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. torito_verdejo

    Advantages of Polar Coordinate System & Rotating Unit Vectors

    What is the advantage of using a polar coordinate system with rotating unit vectors? Kleppner's and Kolenkow's An Introduction to Mechanics states that base vectors ##\mathbf{ \hat{r}}## and ##\mathbf{\hat{\theta}}## have a variable direction, such that for a Cartesian coordinates system's base...
  2. K

    Plotting a rotating ellipse trajectory (the Moon orbiting the Earth in 2D)

    I'm tasked with drawing the trajectory of the Moon around the Earth (in 2D), taking into account the fact that the trajectory also undergoes precession, so the elliptical orbit rotates about it's center (I think it should rotate around the Earth-Moon barycenter, but for the first step I...
  3. kuruman

    How to Zip Through a Rotating Tunnel Without Bumping Into the Walls

    Continue reading...
  4. A

    Maintain a constant velocity for a rotating wheel

    I want to make a small mechanical calculator with no electric parts, nothing too advanced but a few binary gates capable of some basic functions. All the examples I were able to find were amazing but were still dependent on electricity to drive the mechanism. One possibility is a spring but I...
  5. N

    Lenz's Law for a rotating PM motor rotor spinning in a thin CU tube

    The motor is required to operate at its resonance frequency and I am looking to add a thin-walled (0.010") copper tube inside the stator bore to add some damping. The current motor air-gap is 0.015". If I install a copper tube in the stator bore bonded to the stator and leave a 0.005" air-gap...
  6. Foppe Hoekstra

    I Is it possible to have synchronized clocks in a rotating system?

    Consider a wagon with length L and constant speed v on the straight part of a rail. The wagon has clocks on both ends that are sync in the co-moving frame (of the wagon and the clocks). Then there is a curve in the rail with radius r. The speed of the wagon in the curve is still constant v, but...
  7. N

    Rotating with slipping to rotating without slipping?

    It seems to me that this transition implies going from kinetic friction to static friction. The kinetic friction would apply a torque that would slow down the object's angular velocity, but I'm not sure how this connects to the object suddenly transitioning into rotating without slipping.
  8. S

    Impact force calculation due to a rotating object in Overspeed Trip Valve Design

    Bolt comes out by distance d=1.58 mm when speed of rotation of shaft reaches its trip value (w_trip). m is mass of trip weight (Bolt assembly) One of the approach which I had considered is work energy theorem. Initial energy = 0.5m*(w_trip*(r+d))^2 Final Energy= 0.5m*(w_trip*r)^2 (Assuming bolt...
  9. S

    Rotating Sphere: Conceptual Question

    As shown in figure there's a homogeneous solid sphere. It is rotating about axis which is passing through point P directed perpendicular to the plane of paper. (In short like a pendulum). I'm neglecting gravity and assuming a force F which is directed perpendicular to the string. (The string...
  10. cianfa72

    I Rotation or acceleration defined without relation to something else

    Starting from this post, we are able to define the concept of (proper) acceleration or rotation without any reference to something else About this definition which is the physical meaning of gyroscopes axes pointing in three mutually orthogonal spacelike directions ? In other words, from a...
  11. M

    B Polarized Electron in a Rotating Reference Frame

    I tried asking a similar question in cosmology but got no answer there so here goes... Suppose I am on a windowless spacecraft in the middle of an intergalactic void. I know that the spacecraft is spinning from measuring the centrifugal forces but have no way of observing the outside...
  12. R

    Does thrust get cancelled by a rotating crank?

