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. 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...
  2. Mind----Blown

    General approach to find principal axes of rotation?

    Suppose i have an equilateral triangle and i want to find the principal axes of rotation passing through one of the vertex. How can i do that? I am thinking along the following lines but I'm not too sure: 1)Since the equilateral triangle has symmetry about a median, that definitely is one...
  3. meshein

    Senior Project - Quadcopter with Rotating Arms

    Hello, I am working on a senior project right now and I need some assistance with determining the torque required to rotate an arm with a propeller attached to it. Any of the 4 arms can be rotated to control the direction of travel. The idea is the control system will have a consistent set...
  4. pallab

    I What is the velocity of light in uniformly rotating frame?

    S'-frame of reference has uniform angular velocity with respect to the frame S. what is the velocity of light in S frame w.r.t S' ?
  5. D

    Rotating bullet vs non rotating bullet

    I'm writing a program to simulate bullet impact on various materials. I need to know, will the angular momentum of a spinning bullet give it more impact force? I would assume that if so it would also mean that spinning bullets overcome more air friction if I am supposed to add the angular...
  6. Y

    Derive KE of rotating ring and disk

    <Mentor's note: Moved from a technical forum and thus no template.> A ring's kinetic energy is integral of 0.5v2 dm. Distance X is rΘ, and Θ is defined as distance traveled/radius, so X is r*distance traveled / r. Velocity V is X divided by time, so V is r*distance traveled / rt, and I define...
  7. H

    Rotating rod returning to its initial position

    Homework Statement A uniform rod (mass 'm', length 'l') held horizontally above the ground. It is released from its supports and struck vertically upwards at one end. With what impulse should the rod be struck if it is to return to exactly its starting position and orientation? Homework...
  8. Angelo Niforatos

    Equation of Motion for a Disk inside a Rotating Ring

    [Mentor's note : No template as this thread was moved from the technical forums] Hi all! I am working on finding the Lagrangian for the situation stated in the title. This is actually a Wolfram Mathematica demonstration as well in which they give you the Lagrangian. I am working on re-deriving...
  9. J

    B Rotating Disks: Particle Movement & Spin

    When we rotate a disk, can this process be fully explained by looking worldlines of the particles the rotating disk is composed of, hence their x,y,z,t position "as time passes", or do particles have some kind of "facing direction", hence also spin(not the quantum mechanical notion of spin)...
  10. S

    Inertia tensor of a body rotating about 3 axes

    Homework Statement Hello, I know about the inertia tensor about one axis, but how about a body that rotates around 3 axis x,y and z such as a spacecraft with changes in the attitude. Thanks for you help. Homework EquationsThe Attempt at a Solution
  11. A

    Where to place fixed axis of rotating motor

    is there a fixed angle between the fixed axis of a motor to the torque applied opposite to the motor rotation? an example is if i am using and electric screw driver to rotate a spiral screw in stable piece of wood on the floor, what should be my fixed frame if the screwdriver is I shaped and...
  12. J

    Adjustable spring tensioner for 90deg rotating arm

    Hi all, I'm an electrical engineer and I'm part way through the design of a little project and I'm struggling with a mechanical element which I'm hoping someone may be able to assist with. I'm going to keep the finer details out of this to keep things simple and just focus on the issue. This...
  13. hansyhop

    Motorized rotating platform (180 degrees, periodically)

    Hi, I'm trying to build a motorized rotating platform, and was wondering what drive system to choose. The platform is 1.2 meters in diameter with two cylindrical objects (max 25 kg each) attached on top - total weight to rotate approx 80 kg. I want the platform to be programmed to...
  14. Maxwell's Demon

    Does a rotating magnetic field possess angular momentum?

