Questions tagged [rotational-dynamics]

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

Filter by
Sorted by
Tagged with
0
votes
0answers
13 views

Why is stable rotation achieved about the long axis of a skateboard? [duplicate]

I've been preparing a video for my mechanics assignment and I've hit a deadlock. Basically, I want to prove that the long axis of a skateboard takes the least force to move around it.I figured that a ...
1
vote
1answer
57 views

Where does the energy go? [duplicate]

Suppose a typical case of a girl dancing and after sometime changing her $I$ (moment of inertia to rotate) faster, my doubt is in this case there is a non-zero difference in initial and final total KE ...
0
votes
0answers
52 views

Irodov 1.146 precission

Shouldn't we take centrifugal force by net $w$ which is about instantaneous axis of rotation? Solutions I saw said to take the precission frequency only which I am not able to understand, why?
0
votes
0answers
36 views

Rotating Of Object [closed]

The box is secured on top of a rotating platform. Coefficient of friction between circle platform (1m radius) and rectangle platform underneath it is 0.06. What force is required to rotate the ...
11
votes
8answers
4k views

Why is the concept of torque necessary?

In my physics textbook (“Physics for Scientists and Engineers” by Serway and Jewett) it says that “the cause of changes in the rotational motion of an object about some axis is measured by a quantity ...
0
votes
0answers
39 views

Zero Angular Momentum and Rotations [closed]

It's not difficult to understand why a particle that is not rotating does have angular momentum, just looking at it's basic definition: \begin{equation} \mathbf{L}=\mathbf{r}\times\mathbf{p} \end{...
0
votes
0answers
25 views

Why does the torque in a DC motor decrease linearly?

From what I know, the torque decreases because the net current within the coil decreases, as a result from the Back EMF opposing the supply EMF. However how do we know that this current and hence ...
2
votes
2answers
70 views

Solving this problem using the Work-Energy Theorem [closed]

A block is tied to a string wound on a cylinder of mass $m$ and radius $r$, (Through a pulley) which can rotate about its axis on a massless mount placed on a smooth surface. The system is released ...
0
votes
1answer
28 views

How to calculate the effective mass of a body going up a curb? [closed]

I am currently doing a project in which I am finding the force required for a skateboard to go up a curb and was wondering how I would calculate the effective mass so that I can use it in the ...
0
votes
1answer
16 views

Why does this aircraft gyroscope has two degrees of freedom?

This is a Directional Gyro Indicator, it is a tied gyro, the spin axis is tied to the aircraft horizontal axis. I read that this gyroscope has two degrees of freedom. Please explain why is it so? What ...
-2
votes
1answer
21 views

A complex rolling system [closed]

A point mass m is connected to a rigid cylinder of mass M via a massless, rigid, rod. The system (see the figure below) is then released from its (unstable) equilibrium state and the cylinder starts ...
0
votes
0answers
29 views

High wire artist

We want to derive a Lagrangian equations for a high wire artist which uses a balancing rod for stability purposes. We know the masses and the polar moment of inertia of the artist and the rod.
2
votes
2answers
53 views

Is the Moment of Inertia tensor symmetric due to rotational invariance of space?

In standard classical mechanics' textbooks (for instance Morin) one finds that while calculating the angular momentum vector of a rigid body for a 3-D general case from the definition: $$L=r \times p$$...
0
votes
1answer
27 views

Moment of Inertia in non-rotational context

I have read that Civil Engineers use moment of inertia to characterize the elastic properties (rigidity) of such structures as loaded beams. My doubt is how is Moment of Inertia helpful in explaining ...
0
votes
1answer
38 views

Are these equations correct for the motion of a gyroscope?

I was coding a gyroscope simulation and came up with some equations that I used for it. So firstly, torque = (change in angular momentum)/(change in time) = (vertical moment of inertia)(vertical ...
0
votes
0answers
28 views

Angular velocity of a translating and rotating frame

I want to determine the angular velocity $^{I}w^{B}$ which is the angular velocity of frame B about the Inertial frame I. The position of P relative to O is written as: \begin{equation} r_{P/O} = r_{O'...
0
votes
1answer
22 views

A string is wrapped around a cylinder what would be the length of the string that passes the man's hands if he also moves towards it?