    I have a frame which supports a crank rotating through 360°. The crank lifts a 30 kg weight 540 times a minute ( 540 rpm). As the crank moves 0° to 90° the weight is accelerated to full speed which means it has thrust but then from 90° to 180° it is slowed to a stop then from 180° to 270° it is...
  13. 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.
  14. J

    A Describing a Rotating Vector with Changing Angular-Velocity

    I have a question about a rotating vector field: if there is a vector ##A^i(t_0)## at the origin in coordinate space ##IR^3## , when ##t≥t_0##, the vector rotates with a changing angular-velocity ##ω^i(t)##. then we can get a rotating vector field ##A^i(t)##. then how to describe ##A^i(t)##...
  15. Lardos

    A Ideas for determining the volume of a rotating object

    Hello everybody, I am currently working on an experiment investigating the formation of planets. I have a vacuum chamber in which dust particles form bigger agglomerates through accretion (sticking together). From the imagery I can see those agglomerates which are build up by smaller...
  16. ?

    Rotating sphere which separates into hemispheres

    Hi all, The scenario I'm considering is a solid sphere (of uniform density) rotating with constant angular velocity when it abruptly splits into two hemispheres along a cut which contains the rotation axis. The hemispheres will begin to separate; if, for example, we consider the rotation to be...
  17. M

    Calculate the radius of this rotating wheel

    Homework Statement Calculate radius of rotating wheel. Linear speed of point "a" (red dot) is 2.5 times faster than linear speed of point "b"(blue dot) that is 5cm closer to center of wheel. Right answer is 8.33cm. I drew a sketch without linear line. Homework EquationsThe Attempt at a...
  18. QuarkDecay

    Flow (liquid or gas) across a rotating surface's face

    << Mentor Note -- Two threads on the same question merged into one thread >> How does the maximum Power equation change if there's an angle to the way the wind falls into the wind turbine's blades? Example, when it falls vertically to the blades, it's Pmax= 8/27Sρu13 But if there's for...
  19. L

    Can Rotating Magnets Create a Stronger Magnetic Field?

    hello, i have a doubt i bought some small neodimium magnets 8x3mm and now i would like to try some experiments with them i have a doubt: if i rotate one of these magnet around one of its parallel axis, very fast , near 1000000 rpm, what happens to the magnetic field produced by the magnet? if...
  20. SebastianRM

    Problem 9.2 Classical Mechanics: Astronaut in a rotating space station

    Homework Statement Acceleration experienced by an astronaut in a rotating space station. Homework Equations What force would he experience is his own rotating frame of reference. The Attempt at a Solution Newton's second Law for a rotating frame is: mr'' = F net+ Fcor + Fcf Fnet (In the...
  21. brotherbobby

    Solving Rotating Ice Skaters Problem: Is their Straight Line Parallel?

    Statement of the problem : Two ice skaters circle about a point while holding hands. At a certain moment both let go and move along straight lines. Are the two straight lines parallel? Explain. My attempt : Calling the two ice skaters ##S_1## and ##S_2 ##, they must lie...
  22. brotherbobby

    Rotating physicist and momentum conservation

    We are aware of the well-known problem of a rotating physicist whose angular velocity ω increases as a consequence of angular momentun conservation (##I_1 \omega_1 = I_2 \omega_2, \Sigma \tau_e = 0##). I am assuming that the net external force (##\Sigma F_e##) is also zero along with the net...
  23. S

    EMF induced in rotating rod inside uniform magnetic field

    Homework Statement A 40 cm rod is rotated about its centre inside a region of uniform magnetic field of 6.4 T. Given that the speed of rotation is 15 rad/s, find potential difference between the centre and either end of the rod Homework Equations emf = - ΔΦ / Δt ω = 2π / T The Attempt at a...
  24. T

    A ball (sphere) rotating along a moving incline

    Homework Statement We have a ball of mass ##m##and radius ##r##. it is placed on an incline (We don't know the angle of the incline, nor we do whether the angle is constant along the incline - maybe it is a curved incline) and then released. The COM of ball is ##h## meters above the incline at...
  25. Raihan amin

    Shape of a bubble inside a rotating container

    1. A closed cylindrical vessel filled with water (at room temperature) contains a small air bubble of normal pressure and volume ##V=1~{cm}^3## inside in it.The cylinder is then started to be rotated slowly with a small angular acceleration in a complete weightlessness (at a space station)...
  26. marino

