A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.
0
votes
0answers
56 views
Lagrangian of three connected rods
There is free falling chain - 4 connected rods with mass m and length l:
...
-2
votes
0answers
74 views
Rotating pipe with bug [closed]
A thin mass less horizontal pipe rotates about a vertical axis with angular speed $ω$ as shown in the figure. A small bug, located at a distance $r$ from the axis, starts crawling inside the pipe at ...
0
votes
0answers
34 views
Turning around in wheelchairs or on skates
Looking at wheelchairs and skates I always gets confused, because people can spin in circles, but the objects themselves are not perfect circles, they have lines rather than points at either side. ...
3
votes
0answers
52 views
The secret behind the spinning, asymmetrically weighted, 2D disk-shaped top?
When you spin an asymmetrically weighted, 2D disk-shaped top, the heavy part actually rises to the top. Why is this?
http://www.youtube.com/watch?v=h0SZZTBQmEs
...
0
votes
0answers
32 views
what happens to the angular velocity of star in star-black hole system?
What happens to the rotational and revolutionary angular velocities of star in star-black hole system as the star loses mass ?
1
vote
1answer
48 views
Optical illusion of car wheels, speeding up
Perhaps it is some free moving spinner attached to the wheel, but as opposed to this question: Why does the wheel of a car appear to be moving in opposite direction?
I have seen car wheels that appear ...
0
votes
2answers
32 views
Expected behavior of the gravity under some experiment [duplicate]
I would like to know the expected behavior of the Gravity under the following mentioned imaginary experiment:
What if we dig a well or a boar or a straight hole (say, its diameter is 100 meter) ...
3
votes
3answers
90 views
Static as opposed to Kinetic Friction in Rolling Motion
During analysis of rolling motion, why do we consider coefficient of friction as that of static friction and not kinetic friction?
0
votes
0answers
18 views
Stability of trajectory of disc which moves along a straight curve
Let's have a disc which moves along a straight curve on a plane in a uniform gravitational field. There need to discover the stability of it's trajectory.
I represented the possible deviation of the ...
1
vote
3answers
66 views
Maximum angular velocity to stop in one rotation with a known torque
I have an object I can rotate with a given torque. I would like to stop applying torque once I've reached a defined maximum rotational speed. The maximum rotational speed should be defined so that ...
0
votes
0answers
37 views
Dose the gravitational force produces precession in the spinning top?
I'm new at classical mechanics but the text book says there is the torque in the spinning top which generated only by gravitation. It is hard to explain the situation, I've add the link.
...
3
votes
2answers
95 views
What is the physical significance of the off-diagonal moment of inertia matrix elements?
The tensor of moment of inertia contains six off-diagonal matrix elements, which vanishes if we choose the principle axis of the rotating rigid body and the components of the angular momentum vector ...
45
votes
8answers
5k views
Proof that the Earth rotates?
What is the proof, without leaving the Earth, and involving only basic physics, that the earth rotates around its axis?
By basic physics I mean the physics that the early physicists must've used to ...
2
votes
2answers
48 views
Rotating spring system: Is my intuition correct?
Consider a solid spherical object of uniform density that is rotating on an axis A1. Perpendicular to that axis one can draw another line that passes through the sphere. On this axis, on both sides of ...
0
votes
0answers
39 views
Simple Calculus angular velocity problem [closed]
I have a Lagrange equation and I have no idea why the solution note I looked at have got
$d \over {dt}$$(mr\omega) = Q$,
$d \over {dt}$$(mr\ddot\omega) = Q$.
I had
$d \over {dt}$$(mr\dot\omega) = ...
7
votes
2answers
90 views
How to design a deliberately biased coin?
For demonstrating basic probability concepts, it would be nice to have a coin-like object that lands heads/tails not in 50/50% ratio, but biased in a way that can be revealed in a short experiment. ...
7
votes
2answers
226 views
Huge buildings affect Earth's rotation?
Does constructing huge buildings affect the rotation of the Earth, similar to skater whose angular rotation increases when her arms are closed comparatively than open?
