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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3answers
417 views

Trajectory of rolling billiard ball after collision with another billiard ball with same mass

So, according to this article http://www.real-world-physics-problems.com/physics-of-billiards.html , the trajectory of white (cue) ball is always perpendicular to the line connecting center of ball ...
0
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1answer
35 views

Rotational Dynamics Question [on hold]

An ice skater spins with her arms folded. When she extends her arms outward, what will happen to her angular velocity?
0
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0answers
70 views

Euler's equations (rotating frames)

I'm trying to understand the Euler's equations and I'm having problems with rotating frames and on which specific frame is each quantity measured. On the equation $$ ...
1
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2answers
75 views

Centripetal force: if radius decreases, does ANGULAR or TANGENTIAL velocity change?

Having conceptual trouble with this aspect of centripetal force. Say we have a puck on a frictionless table attached to a string that I am holding through a small hole, so that the puck moves in a ...
1
vote
2answers
38 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 ...
1
vote
1answer
178 views

Rotation motion like the number 8

I'm a college student majoring in culinology and I'm trying to find out the reason or method of the number 8 motion. Responses doesn't have to be in culinology examples, but that would be a great help ...
0
votes
1answer
23 views

torque in boomerangs

I`m new to physics and I have recently learnt about boomerangs, but one thing I am unclear about is the gyroscopic precession. From what I learned is that there is a difference in wind speeds between ...
0
votes
0answers
27 views

Finding Angular Speed Problem [on hold]

A cylinder of mass 5.0 kg and radius 0.20 m is attached to an axle parallel to its axis and through its centre of mass. A constant force of 6.5 N acts on the cylinder. Find the angular speed of the ...
1
vote
0answers
46 views

What is the speed of the skaters in this case? [on hold]

You're choreographing your school's annual ice show. You call for eight 60kg skaters to join hands and skate side by side in a line extending 12m. The skater at one end is to stop abruptly, so the ...
0
votes
2answers
25 views

How to calculate the acceleration, if the mass-center is not on the force-vector's line (2D)

In a 2D plane there is an object, whose center of mass is at the $P$ point. The mass of the object is $m$. We apply a force $\vec F$ at the point $A$. If the center of mass is on the line, defined ...
1
vote
2answers
68 views

Angular momentum of rolling sphere [closed]

A sphere of uniform density $\rho$ and radius $r$ is rolling without slipping on a perfectly flat surface. It is moving in a perfectly straight line and its axis of rotation is parallel to the plane ...
0
votes
0answers
24 views

Force Balance Question [closed]

Hello, I've been asked to show that: $$d\Gamma=\tau 2\pi r^2 dr$$ Where $\Gamma$ is the torque applied to the shaft and $\tau$ is the shear stress of the sample fluid at radius $dr$. So, I know ...
1
vote
5answers
84 views

Rolling without slipping taking the contact point as pivot

I'm confused about this "rolling without slipping" kind of situation. Or better in this case the object is rolling and slipping, just use the label "rolling without slipping" to identify the kind of ...
0
votes
1answer
21 views

Freebody diagram for an object rolling up a frictionless incline?

I am confused about the direction of the force that causes the ball to come to rest once it reaches maximum height for a ball rolling up a frictionless incline. I understand that the translational ...
2
votes
3answers
87 views
0
votes
1answer
26 views

Pulley inertia momentum of inertia

Considering a pulley with some mass, or some friction, How can its inertia (or momentum of inertia) influence the tension on 1 rope? Which part of the rope is influenced? Before or after the pulley?
1
vote
1answer
32 views

Conservation of momentum (angular and translational) [closed]

A ring of radius $R$ and mass $M$ lies on its side on a frictionless table. A bug of mass $m$ rests on the ring. The bug starts walking on the ring with constant speed $v$ relative to the ring. ...
0
votes
1answer
30 views

Strength of the centrifugal force

Let's say you're on a roundabout. When you're radially further away from the roundabout would you feel more centrifugal force or less? I feel like intuitively you'd feel less, but from the actual ...
1
vote
3answers
117 views

How to find the center of rotation? (2D)

First off, I'm assuming that a free floating polygon doesn't always rotate around its center of mass unless the net force is zero (based on the points below). If this isn't correct please tell me. ...
6
votes
1answer
127 views

Optimal size for a bike tire

While I was riding my bike funnily enough I wondered if there was an optimal size for a bike wheel. So I suppose that is my question, What is the optimal size for a bike wheel? If the bike accelarates ...
0
votes
1answer
51 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 ...
0
votes
1answer
26 views

Principal axis of inertia parallel to the ones passing through the center of mass

Consider a rigid body and the (at least) three axes of inertia passing through its center of mass. Will any other axis not passing through the center of mass but parallel to one of the principal axes ...
1
vote
1answer
40 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 ...
1
vote
1answer
41 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
3answers
40 views

Axis of rotation and rotating rigid body

We know that angular momentum of a solid disk rotating with angular velocity w is given by Iw about its center.Now If I chose any axis(parallel to above), will it have same magnitude of angular ...
1
vote
1answer
87 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 ...
1
vote
2answers
29 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
186 views

How can angular velocity be constant even when there is a torque by friction?

A cylinder at rest lying on a rough ground is given an impulse which imparts a translational velocity (no angular velocity) to it. The question goes ahead with finding time after which rolling starts ...
0
votes
0answers
18 views

Locus of a point on Euler's disk

I have been struggling to find how a point on the surface of Euler's disk varies with time in $x$ ,$y$ and $z$ coordinates. The disk has a radius $R$ and a thickness say $T$. It originally makes an ...
0
votes
1answer
24 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 ...
0
votes
1answer
18 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
29 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 ...
0
votes
0answers
33 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 ...
1
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3answers
65 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 ...
0
votes
2answers
305 views

A lever arm-definition question

My question is regarding Torque, and specifically the definition of the lever arm. As far as I've understood, the lever arm is the line perpendicular to the line of action to your reference point. ...
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
3answers
73 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?
2
votes
1answer
67 views
0
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1answer
13 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 ...
0
votes
0answers
31 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 ...
0
votes
0answers
14 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
4answers
59 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 ...
4
votes
1answer
277 views

Sum of forces with liquid in rotation

It's not homework (I'm teacher). I would like to compute sum of forces on this study : The shape is symmetrical like that I'm sure the center of gravity is in the center of the shape. I compute ...
2
votes
2answers
3k views

Is squared motor gearbox ratio proportional to inertia ratio?

I read an interesting article http://m.machinedesign.com/news/motor-sizing-made-easy It is very interesting, but I can not follow the 2nd last paragraph. I don't understand why it is true. ...
0
votes
1answer
20 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
1answer
29 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
36 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
21 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 ...
0
votes
1answer
66 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
50 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 ...