I came across a simpler version of this question where the man was stationary, and pulling on a string which was wrapped around a cylinder. We assume that there is no slipping anywhere. Then we know ...
0
votes
0answers
25 views

Representing vectors in non-inertial rotating frame of reference

Suppose an object is moving in a circular path about a certain point $O$, with an angular velocity $\omega\hat{k}$ or linear velocity $v\hat{\theta}$. This is basically describing an anticlockwise ...
3
votes
2answers
61 views

How to determine the direction of the tendency for a rolling object to slip?

I know that the direction of static friction in a rolling motion has been asked about before, but I am specifically asking how to determine the direction of the tendency to slide. In this picture, the ...
0
votes
0answers
32 views

Determining the initial conditions of a rotating/translating reference frame

I am trying to derive a set of initial conditions consisting of a range (ρ), two angles (β and ε) and their derivatives. Calculating ρ β and ε was straightforward ;However, finding $\dot{\rho}$, $\dot{...
0
votes
1answer
31 views

Angular acceleration of orbit

I have been spending some time examining the relationship between linear and angular quantities, especially in relation to orbits: Consider a normal two body system (like earth-sun) with masses m and ...
1
vote
4answers
66 views

Reaction force and normal force for falling and sliding rod

In the following picture, the blue bar represents a rod that is falling from the sky and the black line represents the ground. Hence, in the picture, the moment of the impact is shown. We assume the ...
0
votes
0answers
13 views

Net angular motion of a body in two cases

So here I have two questions and both have nearly the same structure but the method used for both of them is different, not method exactly but formula. So here is the first question, So here is the ...
1
vote
1answer
46 views

Is a Couple always needed to produce torque?

In my book, its written that: When we produce torque on a pivoted body, we are always producing a couple as the reaction force from the pivot provides an equal, opposite and parallel force. So ...
1
vote
0answers
20 views

Can someone explain Couple with respect to forces acting on it?

Suppose two inequal, parallel and opposite forces $P$ and $Q$ are acting on a body not along the same line, producing a clockwise moment. If the rod on which its acting is massless, then we can extend ...
-1
votes
1answer
26 views

Moment of Inertia of a Disk with a small hole in it

We have a disk of radius b. Suppose we cut a small portion ( a circle of radius a) from the disk. Now the mass of this object (with this hole) is M. What is the moment of inertia of this object about ...
0
votes
1answer
59 views

Why does a rotating mass generate a torque on an attached object?

In this video, we see a little toy vehicle with two wheels that is unstable by itself. However, when an attached gyroscope is running (a rotating mass) it stabilizes itself. When the mass starts to ...
0
votes
1answer
31 views

What is the impulse equation in $n$ spatial dimensions?

Reading this. (page 5) It gives the impulse $J$ in 3 dimensions using cross products etc. for a pair of colliding bodies. Since the cross product does not generalise to higher dimensions, is there an ...
1
vote
0answers
53 views

System vector, angular momentum, and rotation

In Goldstein 3rd edision p. 409, it is stated that $$\partial \mathbf{F}=[\mathbf{F}, \mathbf{L}\cdot\mathbf{n}]=\mathbf{n}\times\mathbf{F}\tag{9.121}$$ if $\mathbf{F}$ is a function of system ...
1
vote
1answer
32 views

Can a torque on the rim of a solid cylinder cause rolling without slipping?

I'm getting stuck on friction (heh). Here's a simplified problem from my textbook illustrating my confusion. We have a solid cylinder of mass m and radius r, lying on its side on a table, with a ...
0
votes
1answer
38 views

Setting up equations for colliding ellipsoids?

I can set up some equations for colliding spheres in D dimensions as follows. Let there be two spheres of mass $m$ and $M$ respectively. Let the first sphere have ingoing velocity vector $u$ and ...
0
votes
0answers
20 views

Determine the coordinates of the point on the $y$-axis that the force passes

I've recently done a problem where I've been given adiagram with some forces applied to a beam trolley, and where I needed to determine the coordinates of the point on the $y$-axis through which the ...
0
votes
0answers
22 views

Velocity of a point on a rigid body, relative to the center of mass, when the entire body is in a circular orbit

Suppose I have an inertial reference frame centered around the point $O$. At a distance $\vec{R}$ from this point, we have the center of mass of a rigid body. This rigid body is revolving around the ...
4
votes
2answers
119 views

What is the physical phenomenon the vector cross product describes?

