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

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2
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1answer
109 views

What happens to a ball spinning with peripheral speed near to the speed of light?

I can't imagine such phenomenon. Would it becomes an ellipsoid, or maybe a straight line?
0
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3answers
50 views

Axis of rotation and rotating rigid body [closed]

We know that angular momentum of a solid disk rotating with angular velocity $\omega$ is given by $I\omega$ about its center. Now if I chose any axis (parallel to above), will it have same magnitude ...
1
vote
2answers
41 views

Why moment equilibrium must be calculated around the center in this problem?

Consider this problem, which ask for the value of the two normal forces (friction is negligible). Easy, right? You calculate the moments around G, the center of mass, and equate them to 0 (the ...
1
vote
1answer
92 views

Spinning a string with a mass on the end. What makes the mass rotate parallel to the ground? [closed]

Say you use your hand to rotate a string with a mass attached to the end. The string will first become taunt and then the mass and string will move upwards until they are spinning parallel to the ...
0
votes
1answer
19 views

Spheres collide with merry-go-round [closed]

Four spheres, with uniform densities $\rho_1, \rho_2, \rho_3, \rho_4$ and radii $r_1, r_2, r_3, r_4$, respectively, roll without slipping with constant velocities $v_1, v_2, v_3, v_4$ along tracks ...
0
votes
1answer
35 views

Angular momentum in rolling about the point of contact

A cylinder of mass 5 kg and radius 10 cm is moving on a horizontal surface with velocity of centre of mass 5 m/s towards right and angular speed 10 rad/s (clockwise) . Find the angular momentum of the ...
2
votes
1answer
47 views

Parallel axis theorem and Koenig theorem for angular momentum

Are the parallel axis theorem and the Koenig theorem for angular momentum linked with each other in rigid body dynamics? The parallel axis theorem states that $$I_{z}=I_{cm}+ma^2$$ Koenig theorem ...
0
votes
0answers
35 views

Relative angular momentum?

Let there be a point $P$. A point $C$ is located at a radius vector $r$ from $P$. $C$ is the centre of mass of a rigid body. The rigid body is rotating with an angular velocity $\omega$ about an axis ...
0
votes
1answer
25 views

Derivative of angular momentum of rigid body

I found this equation that describes the change in angular momentum $\vec{L}$ of a rigid body rotating about a fixed point $O$. $I_o$ is the moment of inertia of the body with respect to the axis of ...
1
vote
3answers
75 views

Rolling without slipping in absence of friction force

I'm confused about a rolling without slipping situation. Suppose to have a disk of radius $R$ on a floor, and exert a horizontal force at a certain distance $r$ from the center of mass. I would like ...
2
votes
1answer
78 views

All the planets in our solar system rotate 'Anticlockwise', except Venus. Why is the only planet that rotate' clockwise'? [duplicate]

All the planets in our solar system rotate 'Anticlockwise', except Venus. Why is the only planet that rotate 'Clockwise'?
0
votes
1answer
19 views

Finding the force exerted by a mass at different contact points

Okay so I've been doing a math assignment on torque which has to have real-world applications so I've gone off on a massive tangent trying to incorporate physics principles and trying to get them as ...
1
vote
1answer
44 views

How to explain gyroscopic precession in a more intuitional way?

When Studying the phenomenon of precession a classic example installation is this: a wheel spins around the Y axis, gravity applies and the upper part starts spinning around the Z axis. The textbook ...
1
vote
3answers
78 views

Why doesn't a swing make a full revolution on a swingset in a park? [closed]

I'm familiar with the concept of swinging of a swing in a park, but I'm confused why it doesn't it swing at a complete circle around the center bar?
10
votes
5answers
4k views

Why does a ceiling fan start up slowly? [closed]

I think it probably has something to do with the capacitor inside but I don't get it why doesn't it just Start spinning instantly when we push the button why does it slowly start to spin and gradually ...
1
vote
2answers
42 views

Dynamics of swinging

My husband built a 20'-high tree swing, using 3/16"-welded-link chains to suspend it approximately 2' off of the ground. To our disappointment, pumping barely gets the swing going, with little height ...
2
votes
2answers
36 views

