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

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1
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1answer
108 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 ...
12
votes
3answers
641 views

What is the physics of a spinning coin?

When we spin a coin on a table, we observe 2 things: It slows down and stops after sometime. It does not stay at just one point on the table but its point of contact with table changes with time. ...
0
votes
2answers
25 views

Damping Constant for a Ball Rolling in a Bowl

How would you find the damping constant of a ball rolling without slipping in half of a sphere? I know how to find the period of the oscillation, but how would I find the damping constant ...
43
votes
11answers
8k views

Why does dust stick to rotating fan propeller?

Why does dust stick to rotating fan propeller? Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers. EDIT 1: Thank you for the great explanations. I am ...
2
votes
2answers
123 views

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 ...
1
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3answers
47 views

Why do we add the spin angular velocity and orbital anglar velocity when asked to calculate total angular velocity of Gyroscope?

Normally when we talk of angular velocity we mean how the angle of a vector changes with time with respect to an origin.Thus the oribital angular velocity of gyroscope makes sense to me.However I find ...
0
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0answers
33 views

Why are centrifuge space-station designs not a reality? [duplicate]

Considering how useful it could be for the crew of a space station to have at their disposal a 1g environment (or more) at the rim to a 0g environment at the axis of rotation, why aren't modern space ...
1
vote
1answer
45 views

Rotational Mechanics: Conservation of Angular Momentum

Consider a case where a person stands on top of a rotating disk. The disc is given to rotate at a constant rate. There are two possible movements of the man: He moves away from the center: In this ...
1
vote
0answers
44 views

Rolling without slipping [closed]

A uniform, solid cylinder of mass $m$ and radius $r$ rolls without slipping down a plane, inclined at an angle $θ$ to the horizontal. You may assume, without proof, that the moment of inertia of ...
1
vote
0answers
119 views

Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
7
votes
3answers
185 views

What is the origin of spin of celestial objects?

In an older question from June 2011, Why does each celestial object spin on its own axis?, apparently revived by the system, a user is asking about the origin of the rotation of celestial bodies. The ...
32
votes
5answers
2k views

Why does everything spin?

The origin of spin is some what a puzzle to me, everything spin from galaxies to planets to weather to electrons. Where has all the angular momentum come from? Why is it so natural? I was also ...
14
votes
4answers
2k views

Why does each celestial object spin on its own axis?

AFAIK all the celestial objects have a spin motion around its axis. What is the reason for this? If it must rotate by some theory, what decides it's direction and speed of rotation? Is there any ...
1
vote
1answer
138 views

What prevents this rotational perpetuum mobile from working? [closed]

As a newbie in physics I imagine this device and I would like to know why it can't increase its energy. Note I study the sum of energy during one second (or less) after set friction ON. The device: ...
0
votes
0answers
51 views

Does non-rotating object have rotational inertia if it is attach to rotating object by a spring

My question relating to the Rotational Inertia as a whole system. As I learn that the Rotational Inertia is defined for a rotating object to an fixed axis but I got problem in the case a rotating ...
0
votes
1answer
88 views

The direction of frictional force in smooth rolling motion

I want to ask about the direction of frictional force in smooth rolling motion which means the rolling object doesn't slide on the surface. Here is the first case, the wheel rolls to the right so the ...
1
vote
1answer
40 views

Ideal Rolling Motion on Surface with Friction

I have a question concerning ideal rolling motion on a surface containing friction. By ideal rolling motion, I mean the tangental velocity of the rolling object is the same as the velocity of the ...
1
vote
1answer
48 views

Acceleration of Center of Mass in Rotational Motion

I have a question regarding the acceleration of the center mass during rotational motion. From my understanding, Fnet = m*a(center of mass). Also, Torque = angular acceleration * moment of inertia. ...
1
vote
1answer
33 views

Torque from Gravity

If a sphere rolls on an inclined plane, why does the component of gravity parallel to the plane not exert a torque on the sphere? Shouldn't infinitesimal mass particles experience a force of ...
0
votes
1answer
38 views

Can rotational motion can be reduced to translational motion?

Susskind says that all physics (dynamics of the world) can be described in terms of particle positions and their first order derivatives. Does such translational motion gives rise to rotation?
4
votes
2answers
2k views

Which force makes a wheel roll down the hill? What causes friction?

