A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.
2
votes
3answers
289 views
Does a toy top weigh less when it is spinning?
I am under the understanding that a toy top will weigh less when it is spinning. The Russians made a spinning type transport back in the 70s to lessen its payload over the tundra. Is this an effective ...
1
vote
0answers
47 views
Fading transition and rotation of and object in 2D
I'm looking for sources about I guess dynamics subject.
The model I'd like to solve is reduced to a question of:
How does a force applied on a certain point of an object results in both fading ...
0
votes
1answer
87 views
What happens at the end of Coriolis Deflection
Consider we launch a cannonball due south from a point at 45 degrees latitude in the Northern Hemisphere (e.g the point defined with the co-ordinate system on this diagram). The cannonball travels for ...
0
votes
0answers
66 views
Help course exercises vol.1 Cap Berkeley. 6 [closed]
1 2. Angular momentum of tetherball. The object of the game
tetherball (Fig. 6.24) is to hit the ball hard enough and fast
enough to wind its tether cord in one direction about the
vertical post to ...
0
votes
0answers
301 views
How to find Rotational and Translational Equilibrium of Hanging Masses on a Bar?
I am making a hanging mobile which needs to be done mathematically by calculating torque. The problem is, I can't seem to figure out how to solve for the distance of the two masses from the pivot ...
1
vote
1answer
182 views
Non-commutative property of rotation
Addition of angles are non-commutative in three dimensions. Hence some other angular vector quantities like angular velocity, momentum become non-commutative. What is the physical significance of this ...
28
votes
4answers
3k views
How can earthquakes shift the earth's axis?
One often comes across news articles that claim that an earthquake shifted the earth's axis.
http://news.google.com/?q=earthquake%20shifted%20OR%20shifts%20earth%27s%20axis
If you ignore the ...
0
votes
3answers
267 views
Aircraft Level Flight Trajectory
An aircraft climbs to 15000 feet and enters 'level flight' phase. My basic knowledge of physics says that forces on the aircraft at this time are balanced - as seen in this diagram.
...
1
vote
3answers
584 views
What determines the direction of precession of a gyroscope?
I understand how torque mathematically causes a change to the direction of angular momentum, thus precessing the gyroscope.
However, the direction, either clockwise or counterclockwise, of this ...
10
votes
3answers
504 views
Which direction will Coriolis forces deflect a bubble?
If I throw a ball straight up, it deflects slightly to the west due to Coriolis forces. If instead I watch a bubble float up in water, is the bubble deflected west, east, or neither?
I think the ...
1
vote
3answers
438 views
What is the principle behind centrifugation?
What is the principle behind centrifugation?
I understand the idea that you spin something around the centripetal force will cause an apparent force on the spinning system.
However I don't quite ...
8
votes
2answers
621 views
Hamiltonian is conserved, but is not the total mechanical energy
I wondering about the interpretation for the energy difference between the Hamiltonian and the total mechanical energy for systems where the Hamiltonian is conserved, but it is not equal to the total ...
11
votes
5answers
798 views
What causes the back of a bike to lift when the front brake is applied?
What causes the back of a bike to lift when the front brake is applied? (Like in an endo.)
Also, if I were to replicate this effect with a wood block with wheels that crashes against a wall (only the ...
1
vote
0answers
242 views
Rotational Dynamics Problem-Rod slipping against block
A uniform rod of mass m and length l is pivoted at point O. The rod is initially in vertical position and touching a block of mass M which is at rest on a horizontal surface. The rod is given a slight ...
0
votes
0answers
131 views
Torque required to spin a disk along its diameter
How would I calculate (or simulate) this? I am only interested in the aerodynamic drag caused by the surface moving, not any other forces.
As far as I know, the only variables needed are the drag ...
2
votes
1answer
240 views
Why is an electric motor more efficient at higher loads?
My question is driven by the plot below. We see that acceptable operating range of a motor is between 50-100% of the rated load. Below 40% or so the efficiency of the motor drops off dramatically.
...
7
votes
4answers
460 views
Which direction will the yoyo move?
This question has been around the net for a while, and I haven't seen a good explanation for it:
A yo-yo is initially at rest on a horizontal surface. A string is pulled in the direction shown in ...
0
votes
3answers
101 views
Why is $F = mg - T$ in this case?
The situation is as follows:
I am told that $F_{net} = mg - T$ in this case, but doesn't that not take into account that $T$ isn't applied to the center of mass? Newton's second law is defined for ...
2
votes
3answers
21k views
Linear acceleration vs angular acceleration equation
I'm learning about angular velocity, momentum, etc. and how all the equations are parallel to linear equations such as velocity or momentum. However, I'm having trouble comparing angular acceleration ...
