# Tagged Questions

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

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### Changing Earth angle of revolution

As a follow-up to the two previous questions What is the scientific reason for the inclination of earth's rotation axis? and How much effort would be required to fix the Earth's rotation? ...
684 views

### Rotating/Translating Disk

I was trying to understand an aspect of rotational dynamics and thought of a problem to help me learn. I'm sure this problem has been considered by countless people in the past, but I'm having some ...
185 views

### understanding why inversion cannot be accomplished by a rigid change

Goldstein pg 151 says "it is clear that an inversion of a right-handed system into a left-handed one cannot be accomplished by any rigid change in the coordinate axis..." I am trying to understand ...
4k views

### How to calculate cord tension in a vertical circle?

Mass m is connected to the end of a cord at length R above its rotational axis (the axis is parallel to the horizon, the position of the mass is perpendicular to the horizon). It is given an initial ...
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### Pulley rotation

In the given arrangement, the friction in between the string and the pulley is enough to avoid slipping and the string is light. On the basis of these assumptions, would the tension in the upper part ...
92 views

### Could 1 force cause a pure moment?

A friend of mine told me if there is only one force, it cannot cause only rotation. I wasn't convinced so I proposed a thought experiment, and now we are both confused. Suppose that we put a rod ( ...
116 views

### What causes the differential rotation of the Sun?

I have read the explanation in Wikipedia, but the problem is that because I'm a beginner in physics, I don't quite understand what more than half of the words mean! Here is the explanation from ...
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### Conditions to impose for accelerations in systems of rotating pulleys [closed]

I do not understand completely how do rotating pulleys with ropes (that do not slip) work. Besides the equations of motions, which are clear to me, some conditions on the accelerations are needed to ...
648 views

### Solving for motion of rotating rod using only Newton's laws?

I have a question that's been bothering me for years. Given a rod of uniform mass distribution with total mass $M$ and length $L$ that lies on a horizontal table (with one end fixed to the table ...
153 views

### Rotation and fictitious forces

A bug eats through an apple and forms a vertical, infinitesimally thin canal parallel to the vertical diameter at a distance $\frac{R}{2}$ from the center. The apple rotates at angular velocity around ...
579 views

### Angular acceleration of stone disk

I have a conceputal question regarding the following problem: A round massive stone disk with diameter $0.600 m$ has a mass of $50.0 kg$. The stone rotates at an angular velocity of $115.2 rad/s$, ...
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### How is temperature and pressure related to when a tire rolls on the ground?

Actual observation: Here in Canada it is known that even when 'dry' snow is on the ground, and when a car drives over it that small amount of dry snow instantly becomes ice (NOT GOOD). Anyways as an ...
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### How do I treat the Lagrangian in the case of a rigid body?

Here's Exercise 1.11 from Goldstein's Classical Mechanics 3rd edition (the first one after having derived the Lagrangian basically): Exercise 1.11: Consider a uniform thin disk that rolls without ...
184 views

### Rotating a Gaussian Beam

I need to see how the phase fronts of two gaussian beams interact when they intersect with each other. The two beams are tilted towards each other so that they intersect each other at their respective ...
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### Difference of the O(N) Non-linear Sigma model and SO(N) Non-linearSigma model

The Hamiltonian $$H=J\sum_{i,j}\vec{n}_i\cdot\vec{n}_j$$ is invariant under a global rotation $\vec{n}_i\rightarrow R\vec{n}_i$, where $\vec{n}$ is a $N$ component rotor ...
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### 3 Axis Gryroscope with forced Precession and Limits of Motion

I am working a problem concerning a 3 axis gryoscope, the spinning mass is a magnet (dipole). This is part of a optical sensing device. The inner gimbal is for pitch rotation, and the outer gimbal is ...
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### Videos of changing the orientation of an astronaut in space

Kane, Headrick and Yatteau describe in their paper "Experimental investigation of an astronaut maneuvering scheme" possible maneuvers to change the orientation in space without external torque. Is ...
3k views

### Forces and torques about the CENTER OF MASS of a physical pendulum

I'm currently stumped by the following situation. Say we've got a rectangular physical pendulum (think ruler with a hole-punch at one end). It's trivial to analyze the motion of the pendulum with the ...
8k views

### Force applied to wheel in pure rolling motion at contact point with road [closed]

Suppose a wheel with radius $R$ is resting on a non-inclined surface. A torque $\tau$ is applied to the wheel center. In an attempt to prevent wheel from spinning, the ground applies a static friction ...
2k views

### Could life survive a pole shift caused by an asteroid collision?

Could life on earth survive a large pole shift caused by an asteroid collision? I became aware that there are people who believe that the earth's pole suddenly shifts. That is, its rotational axis ...
464 views

### Is this expression for the kinetic energy of a spinning disk revolving about a second axis correct?

My question is motivated from a question from another user. You can see the configuration of the rotating system here: One disk/ring in double rotation and sum of energy. I am not interested in all ...
355 views

### Will the day/night cycle change when the poles melt?

I want to know what will happen to day and night if poles melt i think it will change but I don't know why? ^_^
119 views