    Does a rotating magnetic field possesses angular momentum in the direction of rotation? I suppose this comes down to a broad question about the physical nature of fields in general. I love the Einstein-de Haas effect, where an iron core spins in the opposite direction of the induced spin...
  15. J

    I Time dilation of a rotating disk

    Iv'e been recently interested in time dilation, but the relative time difference between two observers confuses me (i.e. that a high speed observer, and a stationary observer will each perceive the other's clock to run slow.) I thought of the following experiment to help me understand, but I'm...
  16. W

    Friction on a flat rotating surface

    If I push an object such as a cylinder of wood along a flat table (flat face of cylinder in contact with the table) through it's center of mass, the friction or energy required is not dependent of the surface area the block makes with the table, Friction = μ N, correct? And the energy required =...
  17. Arun John

    Mechanism for vertical movement while rotating horizontally

    Hi I am trying to make a CAD model of something like the scoop sampler shown below. http://www.rolls-royce.com/~/media/Files/R/Rolls-Royce/documents/customers/nuclear/scoop-sampling-tcm92-50918.pdf This is for a university coursework that requires design of bespoke machines. I am trying to...
  18. bananabandana

    Transforming to a Rotating Reference Frame

    Homework Statement Let ## \mathbf{r} ## be the position of a point in a rigid body relative to some origin ##O##. Let ##\mathbf{R}## be the position of the centre of mass from that origin. ##\mathbf{r^{*}} = (\mathbf{r}-\mathbf{R})##. ## d\boldsymbol{\phi} ## is the infitesimal vector directed...
  19. bananabandana

    Angular momentum of a rotating door

    Homework Statement A door ( a rod of length ##L##, mass ##M##) rotates with angular velocity ##\omega## about a point ## H ##, and approaches a stop at ##S##. ##H## and ##S## are along the same line, and separated by a distance ## s ##. Show that the angular momentum of the door about the point...
  20. pkt

    B Our atmosphere and the rotating Earth

    What force keeps our atmosphere spinning with the rotating earth? How can there ever be a calm day?
  21. H

    Studying Rotating black hole & Hawking radiation research

    I am an undergraduate student of a university, I have taken the research topic as Study of Rotating black holes and Hawking radiation which I am really interested. Research description as follows. The geometric invariant are computed in various black hole geometries in several different...
  22. StrangelyQuarky

    Conical pendulum in rotating frame

    Homework Statement A pendulum of length l at the north pole is moving in a circle to the east at an angle \theta to the vertical. It has some period T_E as measured in the rotating Earth frame. The experiment is then repeated except now the pendulum is moving to the west with period T_W...
  23. B

    Lift a hemispherical dome filled with water by rotating it

    Homework Statement A hemispherical dome filled with water, radius 2 m and weight 30 tons, can be lifted by giving it a constant angular velocity around its vertical line of symmetry. The dome has a hole on the top. Decide the minimum angular velocity. My answer is 2.91 rad/s . The correct...
  24. R

    Object Rotating on a Semicircle

    Homework Statement A small 700-g collar C can slide on a semicircular rod which is made to rotate about the vertical AB at a constant rate of 7.5 rad/s. The coefficients of friction are μs = 0.25 and μk = 0.20. Determine the magnitude of the friction force exerted on the collar immediately...
  25. E

    The total tension acting on a rotating rod

    Homework Statement Find the total tension acting on a rod rotating about its end with an angular velocity of w as a function of its length x(length) Homework Equations F = ma[/B] The Attempt at a Solution Let the function be T(x) where x is the length of the rod. Considering an interval...
  26. V

    Should I consider Linear Kinetic Energy in this Equation

    Sorry If the thread name confuse you. I want to know if I want to find the Torque from deriving Kinetic Energy and the system has Object the Rotate and the rotating cause linear motion(v). Let's say it a Rolling Disc on the non-slope plane which has angular velocity ω and that ω cause it to move...
  27. R

    Mass on a Rotating Parabolic Dish

    Homework Statement A 3-kg block is at rest relative to a parabolic dish which rotates at a constant rate about a vertical axis. Knowing that the coefficient of static friction is 0.66 and that r = 2 m, determine the maximum allowable velocity v of the block. Picture attached below Homework...
  28. T

    I Any progress on realistic rotating black holes?

