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

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13
votes
1answer
889 views

Unfurling a toilet paper roll in the ISS

Consider a toilet paper roll floating ISS. Can an astronaut unfurl the roll only by pulling the loose end of paper (and not holding the rest of the roll in place, for example by using his finger as an ...
2
votes
0answers
63 views

Protoplanetary disks, angular momentum and prograde orbits

So you've got a protoplanetary disk and you're going to gravitate yourself some planets together. The disk is made up of the usual planetary system stuff, dust and gas and whatnot, orbiting a common ...
0
votes
1answer
65 views

Rigid Body Dynamics - Calculating Rotation of an Object

I've looked everywhere and I can't quite find a concise answer, especially due to my limited knowledge and understanding of terminology. I found something called Rodrigues' Rotation Formula that ...
0
votes
3answers
116 views

Rotational Inertia of a Disc and Subsequent Acceleration

This is more of a conceptual problem as opposed to anything wrong with the maths! (I think/I hope! :) ) But essentially the problem states: imagine a circular flat disc of radius $R$ and of mass ...
1
vote
1answer
96 views

Thought Experiment: spinning ping-pong ball on International Space Station. What would Happen?

A ping-pong ball with a certain vector and rotation/spin in zero G, 1 atmosphere. If you did this experiment in a space station would the ball 'orbit in a circular fashion. or "Can you throw a ...
0
votes
2answers
250 views

What rotates a door : torque or centripetal force?

I can realize that they are different, but can't understand what actually creates a rotational motion. Rotational motion doesn't mean just moving in a circle. There must be rotation of the position ...
0
votes
1answer
41 views

Transmissibility

An extract from a school maths mechanics textbook reads: To deal with cases in which the body is not rotating freely under gravity, we need to take into account also the work done by other ...
0
votes
1answer
42 views

An unwinding cable problem [closed]

Suppose a light cable is wrapped around a solid cylinder with mass $M$ and radius $R$. The cylinder rotates with no friction about a stationary horizontal axis. The free end of the cable is attached ...
2
votes
1answer
70 views

How to get the linear and angular acceleration generated by a force vector field?

I am working on a physics simulation and I have to calculate the angular acceleration in degrees per seconds squared around the point on the object located relatively to the center of a vector field ...
1
vote
1answer
92 views

Coupled wheel and rod (analytical mechanics)

I am struggling with formulating the equations of motion. Consider a coordinate system with origin in $O$ ($y$ upwards and $x$ to the right), label the center of mass of rod $AB$ with $G$ then: ...
1
vote
1answer
39 views

Running Euler's disk in a superfluid

I was considering the toy Euler's Disk, a video can be found here: http://www.youtube.com/watch?v=mVl2CBG_h2s I was interested in understanding the behavior of the disk particularly in vacuum and in ...
1
vote
1answer
70 views

Rotational KE in double pendulum

Why the Rotational Kinetic Energy term for the point mass of Kinetic Energy for double pendulum is not included in Lagrangian equation? \begin{align} ...
1
vote
2answers
60 views

Cause of rolling friction

Can you explain the cause of rolling friction? Can you tell me why, while driving a bicycle, there is less rolling friction when its tyres are fully inflated?
0
votes
1answer
31 views

When conserving angular momentum, about which point(s) should it be calculated?

In my physics problem I have a ball fired at a non fixed bar, which is moving at some velocity, causing them both to stick together and the bar-ball pair to rotate about the new center of mass. About ...
2
votes
1answer
103 views

Why are the principal axes about the center of mass of a cube perpendicular to its faces?

I have calculated the moment of inertia tensor of a cube about its center of mass: $I=\dfrac{1}{6}Mb^2\{1\}$ where $\{1\}$ is the identity matrix. So the principal moments of inertia are all 1 (1 is ...
0
votes
1answer
36 views

Rotational dynamics about a point or an axis?

In several books and sites, its written that torque is calculated about an axis and angular momentum is calculated about a point. In MIT's Angular momentum lecture pdf, Angular momentum has been ...
2
votes
1answer
97 views

Torque for a door

The question is: A door is hinged at one end and is free to rotate about a vertical axis. Does its weight cause any torque about this axis? Give reason for your answer. I think that the answer ...
0
votes
1answer
100 views

What forces cause the centripetal acceleration in circular motion on a merry-go-round?

Suppose you stand on a merry-go-round spinning at $f$ revolutions per second and you are $R$ meters from the center. What forces act on you? If you were tied to a rope like in Fig 2 you would follow ...
1
vote
1answer
63 views

Can moment of inertia be defined as function of mass?

For continuous bodies, moment of inertia is found as $$ \int dI = \int_{m_i}^{m_f} r^2(m) .dm$$ . Now, $$\int dx = \int_{u_i}^{u_f} f(u).du \implies X_f(u) = \int_{u_i} ^{u_f} f(u) .du + X_i(u)$$ , ...
0
votes
2answers
43 views

Computing lengths for a rotating rod

I know how to assign dimensions to a profile that is under static loading - that is under Mechanics of Materials, I would agree on a beam section, calculate the maximum moment and use the elasticity ...
0
votes
0answers
15 views

Spinning top don't fall and stationary one can't stay upright [duplicate]

Why a spinning top do not fall but a stationary one falls. What kind of law is governing it.
2
votes
2answers
149 views

Why does an unhinged body rotate about its centre of mass?