1
vote
2answers
62 views
Deriving $T = F\ r = I\alpha$ for a rigid body
For a single point mass :
$\tau=F_{t}r=ma_tr=(m r^2)\alpha = I\alpha$
For multiple point masses bound together : $\sum \tau_i = (m_ir_i^2)\alpha = I\alpha$
But how do we go from that to $I\alpha = ...
3
votes
1answer
52 views
Intuitive explanation for why same force applied farther from a hinge causes larger angular acceleration than if applied closer?
A standard example of a problem involving torque is opening a door - the same force F applied far from the hinge causes a larger angular acceleration than if applied close to the hinge.
I always had ...
0
votes
1answer
30 views
A rod of length $L$ & mass $M$ is rotating in a circle about one end then calculate tension in the rod at a distance $x$ from the support
A rod of length L & mass M is rotating in a circle about one end then calculate tension in the rod at a distance 'x' from the support ?
For its solution why should we take mass of L-x portion of ...
1
vote
1answer
68 views
Physics of the point of contact for a spinning top
I understand how spinning tops don't tip over, cf. e.g. this and this Phys.SE questions. What I'm more interested is in identifying the factors that determine the direction the spinning top moves to?
...
4
votes
5answers
206 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 ...
0
votes
1answer
150 views
How is torque equal to moment of inertia times angular acceleration divided by g?
How is the following relation true
$$\tau = \large\frac{I}{g} \times \alpha$$
where $\tau$ is torque,
$I$ is moment of inertia,
$g= 9.8ms^{-2}$,
and $\alpha=$ angular acceleration.
2
votes
2answers
132 views
Angular momentum conservation while internal frictional torque is present
So this appears in a problem which looks simple enough in its context; It's something like this:
Two discs, A and B, are mounted coaxially on a vertical axle. The discs have moments of inertia $I$ ...
0
votes
1answer
78 views
Calculating the moment inertia for a circle with a point mass on its perimeter
I want to calculate the tensor of the moment of inertia. Consider this situation:
The dot represents a points mass, in size equal to $\frac{5}{4}m$. $m$ is the mass of the homogenous circle. I'm ...
0
votes
1answer
65 views
Finding the moment of inertia through superposition?
Let's say I have a body consisting of two homogenous spheres/balls that touch each other. I also have a body fixed coordinate system which consists of that body's principal axes. I know the the moment ...
6
votes
0answers
143 views
What determines the angle of the cushion on a pool table?
If you look at the cushions (bumpers) on a pool table, you'll see that they're not vertical. They're tilted inwards. About 10 years ago, I came across a physics exam in which one of the problems ...
1
vote
1answer
117 views
Double Compound Pendulum: why use inertia about the center of mass for bottom pendulum?
I'm trying to wrap my head around the kinetic energy of a double compound pendulum, like the one shown in the Wikipedia article on double pendulums.
I know for computing the kinetic energy of the ...
0
votes
1answer
50 views
Rotational Dynamics
In studying rotational dynamics of a rigid body , I can't seem to understand why you can solve the problem correctly only using certain points in a body and not all ? Means angular momentum and torque ...
-1
votes
1answer
117 views
Confusions about rotational dynamics and centripetal force
I am a high school student. I am having confusions about the centripetal force and rotational motion . I have known that a body will be in rest or in uniform velocity if any force is not applied. But ...
0
votes
1answer
39 views
Rotational Kinetic Energy of a rigid root - Full workings Provided?
Part v is where I'm stuck.
Here is the question with my workings
If I can find Torque I can find L and using the equation $L=Iw$ I can find the w value and hence using $w=2pi*f$ I can prove it ...
1
vote
0answers
105 views
Equivalence between a charged rotating cylinder and a solenoid
Suppose we have a cylindrical shell of radius $r$ with surface charge density $\sigma$. Then we start rotating the cylinder at an angular speed $\Omega$. You can show that in this case the surface ...
2
votes
3answers
159 views
Newton's Second Law Equivalent in rotational dynamics
The law that
$$\frac{d\vec{L}}{dt}= \vec{T}$$
where $\vec{T}$ is torque about a frame's origin $o$ and $\vec{L}$ is the angular momentum about that origin $o$.
Can this law be ultimately (always?) ...
