Questions tagged [rotation]

Circular motion about a central point or axis

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Is the rotational velocity of a vortex consistent across its height?

I am trying to locate information about calculating the rotational velocity of a vortex. Is the velocity at the vertex maintained across the entire vertical height of the vortex? As the radius ...
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128 views

Rotations of spin eigenstates in QM

If you have a state $| \psi \rangle = | \uparrow \rangle$ which is the spin eignstate of the spin operator $\hat{S}_z = \frac{\hbar}{2} \hat{\sigma}_{z}$ then if you view this state as a vector in the ...
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What sort of friction does a rolling cylinder experience?

When will static friction act on a cylinder executing pure rolling? Will it act when there is some external force (or without it) and if yes wouldn't it rather be kinetic friction? The cylinder is ...
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1answer
70 views

Would an object floating in a spinning cylinder in empty space be pushed toward the wall of the cylinder? [duplicate]

This is a rather simple question, but it intrigues me nonetheless... If there was a ball in a hollow cylinder that was spinning (on an axis) in empty space (no noticeable gravitational effects), and ...
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209 views

Rigid-body Rotation and Kinetic [closed]

Math and formula issue Default Kinetic energy formula My question is How did we go from 2 to 3 ? I do not understand where the: (V_cm/R^2) comes from. Please give a description, where do i fail.
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Finding average angular velocity, getting a neg answer using my inst. vel. expression. Result ok? [closed]

I have a two part question that first asked me to find an expression for instantaneous velocity given $\theta=8.5t-15.0t^2+1.6t^4$. I found that to be $w= 8.5-30.0t+6.4t^3$. Then the next part was ...
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2answers
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Force on different points on a body not passing through the centre of mass [duplicate]

I was studying about centre of mass and I found that if the line of action of force passes through centre of mass then it will execute pure translation. Moreover acceleration of centre of mass is net ...
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How many different axes of rotation can coexist?

I have questions about rotation. There is a sphere in space. I can apply a force to cause the sphere to rotate around a central axis. An infinite number of possible central axes can be drawn. ...
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1answer
164 views

How do experiments prove that fermion wavefunctions really pick up a minus sign when rotated by $2\pi$?

Theoretically, after a rotation of $2\pi$, a fermion wavefunction picks up a minus sign, and it is after a rotation through $4\pi$ that it returns to its initial quantum state. Now, the wave-functions ...
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2answers
119 views

Confusion in rotating frames [closed]

my question is,if we very slightly put a ball on smooth(frictionless) rotating disc then what would happen to ball. It will fall or not. because with respect to rotating disc the ball will be acted ...
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1answer
70 views

Which way to rotate a coin?

The picture shows rotation of a coin in two ways along a axis passing through the centre of gravity.Which method is easier to rotate the coin?
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350 views

Vector addition and translations

I'm reading Feynman's lectures and I have a question regarding volume I chapter 11. In this chapter, Feynman says that not every group of numbers is a vector. He says that if we apply an operation ...
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Why is linear movement relative but rotation is absolute? [duplicate]

Rotation,at classical mechanics at least, is always absolute but linear movement is not. What is the reason for this distinction? I will add a second question. Why this is not the case in general ...
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623 views

Derivation of yaw, pitch, roll equations for an accelerometer

I'm struggling to find a good resource that explains how roll, pitch, yaw angles are calculated from the X, Y, Z measurements of an accelerometer. I came across this document but the explanation on ...
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2answers
125 views

How can I solve the moment of inertia?

The source tells me to use the formula for a ring, but it is not possible, as the portions are nearer to the axis than a normal ring. How can I find the moment of inertia?
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Can we change one rotation mode of a disk continuously into one of the other two, keeping the angular momentum vector constant?

If we consider a disk (with a uniform mass density), we have one mode of rotation around the axis perpendicular to the disk and two modes of rotation around two axes, perpendicular to each other, as ...
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1answer
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Behaviour of a magnetic compass needle near a magnet

We all know that a magnetic compass needle when suspended by a thread and placed near a bar magnet, will rotate. However like a bar magnet, will the magnetic compass needle get attracted towards the ...
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4answers
269 views

Kinetic energy confusion?