Torque and a pulley

I'm supposed to find, symbolically, what the moment of inertia of the pulley is using $T_1, T_2, r,$ and $a$. Obviously the base equation would be the sum of the torques equals $Iα$. I have the ...
0
votes
0answers
17 views

Angular acceleration of rigid body due to a torque

For the rotation of a rigid body about a fixed axis $z$ the following holds. $$ \vec{τ_z}= \frac{d \vec{L_z}} {dt} =I_z \vec{α} \tag{1}$$ Where $ \vec{τ_z}$ is the component parallel to the axis ...
0
votes
1answer
21 views

What is the velocity vector of a spinning hoop as view from the ground?

Original Question: I've calculated a) and b) My issue is with parts c) and d). I understand that v= 1.56 m/s for the translational motion and that the velocity vector should be the sum of the ...
0
votes
4answers
62 views

What is the moment of inertia really?

Is moment of inertia or second moment of inertia, simply the resistance of a body to rotate it over an axis? What is radius of gyration? What if the axis is via the center of mass or somewhere ...
0
votes
1answer
30 views

Instantaneous axis of rotation of a rigid body

For the description of rigid body motion, any point $O$ of the rigid body could be taken as reference, since the velocity of a generic point $P$ can be written in function of the angular velocity ...
1
vote
2answers
45 views

Definition of translational , rotational , rolling motion

What are the exact definitions of pure translational , pure rotational and rolling motion ? I am a class 11th student ... I find it difficult to exactly make a difference between translational , ...
0
votes
2answers
28 views

Velocity of the points of a rigid body

The most general motion of a rigid body is a roto-traslation. Firstly is it correct that any point (let's call it $O$) of the rigid body can be seen as the point through which passes a istantaneous ...
1
vote
1answer
56 views

Massless ladder against a frictionless wall [closed]

I am confused by a review problem for my physics course. I keep getting a different answer from the solution (which was given to us) and not sure what I am missing. A massless ladder has a length of ...
0
votes
1answer
52 views

Hard drive disk seems to “move” its weight when running

I have an external box for hard drives and plugged a broken hard disk to it. Then I turned it on and took it in hand. Now, if I gently tilt and turn the box on its axis, the weight seems to "go" in ...
0
votes
0answers
35 views

Force required to spin a platform around a non-centered axis

This might be a silly simple question, but I cannot confirm the correct answer. Situation: I am building a wall bed and i need to calculate how much force will be needed to lift it from a horizontal ...
0
votes
1answer
75 views

Why do two rolls with the same mass but different moments of inertia roll different distances?

Imagine two rolls with the same diameter and mass. The mass of one roll is concentrated to the center of the roll while the mass of the other roll is concentrated to the edge of the roll. If the two ...
1
vote
1answer
57 views

How I think about rolling [closed]

Below, I have written how I think about rolling. I wish to know if my idea as to how it works is correct. Consider a rigid ball on a plane which has considerable friction. If you push the ball ...
0
votes
1answer
43 views

Question about friction in rolling without slipping?

I was under the impression that for a ball/cylinder to roll without slip, there must be a static friction force that opposes the direction of motion. Why, in this case, does the friction act in ...
3
votes
1answer
42 views

Force and Torque

Think of a uniform sphere. Sometimes if a force is applied to the sphere it does not only move but also spins. So there is a torque. But is it possible to calculate which part of the applied force is ...
1
vote
1answer
52 views

Stacked motors thought experiment

first of all, I'm not a mathematician, just curious about something that came up to my mind and maybe you can help me out. Scenario: I have a motor that spins let's say at 20K revolutions per ...
2
votes
2answers
56 views

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 ...
0
votes
0answers
40 views

Rigid body rotation about fixed axis with angular velocity not constant in magnitude

I'm trying to understand the properties of angular momentum in the rotation of a rigid body around a fixed axis $z$, when the angular momentum $\vec{L}$ is not parallel to the angular velocity ...
2
votes
4answers
93 views

Is torque necessary in rigid body mechanics? [duplicate]

Suppose we have a weightless, rigid rod fixed at one end, but free to swing at the other, where there is a mass $m$ attached. If we want to determine the tangential acceleration of the mass using ...
0
votes
0answers
21 views

Force by torque or force by speed with make the body jump?