    Idealistic solution for non-rotating BH includes a "white hole part" However, realistic solution is different For the rotating BH we have an ideal solution with an "eternal" ring singularity inside, which can't be realistic, as ring, as I said, is "eternal" and there is no transition of...
  29. J

    Calculating Angular Speed of Rotating Apparatus with Attached Mass and Rod

    Homework Statement The 3.0 kg rod has a length of 80 cm. The 4.0 kg rectangle attached to the rod has negligible dimensions. The entire object rotates counterclockwise about the bottom of the rod. Determine the angular speed of the apparatus at the instant the rod is horizontal...
  30. S

    Putting rotating cylinder on the ground

    Homework Statement We put a cyllinder with unknown mass and radius 0.1m which rotates with angular velocity 10 /s around its geometric axis on the horizontal ground. After what time does the cyllinder roll without slipping? Coefficient of friction is 0.1. Homework Equations F=ma M=J ⋅ ∝ The...
  31. sempiternum

    Vibration in "cantilever" rotating bar

    Hi. I'm working in the design of an auotomatic bar feeder for a CNC lathe. I've established that the bars will not be longer than 1.2 m and will not have a diameter of more than 75 mm. Suppose they are steel bars, which would make them have a mass of about 80 kg So, bars are tightly held in one...
  32. ultron2099

    The Physics of a Fender Bender: Rotating a 4000 lb Van

    Recently I was unfortunately involved in a fender bender. I was traveling at most about 10 mph into an intersection I had just left a Yield sign when a bright red car collided with me. My question, how fast was it likely going? I ask this, because I was driving a van weighing likely 4000 lbs...
  33. hadsox

    Rotating and translating spool across a table

    Homework Statement A uniform spool of mass M and diameter d rests on end on a frictionless table. A massless string wrapped around the spool is attached to a weight m which hangs over the edge of the table. If the spool is released from rest when its center of mass is a distance l from the edge...
  34. V

    How Does a Pendulum on a Rotating Arm Behave When the Arm Stops?

    Scenario There is a pendulum suspended on an arm that rotates a certain angle in the horizontal plane. The arm suddenly stops, how far will the mass of the pendulum be displaced in the horizontaly? Thought so far: The momentum of the arm will be L=IW and when it stops I think the mass at the...
  35. cheapstrike

    Rolling of a Disc: Find Acceleration with Friction

    Homework Statement A cart with mass M has four wheels (idealized as uniform discs), each of radius r and mass m, arranged symmetrically with respect to the cart. Find the acceleration of the cart when a horizontal force F is applied on it. There is no slipping between the wheels and the...
  36. G

    Work-Energy for Bead on Rotating Stick

    Homework Statement Verify the Work-Energy Theorem W=ΔK for a bead of masd m constrained to lie on a frictionless stick rotating with angular velocity ω in a plane. Homework Equations W =∫ F⋅dr, K =m/2 v^2 [/B] The Attempt at a Solution Adopting polar coordinates the velocity is v = r' +r*Θ'...
  37. shintashi

    What happens with counter rotating gyroscopes?

    I was thinking about retro rockets for landing and how precise they have to be, but then i thought "you could stabilize the whole thing with a gyroscope". Then I realized the gyroscope would cause the landing shipment to rotate in the opposite direction. So I thought, hey, why not two...
  38. H

    Angular acceleration of a cylinder rotating around another

    For Q11(b), what is the relation between the angular acceleration ##\alpha## of the bottom right cylinder and its horizontal acceleration ##a_x##? I get ##\alpha=\frac{a_x}{\sqrt{3}R}##, which is half the given answer (7.84) below. After the bottom right cylinder rolls around the top cylinder...
  39. person123

