Tagged Questions

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

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4
votes
2answers
261 views

the sound of rotating helicopter blades

Why do helicopter blades make this pulsing, oscilating, slapping(?) sound? Since their movement is smooth, shouldn't the sound be a similar, constant shush, perhaps increasing or decreasing in ...
3
votes
2answers
84 views

What is the cause of planetary rotation. Not orbital rotation [on hold]

I am a senior in high school, sixteen years old. I think differently than most in my class. Recently, I have been thinking about alternative causes for the rotation of our planets. My current thought ...
0
votes
2answers
86 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 ...
0
votes
1answer
19 views

Friction and Conservation of Angular Momentum [on hold]

suppose we have 2 discs placed on a smooth horizontal surface.... one of the discs say A is rotating with W angular velocity and is slowly brought near the other disc kept fixed about its centre but ...
0
votes
1answer
33 views

A question about a body moving in horizontal circular motion

I have some related questions about a body moving in uniform horizontal circular motion: The body moves with a constant angular velocity on a rough horizontal surface. It is attached to a string that ...
2
votes
1answer
35 views

Increase in kinetic energy as some one walks to the centre of a merry-go-round?

When someone walks to the directly centre of a merry-go-round the total kinetic energy of the merry-go-round and person system increases. (assuming the kinetic energy due to the person walking to the ...
0
votes
1answer
26 views

Conservation of angular momentum with different signs of angular momentum? [on hold]

Consider two solid disks, each of radius $r$ and mass $m$ and exactly identical. One of them, say the one on left is rotating about its center anticlockwise with angular velocity $u$ while the ...
0
votes
1answer
48 views

Rotational Inertia of Two rods [closed]

In the figure, two particles, each with mass $ = 0.82 \text { kg}$ are fastened to each other, and to a rotation axis at $O$, by two thin rods, each with length $d = 5.3 \text { cm}$ and mass $M = 1.0 ...
1
vote
1answer
40 views

How to model a very simple spinning wheel

First off, I'm not a physics person, just a lowly software engineer with below average math skills. What I've written is a simple animation of a spinning wheel using C++/GTK/Cairo. It allows the user ...
0
votes
0answers
49 views

Space elevator: a Earth rotation brake model

I read this article regarding the space elevator. A very long carbon nanotubes string, running from the Earth surface into the vacuum of the space. Tension and friction, an enormous quantity of ...
0
votes
0answers
16 views

How to drive quadrotor moments of inertia

I would like to know how moments of inertia (i.e. around x,y and z axes) of a quadrotor is driven given the total mass and the distance from the origin of the quadrotor to the center of a $i$th ...
3
votes
3answers
55 views

what make the bottom portion of a wheel in rolling motion move?

As I just learn about the rolling motion which is the combination of pure translation and pure rotation. The top portion of the rolling body has the speed of double speed at the center of the object ...
3
votes
0answers
27 views

Does a tire need to slip to generate force?

Recently, I have been doing some research on racing and tire modelling. While I was doing this, I encountered many curves like those shown below. While I understand the need of slip angles to ...
1
vote
2answers
86 views

Why does the Earth rotate on its axis?

I know that the earth moves around the sun because of the gravity force, because the spacetime around the sun is curved. But why does the earth rotate on its axis, and which parameters can affect ...
4
votes
2answers
72 views

Can a Storm make the day longer/shorter?

I've read that changes in jet streams can affect the speed that the Earth rotates on its axis, thus making the day longer or shorter?
3
votes
0answers
58 views

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 ...
0
votes
2answers
42 views

Angular momentum in rotational motion

Ok so there are two things that have completely confused me- things related tangular momentum. 1.)First of all, how can a body possess angular momentum even though it is not rotating. according to ...
2
votes
2answers
46 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 ...
0
votes
0answers
21 views

Unbalancing a system of rotating masses - what happens?

