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

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3
votes
1answer
68 views

What would a star look like that was the most oblate possible?

This question about why is the Sun so spherical and it's corresponding answer made me wonder: If some younger stars that are rotating faster than the sun are more oblate, then how oblate are they? ...
1
vote
0answers
53 views

Solving for position of a SO(3) rotating object, given the integrable functions for components of angular velocity along the principle axes

Assuming that you have approximated or solved the Euler's Equations for components of angular velocity along its principal axes of inertia $x$, $y$ and $z$ - i.e. in the coordinate system that is ...
0
votes
1answer
62 views

Determining the instantaneous axis of rotation [closed]

Given the following image The question states ,a heavy disc with radius R is rolling down hanging on two non-stretched strings wound around the disc very tightly.THe free ends of the strings are ...
1
vote
1answer
29 views

Can you explain the dynamic of forces working in the video?

Lifting Against Gravity - Breaking Magic - Discovery Channel In this video, a woman uses Physics, so that it is difficult to lift her off the ground. The explanation that the video gives at the end ...
2
votes
2answers
90 views

A Confusion in Rotational Dynamics

I am trying to analyse the following situation using classical mechanical concepts. Consider a a straight rod $AB$ of mass $M$ and length $L$ placed on a frictionless horizontal surface. A force $F$ ...
0
votes
1answer
129 views

rotation behavior of spinning discs

If two discs of equal mass and volume are parallel to each other but not touching, share an axle but using magnetics do not touch it, are not under gravity, and begin to rotate in opposite directions ...
5
votes
0answers
48 views

How can I approximate Jacobian Elliptic Functions in terms of basic integrable functions for the SO(3) rotation of a rigid body?

So, the rotation of a 3d body can be described with Euler's equations of motion giving the rotational velocity in components along the principal axes of inertia. As showed in f.ex. this paper, ...
1
vote
1answer
65 views

Potential Energy Problem

We know that, the potential energy of a particle is $U= mgh$ where $h$ is the height from the zero potential point. But what I have found in Resnick's Fundamental of physics book is that they ...
0
votes
0answers
23 views

How is the torque on the support?

I simulated with Spaceclaim Ansys this device I would like to study: https://youtu.be/Pjc4dIf1aWI The friction is very low. I drawn a fixed black circle on the orange disk to look at the angular ...
4
votes
1answer
97 views

Total angular momentum of earth

I have problem understanding total angular momentum of earth about center of sun. Consider that earth has an orbital angular velocity of $\omega_0 \hat z$ and a spin angular velocity of $\omega_s \hat ...
0
votes
1answer
506 views

Time period of bar pendulum

Why does the time period of a bar pendulum first decrease and then increase when the distance of the axis of rotation from the centre of gravity increases?
0
votes
1answer
33 views

How does axial offset effect rotor balance?

I have a thin rod rotating about its primary axis, rigidly attached to mass $m_1$ whose center sits at distance $r_1$ from the axis and position $y_1$ along the axis. $m_1$ exerts a centripetal force ...
2
votes
1answer
60 views

internal torques

In translational motion, for an isolated system, the net internal force acting on the system is zero according to Newton 3 law. But for rotational motion of an isolated system, does the internal ...
0
votes
2answers
51 views

Net force when deriving relation between torque and angular acceleration

When deriving the equation $\tau = I\alpha$, it is assumed that $$F_{tangential}=ma_{tangential}$$$$a_{tangential}=\alpha r$$$$\tau=rF_{tangential}$$$$\tau=mr^2\alpha=I\alpha$$ However, doesn’t ...
7
votes
1answer
136 views

Without using rotational mechanics, why does a gyroscope precess the direction it does?

When a top is spun, it will precess in some direction, either clockwise or counterclockwise. It's possible to find out which way using $\boldsymbol{\tau} = d\mathbf{L}/dt$ and $\boldsymbol{\tau} = ...
0
votes
0answers
25 views

Need reference on the minimum moment of inertia [duplicate]

I would like to know if there is a book on classical mechanics about the following Moment Of Inertia About Centre of Mass
0
votes
0answers
95 views

Does reducing the surface area between 2 objects reduce friction?

