What is Rotating: Definition and 1000 Discussions

A rotation is a circular movement of an object around a center (or point) of rotation. The geometric plane along which the rotation occurs is called the rotation plane, and the imaginary line extending from the center and perpendicular to the rotation plane is called the rotation axis ( AK-seez). A three-dimensional object can always be rotated about an infinite number of rotation axes.
If the rotation axis passes internally through the body's own center of mass, then the body is said to be autorotating or spinning, and the surface intersection of the axis can be called a pole. A rotation around a completely external axis, e.g. the planet Earth around the Sun, is called revolving or orbiting, typically when it is produced by gravity, and the ends of the rotation axis can be called the orbital poles.

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  1. J

    Dog running on rotating ball

    <Moderator's note: Moved from a technical forum and thus no template.> We place a ball on an an inclined plane (angle ##\theta##). At the top of the ball is a small dog who always stays at the top over the course of the ball's movement. What is the motion of the ball? ------------------ I...
  2. S

    Pendulum on the end of a rotating disk

    Homework Statement There's a pendulum with mass m and longitude L strapped to a disk with radius R that rotates with an angular velocity ω. Calculate the angle that the pendulum is shifted (Φ) depending on ω. You are given m, L, R, ω, g. Calculate Φ depending on ω. Little drawing (with my...
  3. B

    Berry phase of 1/2 spin in slowly rotating magnetic field

    Homework Statement Homework Equations This is the way to solve when magnetic field B is arbitrary direction one. The Attempt at a Solution I got a eigenvalue of this Hamiltonian and eigenstates. but i have no idea how to set a coordinate to value the gradient
  4. T

    Rotating a spherical conductor in an Electric field

    Hi, I recently came across the familiar image of a metal sphere in an electric field: https://i.stack.imgur.com/x58Ia.jpg I noted how the free-charges on the surface of the sphere align with the electric field lines as opposite charges are attracted. Then I wondered, 'what if the sphere was...
  5. A

    Precession of Rotating Rigid Body from Two Different Frames

    Homework Statement A) Use Euler's equations to solve for the precession rate of a rapidly spinning gyroscope precessing uniformly (no nutation). Use I3 for the moment along the z axis along the axle. Beginning with Euler's equations, derive the coupled equations: ## \dot ω_1 + Ω ω_2 = \frac...
  6. unseeingdog

    How to make a rotating rod turn the axle of a small motor?

    I'm building a small wind turbine and I'm trying to find a way to make the metal rod where the propellers are rotate the axle of a small motor. The diameter of the rod is 1/4" and the diameter of the axle of the motor is about 2 to 3 millimeters. This is more or less the design of the turbine...
  7. O

    Does a rotating rigid body has single angular velocity.

    Hello all, First of all I want to mention what believe or result of concepts, do I hold about rotational motion especially about the angular velocity. I assume that unlike velocity angular velocity has to worked out from some point. I will begin with particle for this I assume that the question...
  8. B

    Ball in Rotating Disk: Kinetic Energy, Work, and Power

    Homework Statement A disk of radius ##R## has a cylindrical hollow that goes through it crossing its center ##O##. The disk rotates around its central axis with constant angular speed ##\vec{\omega}##. A little ball of mass ##m## and with the same radius of the hollow, is at a distance ##R_0##...
  9. N

    Net force from a varying speed non-uniform rotating wheel?

    If we have a circle rotating then a force v^2/r is produced but no net directional force exists as it cancels. But if the wheel was not uniform in mass ie not rotating around its centre of mass AND the speed changes over time as a function of the "wheel"'s rotation angle could a net force be...
  10. B

    Finding torque in rotating cylinder (viscosity )

    Homework Statement Homework Equations τ= η* (dv/dy) The Attempt at a Solution so dA= 2π* r * dr T = ∫ τ * r * dA T = ∫((0.01* r* 2*π * 450 /60)/(5*10^-3) ) * r * 2π* r *dr = (592.1762)*(r^4/4) T = 0.11 Nm is this correct?
  11. I

    Bead Sliding on Rotating Rod after Motor is Turned Off

    Homework Statement A bead of mass m slides in a frictionless hollow open-ended tube of length L which is held at an angle of β to the vertical and rotated by a motor at an angular velocity ω. The apparatus is in a vertical gravitational field. a) Find the bead's equations of motion b) Find...
  12. S