0
votes
0answers
37 views
Mechanics: collision & rotation [closed]
A rod AB of mass M and length L is lying on a horizontal friction less surface. A particle of mass m traveling along the surface hits end A of the rod with a velocity 'v' in a direction perpendicular ...
0
votes
3answers
81 views
Mass equals Moment of inertia when constant density?
I have found equation for moment of inertia $(J)$. I'm calculating $J$ for hemisphere, with rotational axis $Z$.
$$ J = \iiint\limits_V r^2 \cdot \rho \cdot dV $$
But if $\rho$ is constant ...
0
votes
2answers
72 views
What's the motion of this yoyo under external force will be?
A yoyo on a horizontal table is being pulled by a string to the right, the table is not frictionless. If we only know that the object doesn't slip, how do we know if the string is winding up or ...
1
vote
1answer
106 views
Moment of inertia of a yo-yo
Considering the yo-yo like two CDs with a hollow cylinder between them, what is the moment of inertia of that object?
The axis that I must choose can't pass through the CM and be parallel to ...
3
votes
2answers
191 views
what's the physical significance of the off-diagonal element in the matrix of moment of inertia
In classical mechanics about rotation of rigid object, the general problem is to study the rotation on a given axis so we need to figure out the moment of inertia around some axes. In 3-dimensional ...
1
vote
1answer
169 views
Cylinder rolling down an inclined plane held by a string
A cylinder of mass M and radius R is in static equilibrium as shown in the diagram. The cylinder
rests on an inclined plane making an angle with the horizontal and is held by a horizontal string
...
0
votes
2answers
138 views
Rolling ball which slips
A bowling ball of mass $M$ and radius $r_0$ is thrown along a level surface so that initially ($t = 0$) it slides with a linear speed $v_0$ but does not rotate. As it slides, it begins to spin, and ...
2
votes
1answer
108 views
Optimal door opening
This is a problem that has been periodically bugging me, so I finally decided to work on it. I haven't done any physics since high school, so I'm a bit out of practice:
Consider a doorway with two ...
4
votes
4answers
148 views
Wheel locks and spinout
Imagine driving in a straight line on a ice lake, when you hit the brakes, if your goal is to stay in straight path with no spinout, which wheels would you choose to have locked: front or rear? ...
2
votes
2answers
175 views
Foucault pendulum
The equations of motions for a Foucault pendulum are given by:
$$\ddot{x} = 2\omega \sin\lambda \dot{y} - \frac{g}{L}x,$$
$$\ddot{y} = -2\omega \sin\lambda \dot{x} - \frac{g}{L}y.$$
What are the ...
4
votes
2answers
105 views
Thrust center in space
I have this dilemma: Suppose you have a space ship somewhere in deep space, where there is no drag force or substantial gravity. If the ship has a single engine situated in such a way that the center ...
2
votes
1answer
239 views
Is angular momentum always conserved in the absence of an external torque?
Consider either the angular momentum of the earth around the sun or equivalently swinging a ball horizontally on a string.
I know that with respect to the point of rotation of the swinging ball, ...
1
vote
0answers
69 views
Limitations on the choice of axis of rotation regarding rolling wheels
Consider a situation where a wheel is rolling without friction on a level surface. Call the center of the wheel $C$, the point where the wheel contacts the ground $G$, and some arbitrary other point ...
6
votes
0answers
106 views
Nuclear Magnetic Resonance (NMR) Conceptual Questions
Let $M$ be the magnetic moment of a system. Below are the Bloch equations, including the relaxation terms.
$$\frac{\partial M_x}{\partial t}=({\bf M} \times \gamma {\bf H_0})_x-\frac{M_x}{T_2} $$
...
3
votes
2answers
259 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 ...
0
votes
1answer
972 views
DC Motor Torque Constant
I am very new to DC motors and to stackexchange. Please correct me if anything I said does not make sense.
For DC motors, the equation looks like this:
$P = \tau\dot{\theta}$
where $P$ is power, ...
2
votes
1answer
106 views
Transform torque from Euler angles to infinitesimal Cartesian rotations
For a certain pair of rigid bodies, I have the gradient of energy in terms of Euler angles. I want to transform this gradient to the gradient of energy in terms of rotations about the $x, y, z$ axes ...