Lets say you have a rod spinning on its long axis and this rod has a few smaller beams attached perpendicular to the rod at varying angles and with varying masses. Here's a picture from wikipedia to ...
2
votes
2answers
23 views

A hypothetical question on mechanics

Being located in a tropical region, I am quite acquainted with the Ceiling fan. I have a question about it. If the top, that is, the axle (I'm not sure of the terminology: I mean the part which is ...
11
votes
3answers
932 views

Changing Rotation Direction in Mid Air

Not sure whether this is an appropriate question for this site, but could anyone explain the physics behind how this skier is able to change his direction of rotation mid-air? ...
-1
votes
0answers
35 views

Bizarre spinning top explanation?

If you watch the first three clips over here (mit opencourseware) - ...
-3
votes
3answers
60 views

Can a rigid body have three independent rotations, about three different, mutually perpendicular axes?

It appears that this is kinematically possible, but it looks quite unprobable. For example, it is quite improbable for a sphere to have rotation about three mutually perpendicular axes (supposedly ...
1
vote
1answer
30 views

How to convert translational energy to rotational energy [closed]

Suppose that I have a body moving at certain velocity in straight line. So it has translational energy. I want to convert this energy to rotational energy of another body, by making the velocity of ...
0
votes
2answers
31 views

Cylinder falling down the edge of a table [closed]

Suppose, a cylinder is placed on its lateral side at the edge of a table, carefully, so that the slightest push makes it fall from the table. We give it a gentle push. And it begins to fall. How is ...
1
vote
3answers
94 views

Distinction between torque and force

Distinction between Torque and Force. Consider a case when a cylinder is rolling down a rough inclined plane. While analysing its motion, we consider the force applied by the friction and the ...
2
votes
2answers
150 views

Does General Relativity correctly explain the ellipsoidal shape of the earth?

Does General Relativity theory correctly explain the ellipsoidal shape of the earth? It seems it does not because the Thirring expression¹ for the force of a spherical shell—of mass $M$, radius $R$, ...
3
votes
1answer
41 views

In 2-dimensional and 3-dimensional universes, stellar systems and galaxies are flat and disky. But what about in 4-dimensional universes?

I just watched that interesting video: https://www.youtube.com/watch?v=tmNXKqeUtJM In 2 dimensions a cloud of particles rotating in a plane is flat by definition since it's in 2 dimensions. ...
1
vote
0answers
40 views

Maximum friction force for a wheel to be able to roll

The wheel with mass $M$ and radius $R$ below is free in space (it is not on the ground). A torque $\tau$ is applied to it through an engine. A horizontal force $F = \frac{\tau}{R}$ is also applied to ...
0
votes
0answers
22 views

How does ground interact with a box rotating around its corner?

I have some questions about how forces $F$, friction force $F_{friction}$, and normal $N$ interact in order to the box below to turn (clockwise) about the red corner, without sliding. The force $F$ is ...
0
votes
0answers
38 views

How to deal with a box on a plane from the perspective of a non-inertial frame of reference?

I'm having a hard time trying to understand how combined translation and rotation happens when analyzed from a non-inertial frame of reference. I've read that second law of motion doesn't apply to a ...
0
votes
1answer
44 views

Is a motor HP torque directly proportional to the increasing HP among identical motors?

Say you have three motors: Motor 1 = .10 HP with x torque, performing task A Motor 2 = 1 HP with y torque, performing task A Motor 3 = 10 HP with z torque, performing task A If the motors are the ...
0
votes
1answer
56 views

Why is the slippage constraint for one moving cylinder and one fixed cylinder $r(\phi - \theta)=R \theta$? [closed]

Why is the slippage constraint for one moving cylinder and one fixed cylinder $r(\phi - \theta)=R \theta$? Every time I write it down on paper I get the result $r\phi = R \theta$. I am not sure if I ...
1
vote
1answer
20 views

How to calculate angle of inclination attained by a weigh balance on unequal loading?