    I Rotating Black Holes: A Century of Discovery & Beyond

    Schwarzschild black holes were discovered over 100 years ago, in 1915. Quite soon, in a couple years (by 1916...1918), charged Reissner-Nordström holes were discovered. Yet rotating black holes were only discovered by Kerr in 1963 - 48 years after Schwarzschild holes. Why? What was known about...
  13. S

    Moment of Inertia of a Rotating Disk

    Homework Statement Given a non magnetic rectangular plate with dimensions: L & B. The plate is of uniform density and thickness.There are 2 points equidistant from the center of the rectangular plate along the line at B/2. Both of these pivot points have 2 disks of radius R and mass M that are...
  14. Pushoam

    Angular momentum of a rotating skew rod

    Homework Statement Homework EquationsThe Attempt at a Solution The angular momentum of the upper particle is given as ## \vec L_u = m \vec r \times ( \vec \omega \times \vec r )= m[ \vec \omega r^2 - \vec r ( \vec r . \vec \omega )] = m[ r^2 \vec \omega - \omega r \cos \alpha \vec r ]##...
  15. P

    Optimizing Rotating Mass Balancing: How Many Mass Blocks are Needed?

    Hi all, I will like to check if I need to balance an out of balance shaft, why is the minimum number of mass block = 1 Is it because i can place the mass on the opposite end of the unbalanced mass at the same distance r away from the axis? and how many are required to dynamically balance the...
  16. Y

    Calculate torque of an 80kg object rotating on its central axis

    Hello, I have a setup whereby: 1) A circular plate with fillet under-edge, is sitting like a bowl on some ball bearings. Imagine if you will a bowl sitting in a way that it can rotate and move a bit up and down etc. without much friction. 2) The plate needs to rotate at about 4 or 5 rpm. 3) The...
  17. J

    Calculate the power generated by a rotating rod turning a generator

    I want to know how much electrical power I could potentially generate by putting a generator at the pivot point of a rotating arm. I figured I can calculate the Kinetic Energy of the arm by: E=(Iw**2)/2 where I is moment of Inertia and w is angular velocity. I = (mL**2)/3 for a uniform rod in...
  18. A

    Finding tangential acceleration of a rotating disk

    Homework Statement Attached. Homework Equations Tangenital acceleration=r*alpha 3. The Attempt at a Solution I thought it was just simply r*alpha=2*20=40 (choice A), but the answer key says choice B. I'm not really sure where I could have went wrong or its possible that there is a mistake...
  19. J

    Maximum height a waitress can push on a glass without it tipping

    Homework Statement I have attached the problem. I will write it out: A waitress attempts to push a glass of water of heigh 15.o cm and diameter of 7.00 cm on a dining table, as shown in the figure. If the coefficient of static friction between the glass and the table's surface is 0.350, what...
  20. F

    Total moment caused by an unbalanced mass on a rotating shaft

    Homework Statement [/B] The shaft connecting A to D is rotating at 1200RPM clockwise and has a concentrated unbalanced load of W2 of 8lb at a radius of 2in. in a plane midway between points A and D as shown in the figure. A constant vertical load W1 of 1500 lb is applied by means of two...
  21. S

    Torque for rotating table

    Is there a way to figure out the torque needed to rotate a work table without knowing the center of gravity? I have a customer who wants to use a 40:1 ratio gearbox to rotate a work station, the gearbox would have an 11" wheel on the input shaft for the operator to turn. However the...
  22. peroAlex

    Induced Voltage of Rotating Circular Loop

    HI! I'm a student, this question appeared in one of the old tests. I though I had it solved but apparently I must have missed somewhere. Could someone please at least hint me where I went wrong? Homework Statement Thin wire is wound into circular frame with radius ## r = 0.05m ## which...
  23. N

    Velocity from position vector in rotating object

    Homework Statement I am trying to solve for change in velocity for the center of a rim with respect to the contact patch of a tire that has some degree of camber. The equation finalized is shown in the image below, equation 2.6. http://imgur.com/a/oHucp Homework EquationsThe Attempt at a...
  24. V

    Hydrostatic equilibirum in slowly rotating star

    Hello, in article Slowly relativistic stars by James B. Hartle (http://adsabs.harvard.edu/full/1967ApJ...150.1005H) is equation of Newtonian hydrostatic equilibrium, eq. (5). $$const.=\int_0^p\frac{dp}{\rho}-1/2(\Omega \times r)^2+\Phi,$$ where ##p## is pressure, ##\rho## is desinty, ##\Omega##...
  25. Pushoam

    Force on a bead due to a rotating wire

    Homework Statement A bead slides without friction on a horizontal rigid wire rotating at constant angular speed ω. The problem is to find the force exerted on the bead by the wire. Neglect gravity. Homework EquationsThe Attempt at a Solution The physical force acting on the bead is the force...
  26. Pushoam

    What shape does the surface of a rotating liquid take?

