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

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1answer
166 views

Net torque on an object

Suppose that a cord is wrapped around the rim a disk of radius $R$. The disk is allowed to rotate around its central axis (the line passing through the center and perpendicular to the disk surface). ...
0
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1answer
147 views

Speed of ejection of liquid from revolving tank

Consider a cylindrical tank filled with a liquid. Suppose it has an orifice somewhere at the bottom and the liquid is spurting out of this hole at a certain velocity. Now, if you rotated this cylinder ...
4
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4answers
4k views

What determines the direction of precession of a gyroscope?

I understand how torque mathematically causes a change to the direction of angular momentum, thus precessing the gyroscope. However, the direction, either clockwise or counterclockwise, of this ...
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2answers
578 views

Between a solid and a hollow cylinders of the same mass, which one has the greater rotational kinetic energy?

I know that rotational kinetic energy is $\frac{1}{2}I\omega^2$. Therefore, the rotational kinetic energy will depend on the moment of inertia. I came to the conclusion that since both have the same ...
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1answer
222 views

Centrifugal force acting on ring [closed]

Take for example the case of a rod rotating about an axis passing through its centre of mass and perpendicular to it. It has a ring hung from one of its sides. The rotation of the rod causes the ring ...
0
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1answer
103 views

Centrifugal force in inertial reference frame [duplicate]

Take for example the case of a rod rotating about an axis passing through its centre of mass and perpendicular to it. It has a ring hung from one of its sides. The rotation of the rod causes the ring ...
2
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2answers
204 views

Centripetal force in frame of reference of body moving In a circle

Suppose a body is moving in a circle about a fixed point. In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt? More generally, does a body only ...
4
votes
3answers
290 views

Moment of Inertia, why $r^2$and not $r$?

So my engineering mechanics book includes a brief discussion on area moments of inertia. Unfortunately, the ensuing chapter is predominately computational in nature. I don't have a thorough grasp of ...
2
votes
4answers
400 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 ...
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4answers
1k views

Rotation of a slipping ladder

Imagine a ladder leaning against a wall. All surfaces are smooth. Hence the ladder will slip and fall. While falling it rotates because there are external torques acting on it. My question is about ...
0
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1answer
54 views

Force experienced on two particles in a rotating system?

I've a system of two particles of the same mass who rotate in a circle about the centre of mass of the two particles. Is the force experienced by the particles $F=MV^{2}/r$ or should I use ...
2
votes
2answers
523 views

Angular momentum of a translating and rotating body

If a rod is rotating about one end, does it have pure rotation or do you also consider the translation of centre of mass when calculating its angular momentum? Also, how would one calculate the ...
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1answer
171 views

Why is body frame angular velocity nonzero?

This question is relevant to Euler's angles and Euler's equations for a rigid body. Why aren't $\omega_1$, $\omega_2$ and $\omega_3 = 0$ in the body frame? How can we measure $\vec\omega$?
3
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1answer
112 views

Paradox of angular velocity

For a torque-free symmetric top, the Inertia tensor has an inverse $I^{-1}$, and $L=I\omega$. Which implies that $\omega=I^{-1}L$. But since $I, L$ are constants, $\vec\omega$ is a constant. However, ...
0
votes
3answers
214 views

Will an object resting on a rotating platform move in a frictionless world?

Imagine that a pebble is placed on a uniformly rotating, frictionless disk. What will happen to this pebble? Will the disk slide under it and the pebble stay as is? Or will there be a centrifugal ...
5
votes
1answer
484 views

Rod slipping against block due to gravity? [closed]

A uniform rod of mass $m$ and length $l$ is pivoted at point O. The rod is initially in vertical position and touching a block of mass M which is at rest on a horizontal surface. The rod is given a ...
5
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7answers
1k views

What is the proof that a force applied on a rigid body will cause it to rotate around its center of mass?

Say I have a rigid body in space. I've read that if I during some short time interval apply a force on the body at some point which is not in line with the center of mass, it would start rotating ...
1
vote
1answer
511 views

Back motor effect of loaded generators?

The back motor effect (see Counter-electromotive force) is the counter torque which opposes the rotational motion of the coils in a generator when the generator is under load. The back motor effect ...
11
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8answers
5k views

How do you explain spinning tops to a nine year old?

Why don't spinning tops fall over? (The young scientist version) My nine year old son asked me this very question when playing with his "Battle Strikers" set. Having studied Physics myself, I am very ...
0
votes
1answer
302 views

Will a free-falling rod (without drag) rotate?

When we consider a bicycle is turning on a flat plane, we know that there is friction, which provide centripetal force on the bicycle. And we know that the bicycle is no longer perpendicular to the ...
8
votes
2answers
293 views

rotational oblateness

I am trying to compute the amount of oblateness that is caused by planetary rotation. I picture the force of gravity added to the centrifugal force caused by the rotation of the planet as follows: ...
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vote
0answers
237 views

Degeneracy in molecular rotation energy levels for asymmetric molecules

I was recently reading Atkins' Physical Chemistry, the topic of rotational energies of molecules. It states the degeneracies of spherical top, symmetric and linear molecules as being $(2J+1)^2$, ...
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3answers
190 views

Chinook Helicopter Torque

The Chinook Helicopter has 2 rotors to counteract the torque generated by spinning the blade. Theoretically, could you use a smaller "back" rotor that is farther away from the main rotor to achieve ...
1
vote
1answer
221 views

Artificial Gravity [closed]

Consider a structure that is in the shape as shown below rotating about an axis through its middle perpendicular to the long axis in order to provide artificial gravity. What would an ...
1
vote
1answer
485 views

Moment of Inertia and Rotational Dynamics? [closed]

I'm having problems with the intuition behind the Parallel axis theorem and the Perpendicular axis theorem. I'm self studying Mechanics for the British Curriculum but, the book I've is missing the ...
1
vote
1answer
222 views

Question regarding mass hanging from center edge of rotating disc

So, say you have a free to rotate disc, assuming no external torques, and you have a spool, radius 7.93 mm, attached to its centre. Say the spool has a string attached to a point on its edge and ...
6
votes
2answers
399 views

Does rotational energy have effect on gravity/metric?