### Does zero net force imply zero net torque?

I wonder, for a rigid body, if the vector sum of all external forces acting on it is equal to zero ($F_{\rm net} = 0$), does this imply that the vector sum of all external torques acting on it about ...
119 views

### Does angular momentum of a system whose moment of inertia is changing remains constant?

An equilateral triangle $ABC$ formed from a uniform wire has two small identical beads initially located at $A$. The triangle is set rotating about the vertical axis $AO$. Then the beads are released ...
214 views

### Why does Torque exist?

Specifically, why does force increase with the moment arm? What about making a perpendicular force farther away from the axis of rotation increases it? How does the moment arm cause that?
1k views

### Why is moment of inertia dependent on $r^2$ and not on $r$ ? (physical reason)

Moment of inertia is the mass equivalent in rotational dynamics. I know , by mathematical arguments, moment of inertia of a particle is $$I = \text{mass} \cdot r^2$$ . But what is the physical reason?...
2k views

### Angular momentum conservation while internal frictional torque is present

So this appears in a problem which looks simple enough in its context; It's something like this: Two discs, A and B, are mounted coaxially on a vertical axle. The discs have moments of inertia $I$ ...
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### Regarding (regular) kinetic energy and rotational kinetic energy

In my physics class we saw this problem: A disc of mass $M$ and radius $r$ is standing vertically and can rotate freely through an axis thats goes through its center of mass. A small particle of ...
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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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### Conserved quantities in an angular system

Given the following question: A student initially stands on a circular platform that is free to rotate without friction about its center. The student jumps off tangentially, setting the platform ...
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### During a turn, do the rear wheels necessarily trace out the same arcs as the front wheels?

When a vehicle makes a turn, the two front wheels trace out two arcs as shown in the figure below. The wheel facing towards the inside of the turn has a steering angle that is greater than that of the ...
863 views

### Forces acting on a point mass in a spinning rigid body

I have learned that all spinning objects will continue spinning even if no force is acting on it, and the tendency to do so is called moment of inertia. But I wonder about the fact that a single point ...
6k views

### Time period of torsion oscillation

For the oscillation of a torsion pendulum (a mechanical motion), the time period is given by $T=2\pi\sqrt{\frac{I}{C}}$ which is a result of the angular acceleration \$\alpha=\frac{d^2\theta}{dt^2}=-(\...
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### Is it humanly possible to change Earth's axis?

Just what the title states with the qualification that the change must be affected without using other celestial bodies as mentioned in the Clarke/Baxter SF 'Sunstorm'. Obviously given the momentum ...
1k views

### Why do some satellites fall to Earth?

In another question How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?, one can read an affirmative answer. They how do you explain satellites ...
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### Meaning of parallel axis theorem: why is the moment of inertia minimum if the axis passes through the CM?

From parallel axis theorem follows that, given a rigid body, the moment of inertia is minimum if calculated with respect to an axis passing through the center of mass. What is the physical meaning of ...
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### Can torque relations actually be *derived* from Newton's Laws, or is it something extra?

For a long time I have wondered if there is a way to show that the rotational analogs of Newton's Laws are a direct consequence of just those laws, or are we adding more to them? I understand ...
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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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### What motion can emerge in a still universe? [duplicate]

I consider a very large non homogeneous cloud of dust in isolation. No external gravitational interactions. It is initially completely still and start collapsing under the its own gravity. Can the ...
4k views

### Is squared motor gearbox ratio proportional to inertia ratio?

I read an interesting article http://m.machinedesign.com/news/motor-sizing-made-easy It is very interesting, but I can not follow the 2nd last paragraph. I don't understand why it is true. ...
715 views

### What sustains a rigid body's rotation at its constant angular(rotational) speed?

Continuing from the following scenario from my previous question Centripetal force of a rotating rigid body? : Consider someone pushing a roundabout in a playground. Initially the roundabout is ...
645 views

### how to represent the effect of linking rigid-bodies together?

I have 2 rigid-bodies (b1,b2) if i linked one to the other (as if they are conjoined together) , how to represent b1 effect on b2 and b2 effect on b1 Is there any LAW that affect the position/...
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### The rotating movement of an asteroid

I almost all movies where you could see an animation about an asteroid, they move in a very distinct way. I don't know how to explain better, but I think what we can see in the movies is that the ...
688 views

### How long for a frictionless top to fall over?

We've previously discussed why it is that spinning tops do not fall over, see: Why don't spinning tops fall over? However, as the highest rated answer notes, the angular momentum of the spinning top ...
72 views

### 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 ...
223 views

### Conservation of angular momentum in broken pendulum

I'm having troubles in understanding the correct way to use the conservation of angular momentum. Consider the pendulum in the picture with a neglegible mass rope and a slender rode of mass M and ...
186 views

### Change in acceleration due to gravity because of rotation of earth [duplicate]

The formula above is the equation for acceleration due to gravity when earth rotates. G is the original acceleration. Can someone explain how this formula came?
685 views

### In which direction does mud fly off a moving bike's tire & why?

If a bike moves through a muddy area, mud gets on its tires. Then the mud flies off from the tires. Which forces are acting on it? In which direction does it fly off? On my physics test, I wrote ...