I have been searching about the cross product and still can't grasp the physical intuition of it. As far as I know, mathematically the cross product is a tool that creates a new vector perpendicular ...
0
votes
1answer
34 views

Torque direction and interpreting whether the object is speeding up or slowing down

I have understood that the convention is that counterclockwise Torque is taken as positive and clockwise torque is taken as negative, But my textbook says "If the net torque is positive, the ...
1
vote
2answers
32 views

Hoop flattening and centripetal force

Consider a loop (or a hoop) rotating perpendicularly to it's symmetry axis. The loop flattens. It is a model of Earth flattening. https://www.youtube.com/watch?v=Tctr8CIMOZA&ab_channel=UMDemoLab ...
0
votes
1answer
35 views

Why does a long, free-to-move object rotate when I push it from one side?

I was pushing a pencil along a table top when I noticed that it rotates around the other end. Somewhat like this(The dotted red lines show the final position of the pencil). The pencil is not pivoted ...
0
votes
1answer
57 views

Balancing smartphone on corner of table

Today when I saw my phone on table some of its part was off the table, so I wondered what will be the arrangement of phone to balance it on corner of table such that it's minimum area (of screen) is ...
1
vote
1answer
60 views

What does rotational balance mean?

For example, in many badminton rackets, it says that the balance point is rotational. How does this actually work?
0
votes
1answer
33 views

Effect of smooth surface on an already rolling body

If a purely rolling body (say, a sphere) is first rolling on a rough surface and suddenly after a distance the surface is found to be smooth, then what effects does it have on the state of rolling of ...
0
votes
1answer
32 views

Why does the friction at axis of a wheel cause a torque?

To have angular acceleration about the axis, you must have a nonzero torque on the body about the axis of rotation. However, the torque due to the friction at the axis would be zero, since the ...
2
votes
1answer
51 views

Gyroscope Angular Momentum Analysis

I'm puzzled by the following questions on gyroscope in HRK physics 5ed (p. 220) Basically the gravity torque is $~τ=Mg\,L\,\sinθ~$. The angular momentum $L_s=I_s\,ω_s~$ has a horizontal radial ...
0
votes
2answers
41 views

Why does a precessing wheel remain horizontal, instead of flipping?

Suppose I have a wheel with an axle, such that one side of the axle is tied to a rope. I'm initially holding wheel in such a way, that the radius vectors of the wheel are perpendicular to a board. I ...
4
votes
4answers
135 views

What is faster? Pure rolling or rolling with slipping?

So we know that a ball will slide down an incline when there is no frictional force. Once you say there is static friction it causes a angular momentum which in turn causes a torque and makes the ball ...
-2
votes
1answer
29 views

What would the moment of inertia of a hollow cylinder a distance to the axis of rotation be? [closed]

So I understand that the inertia of a hollow cylinder about a tangent axis to its surface is $\frac{M}{2}(R_1^2 + 3R_2^2)$, but what if this axis is not a tangent to the cylinder? So it is some ...
0
votes
2answers
129 views

Is it possible in principle, to analyze problems in rotational mechanics using force and mass, instead of torque and the moment of inertia?

Suppose we are studying the case of uniform circular motion. The analysis of such motion is usually done using Newton's second law as $ma=m v^2/r$. Even when the motion isn't purely circular, like the ...
0
votes
0answers
30 views

Rotational Ehrenfest theorem derivation

I'm trying to derive the rotational analog to the Ehrenfest theorem, but I dont know why I don't get the expression, we want to derive, $$\frac{d}{dt}\langle L \rangle= \langle N \rangle \quad ; \quad ...
0
votes
2answers
47 views

Does an object performing a uniform circular motion rotate even when there is no angular acceleration?

Is a tangential acceleration, and thus an angular acceleration necessary for rotation to occur? If an object in a UCM rotates then would centripetal acceleration be sufficient to induce rotation? Then,...
0
votes
2answers
31 views

Spin, Tumble and Wobble in Regular Polyhedra

Because of its uniform distribution of mass, any line passing through a sphere's center of mass can be used as a spin axis, and the sphere will not exhibit secondary wobble or a chaotic tumble. Under ...

1
2 3 4 5
66