# Questions tagged [rotational-dynamics]

A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.

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### Why is stable rotation achieved about the long axis of a skateboard? [duplicate]

I've been preparing a video for my mechanics assignment and I've hit a deadlock. Basically, I want to prove that the long axis of a skateboard takes the least force to move around it.I figured that a ...
57 views

### Where does the energy go? [duplicate]

Suppose a typical case of a girl dancing and after sometime changing her $I$ (moment of inertia to rotate) faster, my doubt is in this case there is a non-zero difference in initial and final total KE ...
52 views

### Irodov 1.146 precission

Shouldn't we take centrifugal force by net $w$ which is about instantaneous axis of rotation? Solutions I saw said to take the precission frequency only which I am not able to understand, why?
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### Rotating Of Object [closed]

The box is secured on top of a rotating platform. Coefficient of friction between circle platform (1m radius) and rectangle platform underneath it is 0.06. What force is required to rotate the ...
4k views

### Why is the concept of torque necessary?

In my physics textbook (“Physics for Scientists and Engineers” by Serway and Jewett) it says that “the cause of changes in the rotational motion of an object about some axis is measured by a quantity ...
39 views

### Zero Angular Momentum and Rotations [closed]

It's not difficult to understand why a particle that is not rotating does have angular momentum, just looking at it's basic definition: \begin{equation} \mathbf{L}=\mathbf{r}\times\mathbf{p} \end{...
25 views

### Why does the torque in a DC motor decrease linearly?

From what I know, the torque decreases because the net current within the coil decreases, as a result from the Back EMF opposing the supply EMF. However how do we know that this current and hence ...
70 views

### Solving this problem using the Work-Energy Theorem [closed]

A block is tied to a string wound on a cylinder of mass $m$ and radius $r$, (Through a pulley) which can rotate about its axis on a massless mount placed on a smooth surface. The system is released ...
28 views

### How to calculate the effective mass of a body going up a curb? [closed]

I am currently doing a project in which I am finding the force required for a skateboard to go up a curb and was wondering how I would calculate the effective mass so that I can use it in the ...
16 views

### Why does this aircraft gyroscope has two degrees of freedom?

This is a Directional Gyro Indicator, it is a tied gyro, the spin axis is tied to the aircraft horizontal axis. I read that this gyroscope has two degrees of freedom. Please explain why is it so? What ...
21 views

### A complex rolling system [closed]

A point mass m is connected to a rigid cylinder of mass M via a massless, rigid, rod. The system (see the figure below) is then released from its (unstable) equilibrium state and the cylinder starts ...
29 views

### High wire artist

We want to derive a Lagrangian equations for a high wire artist which uses a balancing rod for stability purposes. We know the masses and the polar moment of inertia of the artist and the rod.
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### Is the Moment of Inertia tensor symmetric due to rotational invariance of space?

In standard classical mechanics' textbooks (for instance Morin) one finds that while calculating the angular momentum vector of a rigid body for a 3-D general case from the definition: $$L=r \times p$$...
27 views

### Moment of Inertia in non-rotational context

I have read that Civil Engineers use moment of inertia to characterize the elastic properties (rigidity) of such structures as loaded beams. My doubt is how is Moment of Inertia helpful in explaining ...
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### Are these equations correct for the motion of a gyroscope?

I was coding a gyroscope simulation and came up with some equations that I used for it. So firstly, torque = (change in angular momentum)/(change in time) = (vertical moment of inertia)(vertical ...
28 views

### Angular velocity of a translating and rotating frame

I want to determine the angular velocity $^{I}w^{B}$ which is the angular velocity of frame B about the Inertial frame I. The position of P relative to O is written as: \begin{equation} r_{P/O} = r_{O'...
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### A string is wrapped around a cylinder what would be the length of the string that passes the man's hands if he also moves towards it?

I came across a simpler version of this question where the man was stationary, and pulling on a string which was wrapped around a cylinder. We assume that there is no slipping anywhere. Then we know ...
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### Representing vectors in non-inertial rotating frame of reference

Suppose an object is moving in a circular path about a certain point $O$, with an angular velocity $\omega\hat{k}$ or linear velocity $v\hat{\theta}$. This is basically describing an anticlockwise ...
61 views

### How to determine the direction of the tendency for a rolling object to slip?

I know that the direction of static friction in a rolling motion has been asked about before, but I am specifically asking how to determine the direction of the tendency to slide. In this picture, the ...
32 views