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. D

    What is the period of rotation for three stars forming an equilateral triangle?

    Homework Statement Three stars, each with the mass of our sun (1.9891E31 kg), form an equilateral triangle with sides long. The triangle has to rotate, because otherwise the stars would crash together in the center. What is the period of rotation? Homework Equations F1=GM2/r2 ƩF=0...
  2. L

    How Can You Achieve Zero Bearing Reactions with a Single Balancing Mass?

    Hi, I have a shaft rotating at 1500 rpm. the two bearing supports at either end of the shaft, A and B, have reactions of 5kN and 3kN respectively. I need to add a single balancing mass at a point along the length of the shaft inorder to balance the shaft and hence make the bearing reactions...
  3. R

    Find the volume of the solid obtained by rotating it about the x-axis?

    Hey guys, I know it late its a little past one here. But I'm doing an assignment due tomorrow at I've been stuck on the last question for at least an hour. Find the volume of the solid obtained by rotating the region bounded the curves Y=absolute value of x. and y = square root of (...
  4. M

    Surface of 2 rotating fluids in a cylindrical container

    Homework Statement In a cylindrical container (with radius R) there are 2 fluids (separated like water and oil, fluid 1 lies under fluid 2) with given volumes V_i, given densities ρ_i. You let them rotate with respective angular frequencies ω_i. There is no friction. Find the functions of...
  5. D

    Ricochet of moving / rotating block off a wall

    Thank you in advance for your help! The title pretty much says it all. I'm looking to write a 2D physics sim in which various objects can bounce off solid surfaces (without gravity). The objects are rotating and translating at a constant rate. We are dealing only with x and y - not z...
  6. S

    Rotating Vectors: How to Rotate a Vector 90 Degrees Towards a Specific Direction

    This isn't actually a homework question, I'm programming something, it's just what should be fairly simple math I'm just not certain how to do it. Assume I have a normalized vector which points directly up {0,1,0}. I also have another vector X. I'd like to rotate vector X along the...
  7. A

    Trying to find the Tensile Pressure in a Rotating Disk

    OK, so I'm trying to build a hyper-fast rotating disk, probably of aluminum. The tensile strength is about 200MPa (note that this is a pressure, which makes sense), and I'm trying to calculate the tensile pressure throughout the disk. The point being to find out how fast I can spin the thing...
  8. R

    A rotating container injected with a liquid

    Homework Statement A massive cylindrical container of inner radius ##R## is rotating freely with an initial angular velocity ##w_0##. A liquid of density ρ is slowly injected into the container, until the container is fully filled except the center of the container. The angular velocity of the...
  9. Z

    Magnetic Field of a rotating point charge

    Not sure if this should be in quantum section, please move if necessary. We know that: 1. A stationary particle with charge q is not affected by an external magnetic field. We can assume this particle is not magnetic at all, since it would allign and travel in relation to the magnetic...
  10. T

    Finding the acceleration in a gear rotating and constrained by a link.

    Homework Statement I am having difficulties with finding the acceleration at A. In the picture above are the questions that I need help with.
  11. T

    Find Tension of the rotating rod at a distance x

    Homework Statement A homogeneous rod with a length L and a mass M rotates with an angular velocity ω in a horizontal plane around an axis passing through its end.Find the tension of the rod at a distance x from its axis of rotation .Homework Equations The Attempt at a Solution...
  12. A

    The volume of a solid rotating about a different axis

    Homework Statement y = {\frac{1}{4+x^2}} on the interval [0,2], revolving about y = -1 Use either the disk/washer or shell method to find the volume. Homework Equations v = pi\int (outer radius)^2-(inner radius)^2\,dx v = 2pi\int (radius)(height)\,dy x = \sqrt{\frac{1}{y}-4}...
  13. E

    Massive rotating drum on bearings with friction

    I came across the following example. I have a mass m' that is suspended from a massless rope which is wound around a drum of mass m. The radius of the drum is R. The drum is fixed on a shaft with bearings of radius r. I am supposed to determine coefficient of friction between the bearings and...
  14. S

    How Does Friction Affect a Car on a Rotating Platform?

    Homework Statement I'm not given any signs of a correct answer in the book, so could I get a check here? A car is driven on a large revolving platform which rotates with constant angular speed w. At t = 0 a driver leaves the origin and follows a line painted radially outward on the...
  15. M

    Why does rotating a ball on a string faster makes it horizontal

    Hi suppose i had a ball on string and started to rotate it in a circle around my hand. When i increase the speed of the ball it becomes more and more horizontal. The only forces i can think of at play is centripetal and centrifugal forces and the force exerted by my hand and gravity. Is somehow...
  16. A

    Problem involving a rotating frame. Help

    This is a conceptual problem which I am facing for many days. If we convert a scenario in an inertial frame into a rotating frame, we apply a pseudo force i.e. centrifugal force radially outwards on the particle. Right? Also if a particle is having a circular motion in any frame of...
  17. S

    How does a rotating wheel apply torque/force on a ball?

