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. B

    A rotating system of two point particles with inner torque

    Lets say we have a system of two point particles (1. and 2.) which are rotating around an axis. What is written next in my physics course book is: The torque of a 2.body on the 1. body is M21=r1xF21 and the torque of the 1.body on the 2.body is M12=r2xF12. Understandable. But how? There is no...
  2. alexm

    Motion of rotating rig, find the angle variation with control rod length

    Summary:: We have a rotating arm, offset from the centre of rotation by a certain length, which is controlled by varying the length of a control rod. Need the angle of the rotating arm in terms of length of the rod. The blue line is a fixed column structure. CE and BD form the rotational...
  3. SpaceThoughts

    Low friction horizontal rotating platform

    I am looking for a rotation devise like that one in this video below (from 00:40 seconds). Does anyone know where to get one, or have any idea about how to create a nice and stable rotation setup for experiments without too much hassle and DIY? A bike wheel won't do, a disk is better, because I...
  4. JakubKivi

    Strange turn of a rotating playing card when I throw it

    I have observed that when i throw a card and give it a angular velocity, it "aim" to vertical and depending on direction start to rotate. For example if it rotates to the left in horizontal, vertically left side of cart goes up and right goes down. Conversely for the right rotation. I am not...
  5. erlend

    Hydrodynamic drag of a rotating paddle wheel

    Hello, i have a problem that look's easy to solve but really is not. It involves a rotating paddle wheel submerged in water and i want to know the force exerted on the wheels and the torque required to rotate it. I have made a simple drawing to illustrate the problem below. I want to use the...
  6. Saptarshi Sarkar

    Torque on a rotating system system

    I calculated the total moment of inertia of the system to be ##2ml²Sin²θ##, so the angular momentum is ##2ml²ωSin²θ##. To get the torque on the system I need to differentiate the angular momentum but I don't have any time dependent terms. What should I do?
  7. PeterDonis

    A Exploring Rotating Lepton Model for Hadron Masses

    I just came across a 2016 paper [1] that claims to have computed reasonably accurate masses for hadrons using what it calls a "rotating lepton model" and "the relativistic Newton equation". An earlier 2001 paper by two of the same authors [2] appears to be the first introduction of the general...
  8. M

    Velocity / Acceleration Diagrams of a Mechanism - Piston and a rotating rod

    Summary:: This is a question about finding the acceleration of a point in a mechanism Hi, I have a question about the mechanism shown in the attached picture: Question: We are told that \omega = 6 rad/s and the first part is asking me to find the acceleration of point P on the piston when...
  9. synMehdi

    Understanding Unbalance Force in Rotating Systems

    I would like to undertand more the force caused by unbalance in a rotor, assume that the bearing is represented with 2 springs like above: The geometrical center of the rotor ##O## is equivalent to it center of gravity ##C_g## and the center of the stator (bearing) ##O'## The rotor is perfectly...
  10. Saptarshi Sarkar

    Moving relative to a rotating helix

    I was thinking about a situation related to Galilean relativity but couldn't come up with a solution to the problem. I would be very grateful if someone can explain it to me. So, I was thinking of a situation where I am in the reference frame of a block moving at velocity u along the x-axis and...
  11. JD_PM

    Man rotating in a merry-go-round and grabbing a pendulum

    Where: 1) ##A## is the translational acceleration, ##\Omega## the angular velocity and ##\dot \Omega## the angular acceleration (all relative to the inertial frame attached to the ground ##F##). 2) ##r'##, ##v'## and ##a'## are the position, velocity and acceleration vectors, all relative to...
  12. astrocytosis

    Magnetic field of a rotating cylinder with permanent polarization

    I am struggling to get my work to match the posted solutions to this problem. I understand part (a) but can’t get the integral to work out for (b). I know I have to use Biot-Savart and add up the components from the the surface and volume currents. The cylinder is very long, so I need to make a...
  13. B

    Rotating Flywheel: Tips & Tricks for Beginners

    Hey everyone! Was trying to answer this question and was wondering if this was the right way to go about it? Any help would be really appreciated!
  14. JD_PM

    Man walking on a rotating merry-go-round

    Some information Newton's second law in a non-inertial frame is given by: Where: 1) ##A## is the translational acceleration, ##\Omega## the angular velocity and ##\dot \Omega## the angular acceleration (all relative to the inertial frame attached to the ground ##F##). 2) r', v' and a' are...
  15. Ibix

    A Stress-energy tensor of a rotating rod

    A rod rotates freely (edit: about an axis perpendicular to its length) in empty space. Working in an inertial coordinate system where the rod rotates around a fixed point, the rod is straight, of length ##2L## in its spinning state, and its mass distribution is symmetric along its length. The...
  16. Nacho_rc11

    Rotational dynamics: Rotating rod with two attached balls about a non principal axis

    Firstly I deduced that in this situation the moment of inertia I, is not going to be parallel to w. And I calculated it as a matter of the angle, for the rod and the two point particles attached (with a mass 'm'), and the total moment of Inertia ended up being: I=((R²*sin²α)/2)*(M/6 + m) Being...
  17. Moara