I am 3d printing a system where a gear like object is compressed against similar sized plates on both sides. However, for the first prototype there is too much grinding, making rotation difficult. I ...
1
vote
1answer
95 views

Circular motion, projectile motion

Why does a body thrown upward with certain velocity, at some angle from vertical, travel on a parabolic path but not a circular path? Since there is always a component of weight along the center, it ...
0
votes
2answers
131 views

Does a body always rotate purely about its center of mass? [duplicate]

For example : A rod is held stationary in vertical position on a smooth horizontal ground and then released. Now the center of mass has velocity and acceleration and every point of the rod has some ...
0
votes
1answer
153 views

How can angular velocity be constant even when there is a torque by friction?

A cylinder at rest lying on a rough ground is given an impulse which imparts a translational velocity (no angular velocity) to it. The question goes ahead with finding time after which rolling starts ...
0
votes
1answer
65 views

How to calculate torque needed to rotate a ring held captive between two parts with a known compression force?

I'm wondering how to find the torque needed to rotate a ring that is held captive between two plates with a known friction coefficient, radius, and compression force. I'm assuming that the mass of the ...
0
votes
2answers
101 views

Conservation of Energy vs Conservation of Momentum in Rotational Dynamics

It is clear to me why angular momentum is always conserved, and how in some cases energy is not necessarily conserved within the system (in those cases where bodies deform, or friction is involved). ...
0
votes
2answers
79 views

Simulation of a simple robot [closed]

I'm a programmer and I'm working on a simple robot simulator. Of course, the most important part of this simulator, is the physical simulation of the robot. Unfortunately, I have a little knowledge ...
0
votes
1answer
41 views

What are the limitations of using instantaneous axis of rotation? [closed]

I know when the body rotatates as well as translates IAR or ICR shouldn't be used but I am not able to understand why?
0
votes
1answer
71 views

Resultant center of rotation due to multiple moments

Say, there are multiple moments (M1, M2, M3) acting on a body (of irregular shape) at points P1, P2, P3 respectively. The body is free to rotate about any point. Now, which resultant center (the ...
0
votes
0answers
27 views

Rolling drag and action-reaction forces

I do not understand the two factors acting on the car during driving, they are Rolling drag which it is rolling resistance and action-reaction forces. If the Rolling friction reduces the the car ...
0
votes
0answers
116 views

Angular momentum of satellite including momentum wheel

I have consulted textbooks, online sources and professionals (including spacecraft experts, dynamics lectures and mechanical engineers) without a consensus to the answer to this question. I am ...
0
votes
0answers
21 views

Best learning of gyroscopic motion? [duplicate]

I crave to learn the nitty-gritty of attitude dynamics - namely I want to get really up close and personal with angular momentum, gyroscopic motion, non-inertial effects. I'm looking for a resource ...
0
votes
2answers
803 views

Moment of inertia of uniform solid sphere [closed]

I have derived moment of inertia of solid sphere along diameter but my textbook says that moment of inertia is: $$\frac{2MR^2}{5}$$ What is the mistake in my derivation?
1
vote
1answer
125 views

How is angular momentum conserved?

A uniform vertical turntable (mass M and radius R center O) is at rest on the xy plane and is mounted on a frictionless axle, which lies along the vertical z axis. I throw a lump of putty mass m with ...
0
votes
3answers
129 views

Gear ratio in bicycles using rotational motion

When we change the gears of the bicycle we are riding, we change the the disc we are currently at (which are located at the place where we pedal) to some other disc. This means the radius of the ...
3
votes
2answers
281 views

Rotation matrix of Euler's equations of rotation relative to inertial reference frame

I was playing with simulation of Euler's equations of rotation in MATLAB, $$ I_1\dot{\omega}_1 + (I_3 - I_2)\omega_2\omega_3 = M_1, $$ $$ I_2\dot{\omega}_2 + (I_1 - I_3)\omega_3\omega_1 = M_2, $$ ...
1
vote
1answer
85 views