    I saw a baseball pitching machine that throws baseballs at 40-70mph with one wheel... All that wheel does is rotate and as the ball touches the rotating wheel it accelerates and comes flying out of the machine at an appropriate angle.. My question is: what is the physics behind that wheel on...
  18. K

    Rotating an Electron: Why Different Degrees of Freedom?

    How come we can rotate a molecule at many multiples of the angular momentum quantum (rotational degrees of freedom), but we can only rotate an electron at 2 different steps (spin degree of freedom)?
  19. L

    Hypocycloid Equations required to form system of three rotating circles

    Hi there, I have never worked with Hypocycloids before so I'm unsure which equations I should be using; but I'll try and get across what I am trying to build. Essentially I am trying to create a series of hypocycloids that act in a similar manner to the "spirograph". Goal: Three circles...
  20. X

    Two ways to calculate the final speed of a rotating cylinder

    Situation: A uniform cylinder (Mass M, Radius R) is rotating down a slope of incline θ and distance s, there are two methods which I used to calculate the final speed, one of which considered forces acting on the cylinder and the other using energy, pure rolling assumed throughout, both give...
  21. K

    Friction Measurement of Rotating Equipment

    Hello, I have two questions that I have been pondering for some time without arriving at an acceptable explanation. The questions require a little bit of background information: I work on rotating machinery that operate across a range of (relatively slow) speeds, and generally has a...
  22. S

    Torque for rotating mass inside cylincder

    What I have: A hollow cylinder filled 1/3 its dia with slurry. Wt of cylinder is 987251 pounds Wt of slurry is 358157 pounds Radius of cylinder = 8.5 ft. RPM = 7.5 Time taken = 20 seconds What I need is : Torque required to roate the cylinder Torque required to rotate the slurry inside...
  23. mesa

    Need help rotating a parabola on cartesian coordinate system

    Okay I need to rotate a parabola on a cartesian coordinate system, y=x^2 by 90 degrees about the origin (either direction) without using piecewise, or inverse functions. Basically I am trying to use translations and deformations to accomplish this. Anyone thoughts?
  24. H

    Center of mass of fluid in rotating cylinder

    Homework Statement A closed cylindrical canister with central axis coincident with the Z axis has a height H and a radius R. It is suspended by a rod coincident with the Y axis that passes through the canister, transecting its central axis at a height h above the bottom surface of the canister...
  25. M

    Rotating heavy masses x100 of you're own?

    Hello everyone! A few days ago I was in my university I walked in my engineering building and I saw a HUDGE pendulum! Very big and so heavy! It was hung from the celling wall, not sure of its mass though... Anyhow! I was wondering why is it when we stabilize the mass of an object equally we...
  26. N

    Exploring Apparent Forces in Rotating Plates

    Is the weight force an apparent force ? Sorry but on the web i can't find anything. For example if i should think about a man that is on a rotating plate,what are the apparent force ? I think surely the centrifugal force but should i consider even the wight force of the man ?
  27. T

    Motion of a particle on a rotating frictionless turntable

    Homework Statement 1 A particle is gently placed on a frictionless rotating turntable at a distance r from the centre.Describe particle motion from both ground and table frame. 2 A particle is at a distance r from the centre of a frictionless stationary turntable.The turntable then starts...
  28. B

    Accelerationg of rotating mass *along* the axis of rotation

    Says Wikipedia: "The moment of inertia is a measure of an object's resistance to any change in its state of rotation". Now consider a rotating mass m that I would like to accelerate along its axis of rotation by a. Does this count as a "change in its state of motion"? Will it resist the...
  29. L

    Fields generated by a rotating disk

    Homework Statement We have an uniformly charged disk with total charge q, which is rotating around its axis with constant angular velocity w. Calculate electric and magnetic field in the axis and in the rotation plane. Calculate the radiated power in one cicle. Homework Equations...
  30. P

    How to prove that the sum of two rotating vectors in an ellipse?

    Homework Statement Within the xy-plane, two vectors having lengths P and Q rotate around the z-axis with angular velocities ω and –ω. At t = 0,these vectors have orientations with respect to the x-axis specified by θ1 and θ2. How do I find the orientation of the major axis of the resulting...
  31. R

    Rotating Coupling Pin-Mechanical Failure as a Safety Precaution

    Rotating Coupling Pin--Mechanical Failure as a Safety Precaution Hi everyone, I am RagingCalm, a mechanical design engineer. I hate to ask for help on my very first post but considering that I am utterly stumped and no one at the office can help me...well...ummm...help, please. :frown...
  32. M

    What Does Rotating a Circular Arc Create?

    A 180 degree circular arc (i.e. a half sphere) is obvious: When you rotate this about its two end points, you get a sphere. What about for something less than 180 degrees (e.g. 90)?: I believe this forms an ellipsoid, with coefficients a and b being equal, with respect to...
  33. S

    What is the general approach for calculating tension in different situations?