    Rotating Friction Discs For Project Design

    The situation is part of a mechanism I have been working on. It is used for a bicycle to automatically store and release energy with a spring. It is somewhat similar to a car clutch. Homework Statement Two friction discs are rotating with different angular velocities and different torques. The...
  40. B

    A person walks on a rotating platform at constant speed

    Homework Statement A large flat horizontal platform rotates at a constant angular speed ω. A person on the platform walks in a circular path of radius R0 centered on the axis of the platform with a constant linear speed v relative to the platform’s surface. The coefficient of friction between...
  41. Surya97

    Is there a true upward force on a rotating object?

    I understand that the torque on a gyrating object is defined as the force vector cross multiplied by the lever arm position vector, which produces a resultant vector that is normal to both of the original vectors. However, when an object (let's say a disk) is rotating about an axis...
  42. 1

    Lagrangian of 2 rotating masses on a spring, sliding down plane

    Homework Statement 2 masses are connected by a spring. They are on a frictionless plane inclined relative to the horizontal by ##\alpha##. The masses are free to slide, rotate about their center of mass, and oscillate. 1. Find the Lagrangian as a sum of the Lagrangian for the COM motion and a...
  43. P

    What Causes the Rod to Rotate?

    Homework Statement A vertical rod is rotating about its longitudinal axis at a constant angular velocity Ω. It is allowed to swing freely from the endpoint A. The angle between the rod and the longitudinal axis of the system is denoted by θ. Point A is located on the highest endpoint of the...
  44. C

    Rate of Change of Vector in Rotating Frame

    I recognize the rate of change of a vector in an inertial frame S can be related to the rate of change of the vector in a rotating frame S0 by the equation below taken from my textbook, where Ω is the angular velocity vector. $$\Big(\frac{dQ}{dt}\Big)_{S_{0}}= \Big(\frac{dQ}{dt}\Big)_{S} +...
  45. D

    Calculating Angular Momentum Reduction in Rotating Rockets

    how do i calculate the gradual reduction of angular momentum a rocket initially has before it fires its main rocket engines? I am assuming if the thrust is high enough then the rocket should eventually stop rotating and travel in a straight line. assume the rocket is in a universe without...
  46. Farang

    I Rotating Black Holes: Q&A with First-Time Poster

    Hi there! This has been on my mind for a while now, maybe someone here can help me understand. First of all, non-rotating black hole is an idealized concept, right? Why the term then? Other objects are not normally referred to as rotating stars, rotating planets, etc... And the main question...
  47. James Kent

    How can a device prevent twisting of USB cables when rotating?

    This is a thought experiment: My PC is mounted on my ceiling. I want to run a USB peripheral through a cylindrical retraction device (like a dog leash). The device is going to mount to the ceiling also and rotate about it's vertical axis. This will allow me to pull down the mouse from the...
  48. C

    Angular momentum: rotating ice skaters

    Homework Statement Two ice skaters of mass ##m = 50\,\mathrm{kg}## each are moving towards each other frictionless on parallel paths with a distance of ##3\,\mathrm{m}##. They both have a velocity of ##v_o=10\,\frac{\mathrm m}{\mathrm s}##. Skater 1 is holding a massless rod of length...
  49. Edward258

    Find the temperature increase of a rotating disk

    Homework Statement It's a Blackbody radiation problem: A beam of wavelength λ, in the state of right circular polarization, leads to an absorbent disk.The mass of the disk is m, it's specific heat is C, and its moment of inertia is I .The disk is initially at rest, but after a lapse of time...
  50. T

    Fields of Rotating Dielectric Sphere in Dielelectric Medium

    Homework Statement A dielectric sphere with the electric and magnetic susceptibilities ε1 and µ1 is rotating with angular frequency ω in a constant electric field E~ in a medium, characterized by the parameters ε2 and µ2. The angle between the rotation axis and the direction of E~ is α. Find...
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