# Tagged Questions

A tag for questions about rotational motion, including angular velocity and angular acceleration.

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### Force applies to sphere not on center of mass?

Is this true: When force is applied to a sphere NOT on center of mass (COM) then the sphere will move the same way as when force is ON center of mass, because the sphere is symmetric in every ...
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### Is angular velocity parallel to axis of rotation?

I'm reading the Wikipedia page on angular velocity. It says here of the angular velocity vector in three dimensions that “[t]he magnitude is the angular speed, and the direction describes the axis of ...
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### Rolling as pure rotation

In my book the following statement was written and I didn't understand it well. Can anyone explain it in a more simple way? Figure 11-6 suggests another way to look at the rolling motion of a ...
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### Instantaneous axis of rotation and rolling cone motion

Suppose a cone is purely rolling (no slipping) around a fixed axis. I mean, it is revolving around a fixed axis perpendicular to the ground and passing through its vertex and also rotating, so the ...
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### Equation of Motion for Rigid Body Motion

In a paper, eq 24 I am reading, the author mentions the equation of rigid body motion which is written as the sum of translational motion of the centre of mass, $x_G(t)$ and a rotational term about an ...
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### Why is the inertia ellipsoid of a higher symmetry than the rigid body?

I was always puzzled by this fact. A uniform cube has a sphere-shaped inertia ellipsoid. The sphere has a higher symmetry then the cube. Is there any deep reason or implication behind it?
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### Piston movements in four stroke cycle?

I was reading about a four stroke cycle. Here's what I understood: In the first stroke, the piston starts at the top and moves down. In the second stroke, the piston moves upwards. In the third ...
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### A ball rolling at the bottom of a spherical bowl [duplicate]

This ball oscillates at the bottom of the spherical bowl which has sufficient friction that it doesn't slip throughout the motion and rolls without slipping. At the extreme points of its motion (as ...
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### Why does this model fall apart when angular velocity is small?

I'm doing a physics problem in which a marble spins around a spinning bowl and both have angular velocity $\omega$. It rotates with radius $r$ around the central axis and the hemispherical bowl has ...
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### Angular momentum equations

I do not understand this because angular momentum is $L=I\omega$ ($I$ is moment of inertia;$\omega$ is angular velocity) but it I have also seen equations where $L= rmv\sin(x)$. I do not understand ...
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### Merry go round physics problem?

So I have the following statement. "A merry-go-round is spinning with a fixed angular speed. As a person is walking towards the edge, the force of static friction must increase in order for the ...
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### Rotation axis of a rigid body

I am confused about a trivial concept. Let the rotation of a rigid body, say with one point fixed, be described by the equation $\vec{x}(t)=R(t)\vec{x}(0)$, with $R(0)=I$. Then, at each instant ...
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### How come a rigid body has 6 degrees of freedoms (DOFs) ? Isn't velocity a DOF?

For rigid body we need to know position of three points and their velocities to determine everything. So that would make 12 DOF. Why do text books say it has six DOFs?
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### Problem on rotation

So this is the problem:A wheel of radius of gyration k is placed on a belt moving with a speed v, which is maintained constant by means of an external agency. Assume that the axis of the wheel is ...
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### Two identical disks pulled differently question (Kinetic Energy)

I am currently taking a basic physics course in college and I am having a bit of trouble on this problem that deals with rotational and translational kinetic energy. Let's begin: The question: The ...
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I'm trying to calculate the moment of inertia for a cylinder about a longitudinal axis, but I don't know where I went wrong with my approach. $$I=\int r^2 dm$$ Assuming constant density: $$\frac{M}... 1answer 105 views ### How are these marbles being accelerated? This question refers to an effect visible starting at around 5m45s in this video1. (The question will make little sense if one has not first watched the clip.) The observation At around 5m45s we ... 3answers 336 views ### Sign of torque when rolling an object down an incline Suppose you have an object rolling down the incline at 30 degrees. Given the point of contact is instantaneously at rest, I decided to analyse torques at that point. Therefore, the only force ... 1answer 147 views ### Why is the Earth self-rotating? [duplicate] What drives this happen? Would it be the internal energy or by an external force? I did try to Google the answer, but could not find a good one. 2answers 299 views ### What is the percentage of energy recovery in Kinetic Energy Recovery Systems(KERS) in cars? Kinetic Energy Recovery Systems (KERS) use flywheels to recover energy from the kinetic motion of cars. They use a rotating flywheel that generates energy as it rotates- this generates the electric ... 2answers 269 views ### Non-constant angular velocity In this paper about Backstepping controll of a quadrotor helicopter an algorithm for control is described, but I have hit a dead end. In equation 15 it is described the part of state space for the ... 1answer 227 views ### Cayley-Klein Parameters I have a very simple question(I guess )to ask$$\frac{d\mathbf{m}}{dt}= \mathbf{C} \times \mathbf{m}$$where \mathbf{m} and \mathbf{C} are vectors. Assume that \mathbf{C} is constant over a ... 2answers 465 views ### Angular Momentum of a rigid, extended object Angular momentum of an object is a physical quantity that depends on the chosen point about which to calculate the angular momentum. It is often said that an object that has been thrown up in the air ... 2answers 83 views ### Is rotational motion of the centre of mass impossible? We know that for a system, the center of mass CM moves as a particle as though all the forces on the system were acting on it. So does that mean rotational motion of the center of gravity impossible?... 1answer 182 views ### Kinematics of Euler angles relative to a rotating frame I have a rotating body B and a rotating frame F whose orientations are described by the quaternions q_B and q_F respectively. I also have the angular velocity vectors \omega_B and \omega_F.... 2answers 850 views ### Rolling of a disk and sphere I am confused regarding the fact that when a disk is rolling on an inclined plane without slipping and similarly a solid sphere is rolling on an inclined plane without slipping then the sphere has ... 3answers 2k views ### Instantaneous angular momentum of a disc Suppose we have a disk of radius r and mass m travelling at velocity v. I want to calculate the instantaneous angular momentum with axis through the edge of the disc (on the circumference). ... 1answer 38 views ### Asking about centrepetal acceleration please look at the fig first 1) How can you claim that the triangle ABC is same as the triangle PQR? 2) How can you claim that the angle between V1 and V2 is same as the angle between AC and AB? ... 1answer 2k views ### How does weight/mass affect angular momentum? How does weight/mass affect angular momentum? For my 8th gr science fair project I have to do an experiment on angular momentum. My problem is that we have not been taught any of that in physics yet, ... 1answer 583 views ### Relating angular and linear kinematics In my physics book "University Physics", there is a chapter on relating linear and angular kinematics. I understand the parts where it shows v = r\omega and a_{\text{tan}} = r\alpha. However in ... 1answer 6k views ### What's the right way to calculate the principal moment of inertia? I am writing a program that incorporates calculating the principal moment of inertia for a protein residue based on its component atom XYZ coordinates. I am exceedingly confused about which formulas ... 0answers 90 views ### Foucault pendulum, where did I go wrong? I wanted to approximate the number N of periodes needed, such that the foucault pendelum turns one time. Though my result is off by the factor 2 of the results I found. What I did: I assume the ... 0answers 69 views ### Translational acceleration due to rotation Sorry for boring you, my friends. I am haunted by re-deducing an expression of translational acceleration in ANSYS theory reference about acceleration effect. It takes the form of:$$\lbrace a_t^r \...
Suppose there are $N$ particles embedded in a rigid body which undergoes some random rotation such that: $$\overline{\overline {R}}_{ij} \otimes \vec{a}_{ij} = \vec{b}_{ij}$$ where, $i$ and $j$...