    Hello people, So i found out the tension in a ring rotating with constant angular velocity (in gravity free space) Considering a small element of mass dm - tension will provide the centripetal force, 2Tsin(dθ/2) = dmrω^2 sindθ ≈ dθ dm = m/2πr ds ds = rdθ T = (mrω^2)/2πNow, the other method...
  34. Q

    3D Rigid body dynamics - Rod over rotating disk

    Homework Statement A thin uniform rod is attached to an axis through its midpoint. The axis is standing on a disk rotating with constant angular speed \Omega about its symmetry axis. The rod's midpoint is located directly above the rotational axis of the disk. Let \theta denote the rod's...
  35. A

    Newtons 2nd law for rotating bodies

    Is shown like this in my book: Consider a rotating body with an angular acceleration α. There must be a tangential force component if it is rotating: For a general point on the body we can write: Ftan = mi * ai = mi * ri * α (1) Multiply by ri and sum up you...
  36. R

    Reaction force of freely rotating rod

    Hi guys! I'm working some problems for our exam in a few weeks and there are a few problems we are unable to solve. One of them is: "A homogenous rod is free to rotate without friction around one of it's ends. Find the reaction force in function of the position of the rod." As you can see the...
  37. E

    I can't visualize spin I know that its not just a particle rotating

    I can't visualize spin... I know that its not just a particle rotating about its axis... Then how do I think of it?
  38. T

    Wind resistance on a rapidly rotating flywheel

    I am simulating a rapidly rotating (horizontal axis, if that makes a difference) flywheel. The software isn't too sophisticated w.r.t. wind resistance, unfortunately, and seems to use the diameter of the rotating body as one of the main parameters, in addition to the velocity of the rotating...
  39. P

    Photon Clock & Time Dilation: Rotating the Clock 90 Degrees

    In the case where you have a photonic clock bouncing a photon between 2 mirrors, if the mirrors are alligned on the vertical axis, bouncing the photon up and down and the overall clock traveling along the horizontal axis, then the whole thing makes sense to me. However, if you were to rotate the...
  40. G

    Rotating Vectors Homework: Fill in the Blanks

    Homework Statement This is an answer to a problem involving rotating vectors I can't figure out what the blank spaces mean. Are there 0's there? Are there 1's? Help.
  41. X

    Understanding Rotating Coordinate Systems: A Visual Approach

    Homework Statement http://www.brookscole.com/math_d/special_features/stewart_shared/mathematica_labs/14-multipleintegrals/p05a.pdf Homework Equations The Attempt at a Solution My question concerns the (1) and (2) next to the figure of the rotating coordinate system...
  42. P

    EMF induced in a rotating coil

    Hi, So, I am learning about induced emf. I am having a hard time grasping the idea that we are interested in the angle between the normal to the plane of the coil and the magnetic field. In the standard illustration I actually understand it, but the following question confuses me. We are...
  43. C

    Transform a pde into rotating frame

    Hi, I have an equation of the form; \frac{d}{dt}(W) = \omega \left(x \frac{\partial}{\partial y} - y \frac{\partial}{\partial x} \right) W + g \frac{\partial}{\partial y} W + k x \frac{\partial^2}{\partial y^2} W I want to change it into the rotating frame using the transform; x...
  44. A

    Mass-spring-mass system rotating about its centre of mass relativistically

    Homework Statement 2 equal masses 'm' are attached to a spring of spring constant 'K' and unstretched length 'L'. The system is rotating about its centre of with an angular velocity 'w'. The linear velocity 'v' of m is comparable to that of speed of light 'c'. Find a relation between the...
  45. N

    Force applied on a rotating disc

    I understand that if we apply a force on the axis of rotation of a spinning disc, the torque is in a different direction, and the axis is moved in a direction perpendicular to the direction in which the force upon the axis was applied, determined by the right-hand rule. But what if the force...
  46. snoopies622

    Geodesics in a rotating coordinate system

    In a uniformly rotating coordinate system the trajectories of freely moving objects are influenced by an apparent centrifugal and Coriolis force. Is there a coordinate system or metric (or both) in which these trajectories are geodesics instead?
  47. C

    Why should rotating black holes emit particles?

    In a brief history of time it says theft 'according to the uncertainty principle rotating black holes should emit particles'. However I do not understand why rotating black holes need to emit particles according to the uncertainty principle. I do understand why black holes emit particles it...
  48. J

    Rotating Object Transformation: Length/Circumference Change?

    If a disc-like object was rotating so that its outside edge was at about 0.86c (causing a factor of change of about 2) relative to an observer, then would it contract in length along the axis of rotation, or the circumference (relative to the observer)? in other words, would it get thinner or...
  49. J

    What causes angular momentum in rotating bodies

    Starting at BB everything moves outwards with linear momentum so unless the BB event was rotating where does the angular momentum come from, the Earth rotates, it orbits the sun, the galaxy is rotating and the sun orbiting within it. So it seems that angular momentum is the norm for bodies...
  50. binbagsss

    Angular acceleration direction, rotating rigid bodies

    The direction of rotating bodies about a fixed axis. I am confused on how you should regard the the direction of mg sin x, the angular acceleration and the force exerted on the body by the axis, perpendicular to OG ( where O is the fixed axis location and G is the centre of mass of the body)...
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