    Homework Statement Homework EquationsThe Attempt at a Solution If I consider an arc shaped element of the liquid, then the physical forces acting on it are 4 different tensions , normal force and gravitational force on it. Two of the tensions would be acting along tangential direction...
  27. Pushoam

    Derivative of angular velocity of rotating co. system

    What is time derivative of angular velocity ( measured w.r.t. an inertial frame ) of a rotating co. system w.r.t. the same rotating co. system? I think a person sitting in a closed rotating box feels the an object at rest w.r.t. him as rest. He doesn't observe the angular velocity of the...
  28. Pushoam

    Motion of sleeve attached to a L-shaped rotating rod

    Homework Statement https://www.physicsforums.com/attachments/207518[/B] https://www.physicsforums.com/attachments/207517 https://www.physicsforums.com/attachments/207519 Homework EquationsThe Attempt at a Solution Let me assume that initially the spring is at its natural length. The normal...
  29. G

    Impact force from rotating object?

    Hi, Please could someone help me with the following, I have a rotating cutter and I need to calculate the impact force on an inspection window if one of the cutter blades was to separate from the main assembly. The cutter is spinning at 3000rpm The cutter radius is 115mm (rotating radius)...
  30. X

    I EM Waves in a Rotating Frame: Questions & Answers

    Hello there, I have a question (two very similar questions) about the time and phase delay between rotating objects. I want to describe two extreme cases here: I would appreciate any helps. Case 1 Imagine two observers (people with telescopes maybe) in space that are standing thousands of...
  31. C

    Doubt with rotating body and force

    Hello. Let's image a bar. In one side is attached to a body so the bar can rotate over this axis. There is no friction between the two bodies. The system is at rest in t=0. A force acts forming a 90° angle with the bar. The bar moves and begin to rotates. The force dissapear. There is no force...
  32. anonyaway

    Rotating disc in a rotating magnetic field

    Homework Statement :A metallic disc of radius R is placed perpendicular to a magnetic field B.Both the magnetic field and the disc rotate with an angular velocity w about the axis of the disc.Find the induced EMF between the center and the circumference[/B]Homework Equations Motional Emf=Bvl...
  33. person123

    Solving 2 Rotating Discs Homework: Angular Velocities

    Homework Statement There are two discs with equivalent density and thickness. One has radius r1 while the other has radius r2. r2 is twice as great as r1 The larger disc has an initial angular velocity ω. The two discs then come in contact with one another and friction causes them to rotate...
  34. E

    Question of Leverage in Rotating Mechanisms

    New to physics, so looking to this board for more knowledgeable and experienced help in trying to understand the potential effect of leverage in circular motion. Imagine two disks, A and B, each 1 foot in diameter. Each disk is permanently attached at its center to a vertical shaft which spins...
  35. Faefnir

    Rotating Rigid Body Homework: Find Angular Velocity

    Homework Statement Two masses of 2 kg are attached to the extremities of a 5 cm long, negligible mass and thin bar, free to rotate around a horizontal axis passing through its center. A drop of wax falls on one of the two masses at ##v = 3 \frac{m}{s}## and sticks to it. Retrieve the angular...
  36. Feodalherren

    Frictionless mass sliding in rotating pipe

    Homework Statement A pipe is spinning around with angular velocity ω = 2 rad/s about its fixed end. Inside the pipe is a frictionless block that is free to slide. The block is located 1 m from the fixed end of the pipe when the angle from the vertical, θ = π/2 (horizontal). What is the radial...
  37. A

    I Area of the event horizon of a rotating black hole

    The Kerr metric for a black hole of mass ##M## and angular momentum ##J = aM## is $$ds^{2} = - \frac{\Delta(r)}{\rho^{2}}(dt-a\sin^{2}\theta d\phi)^{2} + \frac{\rho^{2}}{\Delta(r)}dr^{2} + \rho^{2} d\theta^{2} + \frac{1}{\rho^{2}}\sin^{2}\theta (adt - (r^{2}+a^{2}) d\phi)^{2},$$ where...
  38. Asmaa Mohammad

    The average EMF in a coil rotating in a magnetic field

    Homework Statement [/B] Homework EquationsThe Attempt at a Solution Solution for Q1: Solution for Q2: Solution for Q3: Are they correct?
  39. R

    How to Calculate the Torque of a Rotating Cylinder?

