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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0answers
22 views

A ball rolling in a bowl [on hold]

There is a small ball with a radius of 2 mm inside of a bowl with a radius of 22 mm. If the bowl is tilted 1 degree in any direction, what does the slope of the bowl need to be in order to get the ...
0
votes
2answers
77 views

The charm of the gyroscope [on hold]

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. ...
1
vote
1answer
20 views

Degrees of Freedom for an Asymmetric top

How many degrees of freedom does an asymmetric top have if it is rotating about a fixed point?What are the generalised coordinates used then?
2
votes
1answer
58 views

Rotation from Goldstein's Classical Mechanics

I apologize for the ambiguity in my title. It was rather difficult to figure out what is the most appropriate title for my questions. My questions come from chapter 4 and chapter 5 of Goldstein, ...
-2
votes
0answers
20 views

Angular acceleration [closed]

Question: A particle moves with a constant angular acceleration $\alpha$ in a circular path. The time at which magnitudes of tangential and radial acceleration are equal is: $1) \frac{1}{\alpha}$ ...
1
vote
1answer
76 views

Feynman Lectures: Trigonometry Error in Rotational Dynamics?

I'm reading through Vol. 1 Chaper 18, and Feynman says that in the system: (See here for a higher resolution copy - can't embed SVGs), the length of $PQ$ is equal to $r\Delta\theta$: If $OP$ is ...
0
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0answers
27 views

What is the mass to balance this pivot system with a pulley [closed]

How do I calculate Mass 2 for the following system to balance this system.. The brown lines represent a wooden frame which is not attached to the green block. Mass 1 is attached to rope which is ...
1
vote
2answers
34 views

How to model energy loss in a rotating body?

I recently asked a question about modeling instability in a rotating rigid body. I now realize that I was mentally confounding two different effects: The "Dzhanibekov effect" in which a rigid ...
1
vote
1answer
51 views

An electromagnetic induction problem [closed]

The question goes like this : A thin non conducting horizontal disc of mass $m$ having total charge $q$ distributed uniformly over its surface, can rotate freely about its own axis. Initially ...
0
votes
1answer
94 views

Small sphere rolling off the top of a large sphere [closed]

A heavy sphere of radius r = 1.00 meter is fixed with respect to the ground. A small uniform solid sphere is placed at the top of the larger sphere. After a slight disturbance, the smaller sphere ...
0
votes
1answer
66 views

Rotating uniform rod

A uniform rod of mass 1.2 kg and length 1.8 m is pivoted in the horizontal position as shown (black point). The rod is at rest and then released. The acceleration due to gravity is $g = 9.8 ...
1
vote
2answers
65 views

If a ball spinning on a rod hits another ball, what is conserved linear or angular momentum?

Suppose a 1-kg ball A is fixed to a spoke 0.2 m long, which is attached to an axle so that the ball can rotate (v=10m/s, KE=50J, $\omega$=50 rps, L=2, p=0) Now, there is a second ball B (m=1kg), ...
1
vote
0answers
26 views

Gravitational force and time dilation [closed]

Suppose the radius of the earth is reduced by half but the mass is same, then how long will it take to complete one rotation, 24, 48, 12 or 6 h.? please give the mathematical relations and solution. ...
2
votes
0answers
44 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 ...
0
votes
1answer
77 views

Moment of inertia of a cylinder about its base

I've tried to find the moment of inertia of a cylinder rotating about an axis parallel to its base (i.e about the 'End diameter') as one can see here . But when I checked my results with different ...
4
votes
1answer
70 views

How to simulate rotational instability?

I'm trying to simulate (for an educational game) the well-known effect that rotating objects with three nonequal moments of inertia are unstable when rotated around the middle axis. Some explanations ...
5
votes
2answers
119 views

Why don't we talk about angular momentum at all in fluid mechanics?

People usually talk about similar (or maybe not?) things like vorticity or enstrophy in fluid mechanics, but no one talks about angular momentum, why?
7
votes
3answers
942 views

What determines whether a pool ball will bouce backwards after colliding with another pool ball?