    Power generated by a coil rotating in magnetic field

    The problem: a coil of radius r, length l and N turns, rotating with constant angular velocity ω around an axis perpendicular to its simmetric axis and passing for the center of the coil. The coils is submersed in a static magnetic field, intensity B0, perpendicular to the axis of rotation of...
  27. 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...
  28. E

    Slowly Rotating Device For a Camera Pan Timelapse

    Hello all! I just discovered PF as I have been searching about a project I have been pondering for a day. The idea is to have a device that I can set my 3lb camera and have the device rotate left-to-right && || right-to-left over the duration of 2 hours for use in a sunset/cloud timelapse...
  29. F

    Body rotating about its center of mass

    Hello, As many know, when an external force acts on a rigid body and the force's line of action does not pass through the body's center of mass ##c.m.##, the force will cause the body to both translate and rotate exactly about the ##c.m.##. Otherwise, the body will solely translate without...
  30. F

    How to calculate vibrations on a rotating shaft?

    Is there a way to calculate the vibrations on a shaft rotating at a certain rpm? Is there a way to accurately simulate this?
  31. L

    Quick Question: Rate of Change of a Rotating Vector

    Is the equation presented (that the time-derivative of a given vector in such a scenario is equal to its angular frequency vector cross the vector itself) true in the case of a vector whose origin is not on the axis of rotation? The way I'm visualizing this, if we take such a displaced origin...
  32. Dayal Kumar

    Frictional force between two rotating cylinders

    Homework Statement .A cylinder P of radius rP is being rotated at a constant angular velocity ωP along positive y-axis with the help of a motor about its axis that is fixed. Another cylinder Q of radius rQ free to rotate about its axis that is also fixed is touched with and pressed on P making...
  33. Javier Lopez

    Rotating speed of a proton ion

    I tried to calculted the rotating speed fro a proton from the clasical point of view by using its magnetic moment and the moment of a rotating sphere uniformly charged as example here: https://ocw.mit.edu/courses/physics/8-07-electromagnetism-ii-fall-2012/exams/MIT8_07F12_quizsol2.pdf It is said...
  34. physicschick21

    Deriving Ball Movement on Northern Hemisphere with Rotating Reference Frame

    I have a ball of mass m that is situated on horizontal plane on the northern Hampshire. I am asked to show that the ball is moving, clockwise, in a manner of r = v / ( 2Ω*sin(λ) ) where v is the ball's velocity, Ω is Earth's angular velocity, and λ is the terrestrial latitude So here's what...
  35. J

    Torque of a rotating wheel on a static wheel

    Homework Statement A uniform cylindrical wheel of mass ##m_{1}## and radius ##R_{1}## rotates with angular velocity ##\omega_{1}##. It lies a certain distance (along the same axis) from a static wheel of radius ##R_{2}## and mass ##m_{2}##. The wheels are then pushed against each other with a...
  36. C

    Conservation of Angular Momentum on a rotating disc

    I have a disc that is rotating due to air being blown at its outer radius. The incoming relative velocity of the air is high, therefore the effect of friction supersedes the effect of conservation of angular momentum. The tangential portion of this velocity decreases due to the friction as it...
  37. S

    Is Constant Linear Velocity Possible in Rotating Particles?

    If talking about a particle rotating around an axis away from it by r. if the particle is moving with constant angular velocity ω. is the linear velocity constant or no? Now what I know is that since we have Vt= ωr, so r doesn't change, as well as ω, so Vt is said to be constant. but I think...
  38. substitute materials

    Artificial gravity in a rotating space station

    It is often proposed that gravity could be simulated on a space station by rotating around an axis, such that the astronaut experiences the centripetal force of the space station wall, analogously to gravity. It is usually mentioned that the radius of rotation must be very large to avoid...
  39. N