Virtual work (generalized forces) for rotation with Euler angles

Here is what we know from virtual work: \begin{equation} \delta W=\sum_{i=1}^N{\vec F_i\cdot\delta\vec r_{i}} \end{equation} Where $N$ is the number of bodies in the system. I am considering a ...
1
vote
1answer
84 views

When we ride a bicycle, why do we tend to move left when we turn it right? [duplicate]

When we ride a bicycle, we tend to move our body left if we are taking a right turn and vice versa. Why is it like that?
-2
votes
2answers
96 views

Why do cylinders roll easily and not square or rectangular slabs? [duplicate]

Why do cylinders roll easily and not square or rectangular slabs?? Let us say both have same materiel and vertical dimensions? What is the reason for the slab always tends to slide?
3
votes
1answer
122 views

What makes us twist in a somersault?

In a backwards straight somersault you can decide whether you twist early or late. Twisting early means, that you induce the twisting movement before you rotatation hits 180° and twisting late means, ...
2
votes
3answers
162 views

Are angular acceleration and velocity frame dependent?

Basically what I am trying to ask is if a body has an angular velocity $\omega$ or angular acceleration $\alpha$ about an axis then will it have the same angular velocity and acceleration along any ...
0
votes
1answer
566 views

How to Calculate Engine Torque from Wheel Torque?

Not sure if this is the right place to post - so feel free to tell me to go away :) Im trying to understand how I can use wheel torque curves to estimate the engine torque vs engine rpm relationship. ...
1
vote
3answers
90 views

Will computer fan rotate slower if one makes impeller heavier?

Imagine that we replace plastic fan impeller with identical metal impeller (same form, heavier weight). Would maximum fan speed decrease? Common sense suggests that air resistance and bearing ...
1
vote
2answers
287 views

What would happen if we tried to run a motor in space when it is not attached to anything to provide support to it?

I know that a when a motor runs it generates torque and that torque can be used to do useful work. On the other hand, the motor needs strong support that absorbs the reaction torque. In our case let ...
0
votes
2answers
101 views

how is this kind of rolling motion possible?

I was solving this problem : Suppose you put a sphere in a rough ground with velocity of center of mass $v_{cm}= v_o$ in the positive $x$ axis and with anticlockwise angular momentum $\omega_o$ so ...
1
vote
1answer
141 views

How to make a body rotate about a line that does not go through its center of mass by applying external torques

In space, is it possible to make a body rotate a point that does not go through its center of mass by applying external torques?
0
votes
1answer
44 views

Particle slides on incline where incline angle increases with rate $\omega$: why does kinetic energy have a term $(1/2)m(\omega^2 x^2)$?

A particle slides on a smooth inclined plane whose inclination is $\theta$ is increasing at a constant rate $w$. If $\theta = 0$, at time t = 0 at which time the particle start from rest, Find the ...
0
votes
1answer
43 views

Will this system of magnets rotate?

A system of four magnets are placed as shown in the figure. As per the application of Maglev, a stationary magnet produces electro motive force with the magnets in motion. Note That the point O is ...
1
vote
3answers
344 views

Trajectory of rolling billiard ball after collision with another billiard ball with same mass

So, according to this article http://www.real-world-physics-problems.com/physics-of-billiards.html , the trajectory of white (cue) ball is always perpendicular to the line connecting center of ball ...
0
votes
2answers
253 views

A lever arm-definition question

My question is regarding Torque, and specifically the definition of the lever arm. As far as I've understood, the lever arm is the line perpendicular to the line of action to your reference point. ...
0
votes
0answers
46 views
0
votes
1answer
42 views

Can we spin a top in vacuum?

What happens to a top if we spin it in vacuum. Will it have any difference to rotating in a normal surface to a surface in vacuum.
6
votes
0answers
64 views

Why is this handle flipping back and forth? [duplicate]

This gif of some kind of handle being spun in zero gravity has been doing the rounds: Why is it flipping back and forth? It seems odd that it flips, then seems to rotate around one axis in a ...
1
vote
2answers
690 views

How easily can the Earth flip?

The recent media attention to sunspot activity (however warranted) reminds me of a question I had long ago. Given the angular momentum and the slight 'wobble' of the earth, and remembering the ...