    Assuming we have a cylinder that is rotating on a single axis, how would I go about calculating the torque knowing the following information? m = 2000 kg w = 60 rpm r = 5 m
  40. M

    Writing: Input Wanted Design of large rotating asteroid interior space

    Hi. I'm working on a science fiction story about colonization of the solar system. Asteroid way stations are an important part of the story. I want to construct a realistic scenario about how we could convert the Eros asteroid into a shuttling habitat in it's current orbit (or something close...
  41. M

    Mechanical Principles Dynamics of Rotating Systems

    1. For the mechanism shown in FIGURE 1 determine for the angle θ = 45°: (i) the velocity of the piston relative to the fixed point O (VBO) (ii) the angular velocity of AB about point A (i.e. ωAB) (iii) the acceleration of point B relative to A (aBA). (b) Determine the value of the angle θ...
  42. Baptiste Debes

    Magnetic flux through a rotating bar

    Hello everyone, My question is between theorie and practical (so I'm still wondering if it's the right place). I'm reading Serway and Jewett. There's an example about the magnetic flux through a rotating bar and so and induced emf. I understand this emf will be equal to the the opposite of the...
  43. F

    Two wheels rotating connected by a rod

    Homework Statement A cart has two cylindrical wheels connected by a weightless horizontal rod using weightless spokes and friction- less axis as shown in the figure. Each of the wheels is made of a homogeneous disc of radius R, and has a cylidrical hole of radius R/2 drilled coaxially at the...
  44. F

    What Forces Act on a Block in a Rotating Cylinder?

    Homework Statement A cylinder with radius R spins around its axis with an angular speed ω. On its inner surface there lies a small block; the coefficient of friction between the block and the inner sur- face of the cylinder is μ. Find the values of ω for which the block does not slip (stays...
  45. YURIIMkZERO

    Comsol Help -- How to Enable Rotating?

    Hi, I'm a newbie at Comsol and want ask for guide how to fill something with fluid and make sphere rotate. My goal in creating system where one sphere contains second. Second sphere rotates on its axis. Volume between is filled with fluid. I understand how to create spheres, but couldn't...
  46. R

    Visualizing the Motion of Coins on a Rotating Scale

    Homework Statement A uniform rigid scale is held horizontally with one of its end at the edge of the table and the other supported by hand. Some coins of negligible mass are kept on the meter scale. As the hand supporting the scale is removed, the scale starts rotating about its edge on the...
  47. JulianMau

    I Clocks ON a rotating disk: What happens?

    I was reading about the Ehrenfest Paradox and it got me thinking about something (that I think is) similar: Suppose we take a large, flat, and rigid disk, and we attach to various parts of it a number of clocks (some very close to the center of the disk, some along the edge, others in between)...
  48. U

    Force acting during rotating a disc

    Take a disc that can rotate with respect to the rotation axis. For simplicity, let's assume that its mass is homogeneously distributed along the rotation axis, and gravity and frictional forces do not act on the disk. In the first case, the disc does not rotate. All elements of the disk have...
  49. A

    I On the Relativistic Twisting of a rotating cylinder (Max von Laue)

    I'll start with link describing the phenomenon: https://books.google.com/books?id=WTfnBwAAQBAJ&pg=PA43&lpg=PA43&dq=relativity+twist+rotating+cylinder&source=bl&ots=C2SDJNPF2K&sig=ImFDYGm_0qK7JoDU0ulsxLk3sMU&hl=en&sa=X#v=onepage&q=relativity%20twist%20rotating%20cylinder&f=false Quote: "A...
  50. Ron19932017

    Angular veolcity in rotating frame

    Hello everyone, I have some conceptual problems understanding the rotating frame transformation. Take the center of the Earth as inertial frame's origin and another point in Hawaii as rotating frame's origin. In many lecture notes from internet, or Marion chapter 10. The vector describing the...
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