I'm no knowledgeable pool player, but I've noticed that sometimes when the cue ball hits another pool ball, they roll together; and sometimes the cue ball bounces back. And I have a very, very rough ...
0
votes
1answer
44 views

How can a generalised force be dependent on an angle i.e. not a vector?

I'm currently working through an example question in Patrick Hamill's 'A Student's Guide to Hamiltonians and Lagrangians'. The question I'm having conceptual difficulty with is: A particle is ...
0
votes
0answers
45 views

Does a rotating plank only acquire rotational kinetic energy?

I have another doubt with a Kleppner problem :(. A thin plank of mass M and length l is pivoted at one end. The plank is released at 60$^{\circ}$ from the vertical. What is the magnitude and ...
0
votes
1answer
55 views

Does a spinning part affect the moment of inertia of a composite object?

I have been going back through some Kleppner problems and have a doubt concerning problem 6.18. It states: Find the period of a pendulum consisting of a disk of mass $M$ and radius $R$ fixed to ...
1
vote
1answer
23 views

Collision Resolution System Adding Velocity Into System

In my 2-dimensional physics simulation, I have a rectangular rigid body 'a' with infinite mass (the floor), and a rectangular rigid body 'b' with finite mass above it turned at a slight angle. When ...
1
vote
1answer
42 views

If a planetoid was wet and you spun it would the water move to the equator?

If the moon was alone in space and covered in X feet of water and it began to rotate would it displace to a bias along the equator?
0
votes
2answers
53 views

Rotational behavior of objects in zero-g with forces [closed]

I would like to know how this object would rotate in free space out of curiosity. Below is a diagram of the object. It is of uniform density and has a center of mass in the center of the object, ...
1
vote
1answer
43 views

Effects of firing shells on the Earth's angular momentum

During a certain war, millions of shells were fired by country A towards the west, and even more shells were fired back by country B towards the east. The average momentum of each bullet were the same ...
0
votes
1answer
51 views

Finding the minimum radius of the pivoted disc

Here is a question based on Simple Harmonic Motion that I tackled just now. However I think I am having an approach to tackle this but I am not sure about it. Ouestion: A uniform disc of radius ...
2
votes
2answers
476 views

can we calculate velocity of a spinning apparatus by just analysing the frequency of sound emitted by it

I have used a new term spinning apparatus as I was unable to name it. I have tied a thread to a stone and was spinning it and I heard a sound something like that of a rotating propellor of a ...
0
votes
0answers
25 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
1answer
60 views

Falling off a chair, how best to save yourself

If I consider a man sitting on an office chair that reclines backwards iff you lean backwards. What could be done to prevent hin from falling? a) raising his legs till they are parallel to ground. ...
0
votes
0answers
51 views

Illustrate the transition from linear to rotational kinetic energy

I wish to find an everyday situation that illustrates the following: A rod is moving in a direction perpendicular to its axis. One end "gets caught" and the rod starts rotating around this end. THE ...
35
votes
6answers
5k views

How can the Earth keep spinning with a liquid core?

In regards to the 'conservation of angular momentum' being the explanation of why celestial objects spin... If you fill a ball or any other container with a liquid and try to spin it, you will not ...
0
votes
0answers
24 views

rotation of earth and changes in its diameter

could calculated the changes of the Earth's diameter cause the rotation?? I have seen 2 other posts about it but I couldn't understand their calculation and they were a little confusing and couldn't ...
1
vote
1answer
44 views

A pretty dumb question on observation

Very often I have seen, that a bicyclist can balance himself better, while in motion, than he can while at rest(with his legs on the paddles of the bicycle). Now, I know that objects, say, a disc ...
1
vote
3answers
47 views

Uniform Circular Motion and Centripetal Acceleration

In introductory physics books (or at least mine) it limits the equation $a_c=v^2/r$ to the situation where the speed around the circular path is constant. It enforces the idea that the speed is ...
0
votes
1answer
32 views

Calculating the components of angular momentum of a rigid body

You have a rigid body with 1 fixed point in space (the origin). It's self-explanatory how to get the following equation for the angular momentum: $\vec L = \sum_n m_n\vec r_n\times\vec v_n$ ...
3
votes
3answers
203 views

Why does the coriolis effect dissapear at the equator?