    I Rotating Coordinates: Solving for x and y

    If one rotates a tangent plane on a curved surface, this point can be expressed as follows: x' = x cos(theta) - y sin(theta) y' = x sin(theta) + y cos(theta) One solves for x and y and computes based on the deviation of the deviation. My question is: would the answer differ if you choose a...
  40. G

    MHB Rotate Point p: How to Rotate by 75° Counterclockwise

    My problem reads as follows: Point p=(3,3√3) is rotated counterclockwise about the origin by 75 degrees. What are the coordinates after this rotation? I have no idea how to rotate a point, let alone by 75 degrees.
  41. Bizmuth

    Calculating the relevant variables for a rotating habitat

    1. Homework Statement I want to calculate the rotational period and surface velocity of a Bishop Ring with a radius of 80 km and a surface 'gravity' of 0.86 G I should note that this isn't for homework. It's for personal interest. I'm looking for a sanity check on this. The homework forum...
  42. Pushoam

    Solving the Rotating Wire Problem

    Homework Statement A circular wire hoop rotates with constant angular velocity ! about a vertical diameter. A small bead moves, without friction, along the hoop. Find the equilibrium position of the particle and calculate the frequency of small oscillations about this position. Homework...
  43. J

    Velocity of a particle at time t in a rotating frame

    Imagine two frames one inertial (x,y,z) and the other rotating (x',y',z'), their origins are always coincident. The rotating frame is rotating as seen from the inertial frame with a time-dependent angular velocity ##\boldsymbol{\Omega}(t)=(\Omega_x(t),\Omega_y(t),\Omega_z(t))##. In the rotating...
  44. B

    Why wouldn't rotating the handle 90 degrees induce an EMF?

    Homework Statement There is some current in the bottom coil. The question asks which would not be able to light the lightbulb - rotating the insulating handle 90 degrees - increasing resistance of variable resistor -reducing resistance of variable resistor - moving the insulating handle...
  45. Amitayas Banerjee

    Equilibrium Conditions for a Rotating Rod with Two Point Masses

    1. A weightless rod carries towards of masses M and M. The roads Hinge Joint to vertical axis OO', which rotates with an angular velocity ω. Determine the angle φ formed by the rod and there vertical. The attempt at a solution If I am not wrong, the two ways to ensure equilibrium are...
  46. L

    A Momentum calculation of a rotating rod

    ##\ \ \ \ \ ##Calculate the 4 momentum of a rotating rod. We divide it into 4 parts. The part 1 is the work of predecessors. ##\ \ \ \ \ ##In Special relativity, the motion of rod AB (which is an object in non inertial motion) can be described in an inertial reference frame and the motion of rod...
  47. Exath

    Rotation of Rigid Bodies: Rotating stick with disc on top

    Homework Statement [/B] A thin cylindrical rod with the length of L = 24.0 cm and the mass m = 1.20 kg has a cylindrical disc attached to the other end as shown by the figure. The cylindrical disc has the radius R = 8.00 cm and the mass M 2.00 kg. The arrangement is originally straight up...
  48. I

    Force Calculation -- Hydraulic cylinder and a rotating arm

    Hi There, Long time reader, first time poster.. I hope someone out there can help me. I'm designing an add-on to an existing piece of equipment and I'm unsure of how to calculate the resultant force output from a hydraulic cylinder. On a straight push, the chosen hydraulic cylinder has an...
  49. K

    Determining the Speed of a Rotating Disk with a Constant Force

    Homework Statement A solid uniform disk of mass 21.0 kg and radius 85.0 cm is at rest flat on a frictionless surface A string is wrapped around the rim of the disk and a constant force of 35.0 N is applied to the string. The string does not slip on the rim. 1- When the disk has moved a...
  50. T

    Seal rotating parts/reduce friction (hand-tight thread)

    I am working on a design where I have a block which has a cap assembly fitted to it - see images attached. The cap assembly is constructed of: - Main body with two holes through (flowing water in and out) - Locking ring with a threaded outside, grooves for tightening by hand and two o-rings...
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