If I am travelling horizontally, and suddenly a magnetic force acts on me and pushes me inwards (at 90 degrees), so I will follow a circular motion. But if I take a point in the circle and draw ...
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How to remove gravity component from accelerometer X, Y readings

So I have an accelerometer which I am wanting to use in an IMU. When the device is tilted but stationary I want the x, y values to be 0, so effectively negate the effect of gravity along the x and y ...
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Are laws of physics (mechanics) symmetric under uniform rotation?

I'm just starting to learn about symmetry. I understand that physical laws are symmetric under translation and consequently (is it not a consequence?) under uniform velocity in a straight line. I see ...
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1answer
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Rotation inside a black hole

As far as I can see most of the questions about rotation of a black hole refer to the appearance of a hole to an outside observer. What about the region within the Schwarzschild radius? According ...
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0answers
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rotational invariance

I have been treating this Hamiltonian: $$H=v\vec{p}\vec{\sigma}+\vec{A}\vec{\sigma}$$ where $\vec{\sigma}=(\sigma_x , \sigma_y)$. It is relevant for 2D graphene quantum dots with some vector ...
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2answers
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Man rotating in space

Well I think if I were in a gravity free space and was rotating then will I be able to stop myself completely by moving my hands away from my body and hence increasing moment of inertia($I $) and ...
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1answer
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What is the size of the particles which make axis of rotation in a rotating object? [closed]

In a rotating object some particles which are not moving tend to be in a straight line "axis of rotation" and all the other particles of the object tend to be in circular motion around the axis.I want ...
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What is the 4-dimensional matrix representation of rotation operator?

The rotation operator is $$\exp\left(-i\frac{\theta}{2}\boldsymbol{J}\cdot\hat{\boldsymbol{n}}\right).$$ If $\boldsymbol{\sigma}$ is the Pauli matrix, the operator can be written as a matrix form $$\...
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Calculating force through ICOR (instantaneous centre of rotation)

A purely rolling wheel of radius $R$, has its ICOR at the point $P$ (as per the figure). If I calculate the centripetal force on a particle at the top most point of the rim, considering point $P$ as ...
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1answer
144 views

Is Lcm really equal to Icm x w?

Let's suppose a body is rotating about some axis passing through the centre of mass (cm) with a angular velocity $\vec \omega$. The angular momentum about the centre of mass axis is given by $L = I_{...
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1answer
130 views

Rotating rod - Why can't it reach the speed of light? [duplicate]

$v=\omega * r$, so with $\omega = 1 rad/s$ and $r=3*10^8 m$ the v would be c. I know that the information of the rotation travels with the speed of sound in the rod.
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What does it mean if the torque about the COM of a rod is zero but it is non zero about some other point on the rod?

Suppose that there is a thin rod $AB$ lying in a gravity-free environment. There are equal and parallel forces which act on it’s ends A and B. Now w.r.t the COM the net torque is zero. That means ...
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1answer
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When are two rotating shafts at different speeds both at 360 degrees?

I have two rotating shafts that are both oriented at 0 degrees when a test starts. Then assume both shafts instantly start rotating at 60 RPM. One is maintained at a constant 60 RPM and the other ...
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Does the spin1/2 rotation operator rotate spin in real space?

in quantum mechanics, the rotation operator for spin one half is $R_{\alpha}\left(\hat{\boldsymbol{n}}\right)=\mbox{exp}\left(-i\frac{\alpha}{2}\boldsymbol{\sigma}\cdot\hat{\boldsymbol{n}}\right)$. ...
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870 views

Calculate the force needed to move a ball on a horizontal plane?

