}##; 2) By conservation of angular momentum: ##-Rm_1v_1-Rm_2v_2=I_{total}\omega=(I_{disk}+m_1R^2+m_2R^2)\omega## so...

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34. ### I Clock synchronization for ring-riding observers on rotating disk

Hello, reading the wiki entry for Langevin observers on rotating disk - Born_coordinates I'm struggling with the following quoted sentence: But as we see from Fig. 1, ideal clocks carried by these ring-riding observers cannot be synchronized. I do not grasp why, starting from the figure...
35. ### How long does it take for the disk to stop rotating?

Question : Solution attempt : for
36. ### How to calculate the capacity of a disk drive

Good morning, I got a question like this- "A hard drive has 6 platters, each platter records data on both sides. There are 3000 tracks of nine sectors each. Each sector stores 512 bytes of data. How many read/ write heads are there and what are the capacity of the drive(in Gigabytes)?" I...

38. ### Dynamics motion problem with a disk in contact with a bar

First, I tried to model the disk-bar as a crank connecting rod, to the OA bar, and apply this: VP = VB + ω_BP x r_P/B, where P is the contact point between the disc and OA bar. I assumed VP = VP sin 30º i + VP cos 30º j (direction parallel to r_P/B), where r_P/B = sin 30º i + cos 30 j This...
39. ### Determine the moment of inertia of a bar and disk assembly

I have been given an answer for this but I am struggling to get to that point $$ANS = 0.430\, kg \cdot m^2$$ So I thought using the moment of inertia of a compound pendulum might work where ##I_{rod} = \frac{ml^2}{12}## and ##I_{disc} = \frac{mR^2}{2}## (##l## is the length of the rod and ##R##...
40. ### Masses Moving Radially on a Rotating Disk

m = 60kg, ω0 = 2.094 rad/s, I of disk = 130 kgm^2 , outer position ro = 1.5m, inner position ri = 0.3m ∴Fifth object : Ffriction = m.ac μ.m.g = m. v^2 / R => vmax = √ 3. (1.5m) . (9.81 m/s^2 ) = 6.64 m/s => ωmax = 4.43 rad/s so when the fifth object move with greater speed than vmax...
41. ### Dipole moment of a rotating disk

I could do the first part of the question with ease but second part I am not sure how to proceed. Should we calculate the magnetic field at d(where the loop is) and infer something from that for it's motion?? Plz help me out Thanks in advance
42. ### Can we use the disk method in this integral?

Goodd day, I have a question regarding an exercice I have already posted Bvu was very nice and provided this darwing I already have the solution But y question is : can we use the disk method? because as you can see even though the intersection was at x=-1 the sphere goes deep into the...
43. ### Problem integrating with the disk method

Good day I have the following exercice and it's solved using spherical coordinates I totally agree with the solution but I have issue to find out why mine does not work I used the the integration by disk I divided the region of integration to 2 A1 and A2 (A2 is the upper half sphere and A1 is...
44. ### Bronze Age or Iron Age? Age of Nebra Sky Disk is disputed

The age of the Nebra (from Germany) Sky Disk (claimed to be the oldest known representation of the heavens) is in dispute. It was found in the black market, so provenance is unclear. NY Times article here.
45. ### A Life supported by radiation from the accretion disk of a black hole

For a planet to be able to support life, it needs to have a source of energy. In our case this energy comes from the sun. But in this paper, the author argues that a rogue planet (a planet that has been ejected from its stellar system and no longer orbits any star and is wandering in...
46. ### MHB Find the volume by using shell and disk method

Can you check it for me please that I have done it right or not ? Thank you in advance.
47. ### MHB Find the volume by using shell and disk method

Can you check it for me that I done it right or not ? Thank you in advance.
48. ### Electromagnetic Induction of a Disk

Hello I'm having trouble finding the right way to apply Faraday's law to this question. I've found the flux through the disc: ##\phi = \vec A \cdot \vec B = B_{0} \sin{\omega t} \left( \frac D 2 \right)^2 \pi ## and the EMF: ##\varepsilon = - \frac {d \phi} {dt} = -B_{0} \omega \cos{\omega t}...
49. ### MHB Find the area by using disk method

The problem is to solve for the area R. Can you please help me ? I have tried to do it many times. Thank you in advice.