    Wire rotating inside a magnetic field

    For a infinitesimal wire of lengh dx, the induced potential difference in an uniform B field perpendicular to it's motion is : dE=B.Vp.dx, where Vp is the component of the velocity perpendicular to the wire. Looking to the big wire I tried to take an arbitrary point express dE in function of...
  18. N

    MHB Calculate volume of a solid rotating around the y-axis

    Sorry if i made any language errors, English isn't my first language. Question: The limited area in the plane is created when the space between the line y=1 and the graph to the function f(x)=3*x/(x^2+1) rotates around the y-axis. Calculate the volume of the solid.I want to sum up all the...
  19. jisbon

    Free body diagram -- Ball rotating on one arm of a Y shaped pole

    To start off with, I can't seem to interpret the FBD here. Here are my drawing: and what I interpret as. From here, I feel like I can (it's wrong obviously but I'm not sure why) state that sin theta = o/h = o/mg = N/mg , so N= mg sin theta? Thanks
  20. D

    Calculating the average linear speed of all points in a rotating sphere

    Is there a way to calculate the average linear speed of all points in the volume of a sphere rotating on a single axis? Since points closer to the axis of rotation and the poles move slower than points further out, would the average speed be a simple function of r/2 and pi? It would seem that...
  21. Kaushik

    To find the increase in the radius of a rotating ring

    The increase in radius is due to the centripetal force acting on the ring. The centripetal force acting on each point of the ring is directed towards its center. We can find force using ## F_c = M(\omega)^2R## We can use this ##F_c## in the equation of Hooke's Law to find the elongation Could...
  22. Kaushik

    Tension of a rod rotating on a horizontal table about a vertical axis

    If a rod is on a table (horizontally) and rotating about an axis that passes through one of its ends and vertical to the table, what would be the tension on the opposite end of the rod (the end opposite to the axis) . In this post (Check this post out from Socratic QnA), the limits take while...
  23. melissafern

    Find the forces applied to a rotating level

    Homework Statement: Solve parametrically the system with g, load cell measurements and know accelerations. Homework Equations: to be found Consider that we know angles tetha(t) of a mass-less link rotating about its centerr O. - so we also have velocities and accelerations - and values of...
  24. N

    Exploring Induction in Rotating Spheres: Is My Solution Valid?

    Let ##(r,\phi, \theta)## be the radial, polar and azimuthal coordinates respectively. As ##\vec{B}## is confined to ##xz## plane such that ##\theta = \alpha## I assumed ##\vec{B}## on the surface of shell to be ##\vec{B} = a\sin(\alpha) \hat x + \cos(\alpha) \hat z \tag{1}## Surface area...
  25. Santilopez10

    Mass attached to a rotating spring

    a) Our force can be represented as: $$\vec F= -k(r-H) \hat r$$ then the equations of motion are: $$\hat r: \ddot r -r {\dot{\theta}}^2=-\frac{k}{m_1}(r-H)$$ $$\hat{\theta}: r \ddot{\theta} + 2 \dot r \dot{\theta}=0$$ Plus we know that angular momentum is constant then $$|\vec L|=m r^2...
  26. G

    Jumping up on a rotating Earth

    Hello, I know it suppose to be a relatively basic question but still somehow I can't fully understand it. Let assume that a man jumps vertically on the equator, while the Earth is of course rotating. What will happen to the value of his linear momentum in the horizontal axis? It seems to me...
  27. danielp3

    Calculating Torque for Rotating Wheels: Factors to Consider

    Say I have a motor connected to a wheel, the wheel is driven forward. The motor produces a certain torque rotate/turn the wheel right or left, how do I calculate the torque needed to rotate? keep in mind the friction between the surfaces, the mass of the wheel and the tire sag/subsidence due to...
  28. R

    Proving Vorticity of Flow in Rotating Cylinder

    Can someone check if my answer is correct please? Question: If liquid contained within a finite closed circular cylinder rotates about the axis k of the cylinder prove that the equation of continuity and boundary conditions are satisfied by u = ΩxR where Ω = Ωk is the constant angular velocity...
  29. K

    Rotating and tilting charged disk induces a voltage inside a ring

    As I`` m learning for an upcoming exam I found an electrodynamics problem I struggle with. In the first task I need to calculate the magnetic dipole moment of a uniformly charged,thin disk with the Radius R and a total charge Q which rotates with a angular speed omega round its symmetry axis...
  30. J

    I Speed of Light Paradox: Neutron Star Rotation Rate

    Sorry if this is a stupid question but I couldn't find an answer anywhere. According to 2 scientific papers, the neutron star PSR J1748-2446ad has a rotation rate of 716Hz, which equates to a linear surface speed of 0.24c. What if this star was originally rotating, let's say, 5 times (or more)...
  31. brotherbobby

    Tension in a rotating rod at various places

    (The answer given in the text says ##\boxed{T_1\; >\; T_2}## but, as I show below, I think it's just the opposite). I begin by putting an image relevant to the problem above. Taking a small particle each of the same mass ##m## at the two positions, the centripetal forces are ##T_1 =...
  32. Like Tony Stark