I have a small object made of plastic weighing 3kg and I want to make it jump with the help of a dc motor and a plastic rod attached to it (4 such motor with rod are attached on the object at 4 ...
1
vote
0answers
35 views

Torque on rotating rigid body and pivot point [duplicate]

Consider a rigid body performing a rotational motion around a vertical fixed axis $z$ with constant angular velocity $\vec{\Omega}$. The angular momentum vector $\vec{L}$ is not parallel to $z$ (and ...
1
vote
1answer
37 views

How to find the direction of friction in rotation?

Suppose we have a spool which is plucked constantly from top with a force F. The spool is on a rough ground. We have to find the direction of friction, nd direction of motion of spool. ...
1
vote
3answers
46 views

Why the similarity in the Equations of Motion for Rotational and Rectilinear Motion?

These are the equations of motion given constant acceleration, for first rectilinear and then rotational motion. Rectilinear Motion: Rotational Motion: While the variables have changed, and the ...
0
votes
1answer
27 views

Gyroscope is not in center of the mass

I have a controller with a MEMS gyroscope on it. Is there any difference if I put gyroscope not in center of the mass? Will be gyroscope drift increase?
3
votes
1answer
107 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 ...
1
vote
1answer
64 views

Mechanics of a falling cat [duplicate]

It is a general statement that cats tend to fall on their feet. How far is this is a valid statement? If I were to drop a cat from 50 meters height, feet up(hypothetically), would it still land on ...
2
votes
1answer
33 views

Find force centroid from forces and moments [closed]

A force $\vec F = (F_x,F_y,F_z)$ is applied to a point that its position vector is $\vec r = (x,y,z)$ generating a moment $\vec M = (M_x,M_y,M_z)$. Find position vector of the point ($\vec r$) if ...
0
votes
0answers
30 views

Cylinders rotating on a table

Please consult this video for context, its an interesting watch. https://www.youtube.com/watch?v=JuuYFt8bahE In this video Tadashi spins a tube on a table and it rotates around the tube axis and ...
1
vote
2answers
67 views

Rotational work and forces such as static friction or ropes tension

I'm confused about the rotational work, defined as $W=\int_{\theta_1}^{\theta_2} \tau_z d \theta $ Where $\tau_z$ is the component of the torque parallel to the axis of rotation $z$. Consider a ...
0
votes
1answer
54 views

Build rotational Hamiltonian based on Lagrangian of general form

I've been told that one could build rotational Hamiltonian based on Lagrangian of general form: $\mathcal{L} = \mathcal{L} (\vec{\Omega})$. By introducing Euler angles one could rewrite Lagrangian in ...
2
votes
3answers
45 views

Force at axis of spinning wheel

This is a newbie question, but I don't quite understand the forces at work when an object is rotating. So I've read that i can use my right hand to determine the direction of the force at the axis ...
4
votes
2answers
41 views

Does a central force have to be independent of angle?

When defining a central force, some sources, like Wikipedia, say that the magnitude of the force only depends on the distance $r$: In classical mechanics, a central force on an object is a force ...
1
vote
0answers
34 views

Circular motion and car tire angle [closed]

I'm trying to figure out what angle to turn the front tires of my small robotic car (about a foot in length) so that it moves in a circle (on level ground) of radius r (will be accommodating circles ...
0
votes
1answer
25 views

A body acting by zero net external force

I wonder, for a rigid body, if the vector sum of all external forces acting on it equals zero (Fnet = 0), does it also leads to the vector sum of all external torques acting on it about any point ...
0
votes
2answers
45 views

Why is Torque Generated from Friction?

Why does a rolling ball get torque from friction. I understand torque in relation to lever arms and the force being perpendicular to said arm. But what about wheels? There is no lever arm so how is ...