Lets say I have a stationary ball on a horizontal plane. I want to find how much horizontal force is needed to push the ball so it starts moving. The only forces that would be acting on this ball ...
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How do Cogs and belts work? / Cogs and belts in an ICAS exam

Does the left red wheel turn in the same direction as the leftmost orange wheel?If so P and S go up? If not, Q and S go up? My reasoning: If so Leftmost orange cc Left red cc (by assumption) ...
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1answer
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When is the motion oscillatory and when is it not

Sometimes in physics questions we see examples in which applied force is balanced by an innate force of the body. Like in torsion balances and Cavendish's experiment. In this case we say that rotation(...
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1answer
336 views

How to solve rotational dynamics problem? [closed]

I am having tons of trouble solving this problem: Two masses, m1 = 2.0 kg and m2 = 1.5 kg, are tied together by a massless string. The string is draped over a pulley that has a mass, M = 1.0 kg, and ...
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Equation to predict the shape of a planet [duplicate]

I need help calculating gravity and shape of a planet based on a few factors. If a mass rotates quickly enough, the inertia (or "imaginary centrifugal force") will pull the mass outward "...
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1answer
68 views

Uniform circular motion criteria and take off conditions

Does constant angular velocity imply uniform circular motion? When talking about a material point rotating and axis, if we know that it rotates it at constant angular velocity does it imply its ...
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696 views

How to find position of point in 3d undergoing coordinate system rotation

I have solid body at a given point P (a, b, c) in a static coordinate system (xyz) centred at the origin. This solid body however has a certain rotation of its own and I need to figure out the ...
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Is the direction of rotation vector is convention?

Rotation vector is along the axis of rotation and is given by right hand rule. As much I understand, rotation can be clockwise as well as anticlockwise, in other words it has direction. So it can(or ...
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1answer
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Realistic Value for the Rotational Inertia of a Gate [closed]

Don't worry guys i won't ask how to calculate the moment of inertia of the gate. I already have, but i have no experience in real life engineering, so i would be thankfull if any of you could review ...
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1answer
337 views

Forces when a ball rolls down an inclined plane

I apologize if this seems trivial, but I am having some trouble understanding the solution to the following problem: First off, there is a normal force that the plane exerts on the ball, and the ...
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1answer
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When does torque equal to moment of inertia times the angular acceleration?

When does torque equal to moment of inertia times the angular acceleration ? Particularly, for example, if a disc is rotating and translating on a horizontal surface with friction by the help of ...
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Why can’t we convert potential energy into linear kinetic energy but , can convert it into rotational kinetic energy?

Here is a question from my book - A meter stick is held vertically with one end on a rough horizontal surface. It is gently allowed to fall on the ground. Assuming that the end at the floor does ...
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Is it possible that there is a net torque on a body but still it is not rotating or angularly accelerating?

Is there any case possible in which we apply a torque on a body, but that torque does not make the body turn but instead change it's angular momentum or vice versa?
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Lorentz Force Pulsed DC Solenoid Rotating Magnets

Let's say there is a solenoid consisting of 8 loops. The Solenoid has an air core and the wire is welding wire. Each loop has a diameter of 1 Meter. Now imagine a motor shaft running through the ...
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Why does rotation occur? [closed]

So it sounds like a dumb question, as it is very intuitive why rotation occurs. However, can you give me a scientific explanation as to why whenever I exert a force on a body it tends to move, but ...
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A sign problem?

Consider a particle moving in a vertical circle of radius $r$. The particle is acted by gravity and it is connected to the center of the circle by a massless rod. If I decompose the gravity force ...
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1answer
803 views

Pendulum with Friction

There is a pendulum as shown in the image, with a disc, a rod, and a weight on the rod, pivoted at the rod's center. The masses of the rod and weight are $M$ and $m$, respectively; the mass of the ...
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Why wouldn't the part of the Earth facing the Sun a half year before be facing away from it now at noon?

The Earth takes 24 hours to spin around its own axis and 365 days to spin around the Sun. So in approximately half a year the Earth will have spun around its axis 182.5 times. Now take a look at the ...
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Acceleration with respect to rotating frame and a rotating object

Consider a space, free from gravity, etc. and there is a cylinder (radius $R$) rotating about its axis in this space with an angular velocity $\vec{\omega}$. I'm standing (rest wrt cylinder) inside on ...