Actually I need to rotate the beam (pivoted at centre) with constant angular velocity using the priciple of mass imbalance. Could anyone suggest what would be rate of decrease of mass in one pan ...
1
vote
0answers
49 views

How torque and friction cause wheel to roll

I apologize if this question has been answered before, but I did not find the explanation that I needed. If a torque is applied to a wheel situated on a frictional surface, what forces cause the ...
1
vote
2answers
172 views

What causes a tall box to tip over on an inclined plane?

I'm wondering what makes a tall box rotate about its edge (the red point on the figure below) on a inclined plane (with or without slipping), when subjected to forces of gravity and friction only: ...
1
vote
0answers
67 views

Imaginary Angular Momentum [closed]

I recently came across a problem online where it asks you to interpret an imaginary angular momentum about an axis. Apparently there's a chance it has to do with superconducting. What is the correct ...
5
votes
3answers
72 views

What happens inside a body when it rotates?

I'm studying rigid body dynamics lately. I came across the definition of torque, and though I've found a lot of explanations as to why there is an r there (the moment), all of them are mathematical ...
2
votes
2answers
54 views

Could galactic rotation be similar to an irrotational vortex?

Could black holes' near light speed rotation cause galaxies to move like an irrotational vortices?
0
votes
1answer
164 views

Can a ball stay still while laying on a inclined plane?

Suppose there is a ball on an inclined plane. So, there are three forces acting on it: normal force from the plane, gravity (which can be decomposed in the component of gravity perpendicular to the ...
0
votes
0answers
19 views

Reason behind the rotation of Earth? [duplicate]

I want to know what is the reason that the Earth rotates on its own axis? What are the forces responsible for the Earth's rotation?
0
votes
1answer
52 views

Feynman's explanation of parallel axis theorem

In the book Feynman's Lectures on physics volume 1 chapter 19, He explains prallel axis theorem as follows. Suppose we have an object, and we want to find its moment of inertia around some ...
1
vote
0answers
30 views

Can a harmonic drive reducer/strain wave gear be used to gear up instead of down? [closed]

I wish to be able to gear up from a rotating bicycle hub with a revolutions per minute range of 150 to 850 to a ratio of 200:1. Can I do this by simply reversing the direction of a harmonic drive ...
1
vote
1answer
91 views

Question about torque and center of mass [duplicate]

If a yo-yo's string is not attached to anything and the yo-yo is dropped, it is obvious it will fall at $g$. In this scenario, Tension of string = 0. If a yo-yo's string is attached to a cieling and ...
2
votes
2answers
60 views

Torque, and the Law of the Lever

How fundamental is the Law of the Lever? It seems that we simply define torque as being $r \times F$, if that's the case, then torque isn't a derived quantity, is it? Something like the Law of the ...
1
vote
1answer
68 views

Water bottle moment of inertia

I've noticed that I can make a full water bottle spin about its short axis easier than I can make it spin when it is 1/4 or 1/2 full. Also, when it is spun and is not full, the geometric center of the ...
0
votes
1answer
42 views

Would equations for a spinning top be an (x,y,z) vector [duplicate]

I am following the equations on this page, and for torque it is $mgr\sin\theta$, but I am curious about $r$. I am working on a game and I want to correctly model the top, and am curious if $r$ should ...
1
vote
1answer
47 views

Gravitational Potential Energy to Kinetic Energy

When a yo-yo is released from a height $h$, the gravitational potential energy is converted to kinetic energy. However, the yo-yo obviously has less acceleration than $g$, $9.8\frac{m}{s}$. This means ...
0
votes
1answer
20 views

Angular momentum of a sphere moving vertically downwards [closed]

If a solid sphere is moving vertically down with a velocity $v$ and spinning with an angular velocity $\omega$, what is the angular momentum of the sphere about an axis through the sphere which is at ...
1
vote
2answers
90 views

The charm of the gyroscope [closed]

I am fascinated by the gyroscope, like everyboy who was so lucky as to get such a toy as a kid. But probably also grown-ups are not immune to its charm. I am not asking for a theoretical explanation. ...