    Finding the angular velocity of a rotating arm

    A) The slider experiments three forces, all of them are on the ##x## axis (considering ##x## axis as the axis aligned with the arm): Normal force (exerted by the support), elastic force and centrifugal force, which is ##m.(\omega^2 r)## Elastic force is equal to ##Fe=-k \delta =-2 (R-2R)=2R##...
  33. brotherbobby

    Rotating a given vector about an axis

    The sketch above shows the situation of the problem. Clearly, as the rotation is taking place in the ##y-z## plane, the x-components of the two vectors remain unchanged : ##A_x = B_x##. Let the projection of the vector ##\vec B## on to the y-z plane be vector ##(\vec B)_{yz} = B_y \hat y + B_z...
  34. jisbon

    Rotating cylinder rolling without slipping in a B field

    Firstly, I need to determine what the electric field is causing. Using left hand rule, the force due to the field is acting down the slope. Hence my FBD looks like: Where the two arrows pointing towards the right represent the force due to the field and weight of the cylinder. Since ...
  35. Bofors

    Rotating planet moving through space

    Its velocity, Vp, is towards some other part of the universe, It rotates in the same plane as its direction of travel. The linear velocity of an object on the surface of the planet relative to the centre is Vr. That means the object's total linear velocity is Vp+Vr when it's at one point in its...
  36. Meruem

    I Observing a rotating body when approaching it at near light speed

    Summary: Will the motion around us become faster when we travel faster? When we approach a body rotating on its axis with certain speed v, will we see the body rotating in speed slightly more than the v during our motion ?And what happens assuming that we are approaching the same body in speed...
  37. Like Tony Stark

    Particle moving in a rotating disc

    Well, I tried plugging the data in the formula. I know that ##\vec a_b = 0; \vec \omega=3 rad/s ; \vec r## can be calculated using trigonometry. Then I also know that ##v_{relx}= 10 cm/s##, ##a_{relx}=15 cm/s^2##, ##\vec {\dot{\omega}}=-10 rad/s^2##. But how do I get ##v_{rely}## and...
  38. JD_PM

    I Two rings rotating about a common center

    Problem statement: Two rings rotate with equal and opposite angular (relativistic) velocity about a common center. Matt rides on one ring and Eve on the other and there's a moment they meet and their clocks agree. At the moment they pass by one another, each asserts that is the other's clock...
  39. C

    EMF induced in an off-axis solenoid due to a rotating magnet

    The red arrows correspond to the areas the solenoid will be moved too
  40. Like Tony Stark

    Relative rotational motion on a disc

    The first doubt that comes to my mind is "I have to determine the acceleration with respect to what?", because the problem doesn't tell. Then, I have some problems when having to plug the data in the formula of acceleration. ##\vec a_B=0## because the origin isn't accelerated, ##\vec{\dot...
  41. lfdahl

    MHB Volume of the solid obtained by rotating R around the line y=x

    Let $R$ be the region $\left\{(x, y) : 0 \leq x \leq 1, 3^x − x − 1 \leq y \leq x\right\}$. Find the volume of the solid obtained by rotating $R$ around the line $y = x$.
  42. A

    I Relativistic Energy of Rotating Thin Ring: Quick Qs

    Quick question about the relativistic energy of a rotating thin ring, hoop or cylinder. Is there any reason why the relativistic energy would be anything different than ##E=\gamma_t m_0 c^2## where ##\gamma_t## depends on the tangential velocity ##v_t## observed by someone at rest with the...
  43. Prabs3257

    Rotating Pendulum: Is This Eqn Correct?

    is this eqn correct
  44. Seth Greenberg

    What is the angular velocity in the center of a rotating disc?

    I have a disc. The center of the disc is its center of mass and the motion of the disc is purely rotational (no translation). What is the angular velocity in the center of the rotating disc?
  45. Physics lover

    EMF induced in a rotating rod having mass

    I first calculated induced emf and then calculated torque about O.But what will i do further.I think i need to find current and then fidn emf=IR. Help please.
  46. Moara

    Probability of hitting a rotating disc

    There are two points of the smaller disck such that a line is tangent to the two discks and passes by that point. If the particle leaves of any point belongin to the arch that connects these two points, then it will hit the other disck. So I managed to calculate the value of this arch and divide...
  47. Prabs3257

    Why is my answer for the rotating wedge problem not matching?

    I tried making the fbd of both blocks the block on the incline will have mgsin30 friction force, tension and centrepetal force's cos component similarly second block will have mg tension and friction where normal will be centrepetal force after solving my ans is not matching
  48. S

    B What happens to speed and velocity in a rotating frame of reference?

    Let's say I can see a star in a distance of 1ly in front of me. I'm sitting on an office chair and begin to rotate with a speed of 1/s (1 rotation per second). Then I consider me as not rotating and everything else as rotating. Now I will see the star traveling a circular path with the...
  49. gary0000

    Rotating an ellipse to create a spheroid?

    I was able to find the equation of an ellipse where its major axis is shifted and rotated off of the x,y, or z axis. However, I could not find anywhere an equation for a spheroid that does not have its axis or revolution along the x,y, or z axis. How might I go about deriving such an...
  50. 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...
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