If a force is applied to a body which does not act through its centre of mass, it rotates about its centre of mass and not any other point. Why?
1
vote
3answers
332 views

Minimum angular velocity for circular motion (pendulum)

How can I show that there is a minimum angular velocity $\omega_{min}$, different from zero, such that if we chose an $\omega$ smaller than $\omega_{min}$, then it is not possible to have a circular ...
1
vote
2answers
67 views

Flipping a bottle

I tried to do this experiment recently at school and my house. I started the experiment by having three same type of bottles with the labels A,B and C. Bottle A has been fully filled with water, ...
0
votes
0answers
22 views

How to prove the relation between the rate of change of a vector in space coordinate and in body coordinate?

I'm learning Goldstein's classical mechanics, and in chapter 4.9 he prove such an operator equation $(\frac{d}{dt})_s=(\frac{d}{dt})_r+\omega \times$ In the proof, he takes the space and body axes as ...
0
votes
1answer
28 views

How to approximate lag of roll of a bird (or RC airplane)?

When an airplane rolls using ailerons, the ailerons itself are changing their state quite instantly. However, in order for the airplane to actually start to roll, it should take a considerable amount ...
0
votes
2answers
48 views

Rigid bar on the floor of a rotating space station

I read this in a comment to an answer in physics.stackexchange.com. The comment was An easier method might be to just place a straight, rigid beam on the floor. If you find the floor is concave, ...
1
vote
0answers
29 views

Most general gyroscope motion [duplicate]

In Kleppner & Kolenkow's Introduction to Mechanics (Amazon link), the general gyroscope motion including nutation was derived using the approximation condition $\Omega T \ll 1$ where $\Omega$ is ...
1
vote
4answers
245 views

Why is moment of inertia dependent on $r^2$ and not on $r$ ? (physical reason)

Moment of inertia is the mass equivalent in rotational dynamics. I know , by mathematical arguments, moment of inertia of a particle is $$ I = \text{mass} \cdot r^2$$ . But what is the physical ...
3
votes
1answer
534 views

Ball Rolling Down An Inclined Plane - Where does the torque come from?

There is a ball rolling down an incline, with no slipping. If we consider the point of contact between the ball and the inclined plane to be the pivot point (for our torque calculations), then I have ...
1
vote
1answer
82 views

Disk and Ball on a smooth surface [closed]

A uniform disk of mass $\text{2m}$ and radius $\text{R}$ is placed freely on a smooth surface as shown in the figure. A particle of mass $\text{m}$ is connected to the circumference of the disc with a ...
3
votes
2answers
458 views

Can moment of inertia be negative? [closed]

Q: Find out the moment of inertia of a uniform circular disc of radius $r$ & mass $M$ & the axis passes through a point on the circumference. My attempt: Let the axis passes through $O$ on ...
1
vote
4answers
192 views

Rotation systems. Problem interpreting an equation

In this equation: $$ \mathbf a_i\overset{\rm def}{=}\left(\frac{d^2\mathbf r}{dt^2}\right)_i=\left(\frac{d\mathbf ...
19
votes
6answers
9k views

Can a car steer on a frictionless surface?

Do the front tires of a car act like gyroscopes, such that a car could steer on a frictionless surface?
0
votes
3answers
134 views

Forces and acceleration on rotating objects?

Suppose you have an object undergoing uniform circular motion, with force vector pointing towards the center and another force vector tangential. Can it be said that the net force pointing in the ...
4
votes
3answers
395 views

Has a body angular momentum and torque only in a circular path?

In different contexts, my book(Principles of Physics by Resnick, Halliday ,Walker) , they wrote For torque, the path need no longer be a circle and we must write the torque as a vector ...
2
votes
1answer
122 views

If direction of torque is upwards(or downwards), why does the body rotate perpendicular to the direction?

We know torque is given by $$\vec{\tau} = \vec{r} \times \vec{F}$$ . Its direction is given by right-hand rule which says that torque acts perpendicular to the plane where force applied and position ...
1
vote
3answers
72 views

Problem in understanding the process of calculating the rotational inertia

As we know, rotational inertia is the mass-equivalent in rotation. For a discrete body, it is measured as $$I = \sum m_i{r_i}^2 $$ . But when a continuous body comes, $$I = \int r^2 .dm$$ which ...
1
vote
1answer
52 views

Why does a rolling disc topple less easily than a standing one?

If a force is applied perpendicular to the surface of a disc it topples sideways. Also, a stationary disc topples more easily than a rolling one. The explanations I've seen for this state that since ...
0
votes
0answers
29 views

Force and Torque being applied off-center due to magnetic forces [duplicate]

Say that I have two magnetic dipoles, one of which is rigidly attached to a freely movable inflexible body at some point that is not at the body's center of mass, while the other is fixed in space. ...
1
vote
3answers
451 views

Will an object rotate when we apply a force to it?

What would happen if the axis of rotation passes through the centre of mass of an object? Will the object rotate when we will apply a force to the object? Edit: The object is free, is not fixed to an ...
0
votes
2answers
65 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 ...
0
votes
0answers
41 views

Is it possible to reverse gravital direction of center of a mass by gyroscopic movements?

I am sorry that I don't know exact terms to define my question in physics. I watched a few videos that showed moving objects by its own like cubli (gyroscopic moving parts by means of mini motors and ...
0
votes
0answers
34 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 ...
2
votes
2answers
132 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
vote
1answer
70 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
71 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 ...
20
votes
9answers
3k views

Physical meaning of the angular momentum

Still reading Classical Mechanics by Goldstein, I'm struggling on a very basic notion: angular momentum. I physically understand it as the momentum of an object rotating around something given a ...
7
votes
3answers
302 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 ...
1
vote
0answers
130 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 ...