# Tagged Questions

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

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### Spinning a rope when hanging, what is the curve? [duplicate]

Holding a rope from one end and spinning it. As shown in the picture, what will be the curve of it?
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### Rotational speed of a planet [closed]

Is there a way to calculate the rotational speed (or day length) of a planet? If not then what are the factors? I mean what affects the speed? Is it just the distance from Sun (like period of a ...
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### Mercury's rotation, Disturbation's caused by thermodynamics

Mercury rotates sychronized like two-stroke machine. As it's solar day is 2 mercurian years.(Orbital periods) Or should we say 3:2-stroke machine, as it's Orbital period is 87.969 Earthdays, and it's ...
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### Precession of angular momentum vector

I'm studying Classical Mechanics on Goldstein's book, so I'm using his terminology and notation. First I'll explain where the question comes from: consider a heavy symmetrical top and consider the ...
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### 3D Dynamics: determining the moments of inertia on a plate with a couple

My Mechanical textbook (Bedford & Fowler 4th Edition) has a worked out example for determining a couple using euler's equations. This is not a homework question (at least I don't think it is?), ...
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### Is Conservation of Energy maintained when the orbit of a rotating mass increases in diameter?

I read an example where someone was explaining how the law of conservation of energy does not have to be maintained within a rotating mass even though angular momentum is maintained. The given ...
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### When does an object rotates and translate at the same time?

I have a ruler on the table. If I push it with my finger on its end, it will rotate. but if I push it hard enough, it will rotate and slide slightly. Which force cause this sliding motion? and how ...
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### How to conserve angular momentum?

If i want to conserve angular momentum from the point of contact of the solid sphere and the ground how should I do it? I know we can use $mvh = I\omega + MvR$ ,if the collision is elastic. But in ...
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### Uranus, the Physical model to change it's rotation axle

Uranus rotates pretty wierd, it's 90-degrees tilted; Why is Uranus's axis of rotation tilted? The best answer for this is; that at a distant point in its past, Uranus was struck by a very ...
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### Rotational Energy homework question

A hollow sphere rolls along floor and goes up 30 degree incline, how far up does it stop? Ok so from what i know you need conservation of energy: $mgh=$rotational energy And $h$ is $d\sin(30)$, ...
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### How did planets start to spin and do all of them spin in same direction? [duplicate]

How did planets start to spin in the first place? Some say that gases which condense and form planets start spinning and hence planets acquire a spin. And some say that they start to spin to conserve ...
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### Acceleration of body rolling down inclined plane

Acceleration of a body rolling down an inclined plane is given by: $$\frac{g\sin\theta}{1+\frac{k^2}{r^2}}$$ $g$=acceleration due to gravity $\theta$=angle of inclined plane $k$=radius of gyration ...
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### A particle of mass m slides down the frictionless surface from height h and collides with the uniform vertical rod of length L and mass M [closed]

A particle of mass m slides down the frictionless surface from height h and collides with the uniform vertical rod of length L and mass M. After the collision, mass m sticks to the rod. The rod is ...
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### Acceleration of two points problem (mathematical explanation)

There is a square board with two nails in corner A and B. We apply a torque $M$ to the square board. Nail A can move freely on a track only in the $x$ direction and B only in the $y$ direction. The ...
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### Earth's Kinetic energy change

Earth's rotational speed varies. I have checked the data an found following Peaks; Year 1998, 23.May, the Earth rotated in 86400.0023738 seconds. At 9.July the rotation time of the Earth was 86400....
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### integrating small angular velocities

I know that for a constant angular velocity the following is true: $R=e^{W t} R_0$ where $W$ is an angular velocity tensor, $t$ is a time, and $R$ is a rotation matrix I believe the following is ...
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### Determining Attitude from Angular Velocity History

I am doing an analysis concerning the torque-free motion of an axisymmetric body. The angular velocity is given by $\omega(t) = [\omega_t\ sin(\Omega t), \omega_t\ cos(\Omega t), \omega_{z0}]$ From ...
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### Intuitive Explanation of Tippe Top Effect?

A friend showed me a tippe top (a special kind of spinning top) lately and asked me about the physics behind it. I thought about it for a while but cannot quite figure it out. So I will throw the ...
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### Why can we model a body to have a center of mass?

I accept that bodies have a center of mass. I also accept that if I were to suspend a body by only supporting it at its center of mass, then the body wouldn't rotate. In the scenario that we suspend a ...
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### Circular Motion and Simulated Gravity

People have proposed that a spinning cylindrical shape could be used to simulate gravity. Will gravity be simulated if the cylinder is frictionless? My brain can't seem to wrap around the idea that ...
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### Finding torque due to normal force [closed]

A cubical block of mass $m$ and edge length $a$ slides down a rough inclined plane of inclination $\alpha$ radian with a uniform speed. Find the torque of the normal force acting on the block about ...
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### How do you choose direction of moment of inertia?

If a car is accelerating forward from rest with no air resistance while weight and normal forces act on the wheels, in which direction is the moment of inertia? Is it positive, clockwise or anti ...
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### Doubt related to rotational motion (rolling down an inclined plane)

I have doubt regarding when an object ( sphere , ring , cylinder etc ) rolls down a FRICTIONLESS inclined plane. Let us suppose the angle of inclination of inclined plane is θ. Now the forces acting ...
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### Relation between accelerations of block and tangential acceleration of the pulley in pulley block system

I was doing some pulley block problems in rotational mechanics and almost everywhere when the pulley is rotating and some masses are connected by a light rope(for example I am taking a two block and 1 ...
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### How do I calculate the speeds in x, y, and theta of a spinning rod?

Suppose the object looks like the diagram below (it is the horizontal rod). On the vertices, are two velocity vectors $v_r$ and $v_l$. The question is: how is the motion of the rod affected by these ...
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### Lagrange: when a potential force, when a generalized force?

Consider the following case of a drum unrolling a mass that is on a massless string wrapped around the drum: According to my professor, here we must consider the mass $m$ being in a gravitational ...
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### Contradiction in a simple torque/rotation problem - two ways of calculating external force don't agree

Suppose we have a rigid system of two point objects (both of unit mass) connected with a massless rod, with the objects horizontal on the x axis, at a distance $r_1$ and $r_2$ from the origin, ...
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### Lagrangian in rotating reference frame

I have a homework question I am working on, and I'm stuck on the last term in this proof. The problem states: Question: Consider a primed set of axes coincident in origin with an inertial set of axes ...
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### What is the force that stops a rotating wheel eventually? [duplicate]

Assuming a car is traveling on a road. At some point, the driver stops accelerating the car, so it travels some distance and then stops. Since the car's wheels are round and are rotating without ...
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### 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 ...
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### Apply force to one end of a rod in space: why not purely translational? [duplicate]

Why does applying force to a rod in space (or any other isolated systems) on one end leads to both translational and rotational motion? Here's my logic: suppose there is a rod in space. We ...
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### 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? ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 $F$ ...
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### 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 ...
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### 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, ...
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### 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 ...
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### 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 ...
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### 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
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### 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 ...