I'm studying from the book "Classical Mechanics" by Goldstein and from a coursebook my Professor provided me. In the coursebook, it says that "the Coriolis effect disappears at the equator (Where ...
0
votes
0answers
16 views

Pseudo force in rotational motion?

If a cylinder is in combined rotation and translation on a moving surface(say a plank with some acceleration), while solving for the acceleration of the centre of mass of the cylinder, do we consider ...
1
vote
3answers
45 views

Moment of inertia of a cylinder [closed]

When I tried to calculate the moment of inertia ($I_C$) of a cylinder (mass M, height H, radius R) around the rotating axis going symmetrically through its middle, I came up with a different result ...
1
vote
3answers
100 views

Why doesn't a block rotate due to friction?

In a horizontal surface, a block (cube) is sliding due to a sudden push. When the block slides, there is frictional force which is acting on the block. Frictional force will have a torque around ...
1
vote
0answers
34 views

Deriving tensor in Euler's equations for rigid body rotation

The answer to physics.stackexchange.com/questions/104513 gives the following derivation of tensor $I$: $\begin{align} \frac{\text{d}}{\text{d}t} I &= \frac{\text{d}}{\text{d}t} ...
0
votes
0answers
28 views

Wikipedia's derivation of torque related to angular acceleration [duplicate]

Wikipedia derivation of the relationship between a torque and an angular acceleration is given here. Could someone help me to see how the following: $$\vec{\tau} = \left(-\sum^n_{i=1}m_i [\Delta ...
2
votes
0answers
64 views

Difference of the O(N) Non-linear Sigma model and SO(N) Non-linearSigma model

The Hamiltonian \begin{equation} H=J\sum_{i,j}\vec{n}_i\cdot\vec{n}_j \end{equation} is invariant under a global rotation $\vec{n}_i\rightarrow R\vec{n}_i$, where $\vec{n}$ is a $N$ component rotor ...
2
votes
2answers
257 views

Moment of inertia of disc with a hole

Suppose we have a disc with a hole, when computing moment of inertia of this about the disc's centre. Why do we subtract the moment of inertia of the removed part from the moment of inertia of ...
3
votes
4answers
186 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 ...
2
votes
2answers
65 views

Coin on an turntable | Exact description of forces [closed]

Does more static friction between coin and turntable means that more it will slip off Or Just Exactly opposite of it.When I make picture of situation in my brain I am getting first statement but I ...
1
vote
2answers
50 views

Trouble with derivation in an equation for Newton's Law of Angular Motion

I'm an autodidact and can't follow the part after "it is easily seen that"... which is the 31st equation: Shouldn't it be: $m_i\,{\bf r}_i\times \frac{d^2{\bf r}_i }{dt^2}= \frac{d}{dt}(m_i r_i ...
0
votes
0answers
36 views

Rotating and moving reference frame

I've looked through your forums and can't find exactly what I need. I have a two objects whos dynamics can be described in discrete time as follows: $x_A(k+1) = x_A(k) + ...
-1
votes
1answer
52 views

Angular velocity of precession

So in my textbook they say this ${\rm d}\theta$ = |$d\vec{L}$|/|$\vec{L}$| $d\vec{L}$ is the change in angular momentum caused by a torque whose vector is perpendicular to $\vec{L}$, which is the ...
0
votes
0answers
20 views

Relation between Earth's rotation-gravity and moving object on Earth [duplicate]

This might be silly question. It's about the rotation of the Earth and the objects moving on it. If I jump into the air, why did I also move with the Earth though I am in the air? I am supposed to be ...
1
vote
1answer
40 views

Constant power in rotational dynamics

I am having trouble understanding and applying the concept of constant power (e.g. a motor) in rotational dynamics. We have that: $$P=\tau\omega$$ Therefore if we imagine a physical system with a ...