A wheel rolling down a hill has two axis of rotation. One is where the center or mass is and the other is the point of contact with the surface which acts as a fulcrum. I was trying to understand ...
0
votes
1answer
30 views

$y$-component of center of a cube that rolls without slipping [closed]

For the figure above, let $O$ be the origin point for $x,y$-axis, and $+x$ goes to the horizontal right direction while $+y$ direction goes to the up direction. The figure has one circle and one ...
1
vote
3answers
5k views

Factors affecting torque and RPM of a motor

I am not a physics guy, so not even the basic concept of a DC motor is easy for me. My question is as follows: How do these parts of a motor affect its RPM and Torque? I had my research a while ago ...
2
votes
5answers
326 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 ...
1
vote
3answers
79 views

Friction in Slipping and Rolling

I'm solving a rotational motion problem that involves slipping and rolling. The problem's basic setup has a sphere rotated and then placed onto the ground, at which point it begins to acquire ...
22
votes
6answers
2k 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 ...
5
votes
2answers
139 views

I need help figuring out what is wrong in this aspiring perpetuum mobile

Credits: My question is motivated from a question from another user (One disk/ring in double rotation and sum of energy), I just reformulated what I think he tried to ask into, what seem to me, ...
1
vote
0answers
50 views

A particle attached to a string rotating around a cylinder seems to increase in energy. Does it? [closed]

When a particle attached to a string rotates around a cylinder of finite radius, a point on the curved surface of the cylinder to which the other end of the string is attached, the particle spirals ...
1
vote
0answers
26 views

How to determine radius of curvature of cycloid using centripetal acceleration?

Whenever it comes to radius of curvature of complex curves like cycloid, we all take the help of calculus. But I am still in high school and not that competent with calculus, so please do not answer ...
31
votes
8answers
6k views

Why doesn't a bike/bicycle fall if going with a high speed?

Why does a bike/bicycle fall when its speed is very low or close to zero and is balanced when going with a high speed?
1
vote
1answer
27 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 ...
3
votes
2answers
123 views

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

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 ...
8
votes
7answers
12k views

What do people actually mean by “rolling without slipping”?

I have never understood what's the meaning of the sentence "rolling without slipping". Let me explain. I'll give an example. Yesterday my mechanics professor introduced some concepts of rotational ...
-1
votes
1answer
72 views

How do two rigid bodies with different 3rd moment of inertia rotate differently?

If rigid bodies $R_1$ and $R_2$ has exactly same total mass $M$, central of mass, and rotational inertia $I$, but different third moment of inertia $M_3$, how would they move/rotate differently? ...
2
votes
0answers
173 views

Sum of energy from torques of several disks in double rotation [closed]

Here the study: An external system (not drawn) give energy for rotate disks around themselves and around green axis. All disks have energy at start, at $t=0$ friction is ON and at $t=0$ external ...
0
votes
1answer
71 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 ...
2
votes
1answer
96 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 ...
1
vote
1answer
58 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 ...
3
votes
3answers
96 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 ...
0
votes
0answers
57 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
38 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 ...
4
votes
0answers
183 views

Sum of energy for 2 solids in rotation

I would like to compute the sum of energy of the following case: Two solids are turning (disks). Yellow solid is turning at $w1 rd/s$ around its center of gravity and blue solid is turning at ...
4
votes
1answer
166 views

Understanding gyroscopes

Considering the typical situation of a rotating bicycle wheel held by one end of its axle by a rope tied to the ceiling: gravity torque is the time derivative of the angular momentum, and in this case ...
4
votes
2answers
82 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?
1
vote
2answers
115 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 ...
3
votes
0answers
76 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 ...
4
votes
4answers
7k views

Static as opposed to Kinetic Friction in Rolling Motion

During analysis of rolling motion, why do we consider coefficient of friction as that of static friction and not kinetic friction?
1
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0answers
418 views

Should angular momentum change or it should not.?

Let us consider a body with an arbitrary shape. Let its center of mass be assigned coordinates (o,o,z) with respect to an origin.Now let us consider two equal and opposite forces acting on the body ...
0
votes
2answers
59 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 ...
4
votes
4answers
1k views

Why do rolling disc (coin) move in circular path?

We have a coin that is rolled such that it's tilted at an small angle $ \theta $. Question:: What turns around rolling disc so that it traces circular motion (spiral as it's speed decreses)? ...