Intuitively, if energy can be stored in rotational motion, it has to obey $E=mc^2$. Does rotation of typical stellar-sized objects - BHs, pulsars, binaries - have measurable effect on their overall ...
14
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7answers
4k views

What is the electric field generated by a spinning magnet?

Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
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vote
0answers
113 views

Torque on object [closed]

I would like to understand why the torque on white object is 0. I know gravitational forces are very low. But the torque must be at 0. I drawn all the study in first image. Blue color is a torus of ...
1
vote
1answer
1k views

Calculation for force generated by a rotating rectangular blade

When trying to calculate the lift force generated by a simple rectangular blade, I've found the following equation: $$F = \omega^2 L^2 l\rho\sin^2\phi$$ in which $\omega$ is the angular velocity, $L$ ...
7
votes
6answers
9k 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 ...
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vote
0answers
89 views

Seeking help simplifying this EOM equation

I am working on solving the equation of motion for a particular system. It has been a long time since I've worked with matrix equations and need help in simplifying the following: ...
6
votes
1answer
198 views

Distance a curveball travels?

I've seen some discussions regarding the movement of a spinning object, say a curveball. However, all have been largely qualitative. I was wondering if anyone has seen or worked through a ...
2
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2answers
164 views

How does the curve ball drag air around it?

In cricket or baseball there is a type of ball called the curve ball. This is the top spin of the ball.I read that due to spin the ball drags the air around it due to friction in the way shown ...
0
votes
1answer
472 views

Direction of friction in case of a rolling sphere

Suppose a ball rolls on a ground .obviously it is friction that helps to develop the required torque.Hence I find the direction of friction by pointing out the direction of rolling of the ball. But is ...
2
votes
1answer
960 views

Calculating Rotational Inertia Using Parallel Axis Theorem

I am working on the following physics problem and have run into some trouble The figure above shows particles $1$ and $2$, each of mass $m$, attached to the ends of a rigid massless rod of ...
3
votes
1answer
309 views

Why does dust stick so well to fan blades? [duplicate]

After reading and understanding the reasons why dust stick to rotating fan propeller, I am interested to find out why the dust particles stick so well. Spraying powerful jets of water does not ...
1
vote
1answer
570 views

Why is velocity of outermost point on a rotating wheel double the velocity of centre of mass?

'In the answers to one of the questions based on rotation of a disc in my physics book the answer includes the statement 'As we know that the velocity of outermost point on a rotating disc is double ...
8
votes
2answers
2k views

Hamiltonian is conserved, but is not the total mechanical energy

I wondering about the interpretation for the energy difference between the Hamiltonian and the total mechanical energy for systems where the Hamiltonian is conserved, but it is not equal to the total ...
1
vote
1answer
440 views

Calculating torque in 3D?

Say you have a sphere, and you have several torque vectors acting on it, all at different points. Say you have the vector (6i + 3j + 5k) originating from point A, and the vector (3i + 1j + 9k) ...
1
vote
1answer
801 views

Derivation of Newton-Euler equations of motion

I am in search of a simplified version of the derivation of Newton-Euler equations of motion (both translational and rotational) for a rigid body (3D block) that has a body fixed frame and where the ...
-1
votes
1answer
89 views

Normal force at the bottom [closed]

A solid sphere of mass $m$ is released from rest from the rim of a hemispherical cup so that it rolls along the surface. If the rim of the the hemisphere is kept horizontal, find the normal force ...
1
vote
0answers
337 views

Moment of inertia about a tilted axis

I apologize if it seems foolish, I am a beginner. Suppose I am given an object, whose moment of inertia along the x,z axes are known. Suppose it rotates around a tilted axis, ...
0
votes
1answer
237 views

Conservation of angular momentum

We were learning about angular momentum in class today, and although it sort of makes sense, it's much harder for me to think about than linear momentum. So from what I can tell: Angular momentum ...
5
votes
3answers
5k views

Direction of rotation of a celing fan

From Constellation Energy Quick energy efficiency tip: To stay cool and manage your energy at the same time, use ceiling fans to create a “wind chill” in rooms you are using. The wind chill ...
2
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3answers
1k views

Moment of inertia of a planet

Is there a good way to directly measure a moment of inertia of the Earth, or say, other planet?
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vote
0answers
610 views

Magnetic field of solenoid in cone shape

I have done some drawings of the magnetic fields of cone shaped solenoid electromagnets. I based this on the work of Tesla and some explanations of Bashar, known as the Bashar Cones. I found very ...
0
votes
1answer
68 views

Will macroscopic object change its angular velocity after absorbing electron?

Consider macroscopic object (a ball for instance) which has angular momentum equals 0. Now single electron hits the ball and is absorbed by it. Let's assume it hits in direction perpendicular to ...
5
votes
2answers
467 views

How to derive relation for time derivative in a rotating reference frame

I am looking for an appropriate derivation of the $(\frac{d}{dt})_{\text{laboratory}} = (\frac{d}{dt})_{\text{rotating}} + \omega \times $ relationship that enables one to calculate all desired ...
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0answers
66 views

Gravitational Behavior of Rotating Body

Consider a stick with a (ball-shaped) weight at the end of it. Let's say this stick is in a state of rotation (it's floating in vacuum) and is also in a state of translational motion. Consider a ...