1
vote
1answer
104 views
Appearing To Reverse Object's Rotation
Can it be done, and if so, how does one you explain mathematically the ability to cause a rotating object to appear to change the direction of rotation? I believe it has something to do with angular ...
1
vote
1answer
256 views
Kinetic Energy And Rotational Motion
The problem is, "A metal can containing condensed mushroom soup has mass 220 g, height 11.0 cm and diameter 6.38 cm. It is placed at rest on its side at the top of a 3.00-m-long incline that is at ...
1
vote
2answers
79 views
Ice skater increase of energy
This may be a very basic question but I am not seeing how it works.
Consider the standard example of an ice skate rotating about his/her center of mass and pulling in his/her arms. The torque is zero ...
0
votes
1answer
291 views
Finding Rotational Kinetic Energy Of A Clock
The problem I am working on is:
"Big Ben, the Parliament tower clock in London, has an hour hand 2.70 m long with a mass of 300 kg, and a minute hand 4.20 m long with a mass of 100 kg (see figure ...
0
votes
2answers
2k views
Tensions And Pulleys With Masses
The problem I am working on is:
"A block of mass m1 = 1.80 kg and a block of mass m2 = 6.30 kg are connected by a massless string over a pulley in the shape of a solid disk having radius R = 0.250 m ...
2
votes
2answers
163 views
Ideal 2D Unicycle Kinematics
A particle is connected to a massive wheel by a rigid rod. The wheel can roll without slipping on a horizontal surface. The particle is free to rotate around the centre of the wheel.
I believe the ...
1
vote
1answer
211 views
Artificial Gravity - Spinning Station Questions II
In an answer to Artificial Gravity - Spinning Station Questions Vintage wrote:
A theoretical space station of radius 900 meters, doing a complete rotation every 60 seconds (in order to generate ...
0
votes
2answers
261 views
Setting up equations for a Rotational Kinematics system
I'm having a hard time setting up equations for the following problem:
A green hoop with mass $m_h$ and radius $r_h$ m hangs from
a string that goes over a blue solid disk pulley with mass $m_d$
...
0
votes
1answer
111 views
Deducing latitude from Foucault Pendulum
Suppose I set up a Foucault pendulum and observe that it precesses at a rate of 216.528 degrees per day. While I am observing this, a total solar eclipse occurs. Where am I, and what is the date?
My ...
0
votes
2answers
236 views
Force applied off center on an object
Assume there is a rigid body in deep space with mass $m$ and moment of inertia $I$. A force that varies with time, $F(t)$, is applied to the body off-center at a distance $r$ from its center of mass. ...
4
votes
1answer
150 views
Stability of a rotating ring of multiple electrons at relativistic speeds
There was a time when physicists where concerned about electron internal structure. The rotating ring model was one of the proposals to explain how a charge density could become stable against ...
2
votes
1answer
209 views
Why are Euler's equations of motion coupled? Physical explanation
I have a problem with one of my study questions for an oral exam:
Euler’s equation of motion around the $z$ axis in two dimensions is $I_z\dot{\omega}_z = M_z$, whereas it in three dimensions is ...
1
vote
3answers
531 views
Time period of torsion oscillation
The time period for a wave of frequency $\nu$ is given by $T = \frac{1}{\nu}$ or in other words, $T=\frac{2\pi}{\omega}$ where $\omega$ is the angular velocity...
For the oscillation of a torsion ...
1
vote
1answer
42 views
Rotation of diatomic homonuclear molecule
I know that the rotation energy of a diatomic homonuclear molecule is $E_{Rot}=\frac{\hbar J(J+1)}{R^2 M}$. Does the axis of rotation depend on $J$? With respect to which axis does the molecule for ...
3
votes
1answer
127 views
Angular acceleration of stone disk
I have a conceputal question regarding the following problem:
A round massive stone disk with diameter $0.600 m$ has a mass of $50.0 kg$. The stone rotates at an angular velocity of $115.2 rad/s$, ...
0
votes
2answers
251 views
Precession of angular velocity about the body-fixed axis
My textbook mentions that under force-free motion of a symmetric top, its angular velocity vector $\overrightarrow \omega$ precesses about the $z$-axis of the body-fixed coordinate system. This seems ...
1
vote
1answer
164 views
What controls whether a ball will skid or roll?
A billard ball is struck with a cue. The line of action of the applied impulse is horizontal and passes through the center of the ball. The initial velocity $v_0$ of the ball, its radius $R$, its mass ...
3
votes
1answer
275 views
Rotating/Translating Disk
I was trying to understand an aspect of rotational dynamics and thought of a problem to help me learn. I'm sure this problem has been considered by countless people in the past, but I'm having some ...
33
votes
7answers
3k views
Why is the Earth so fat?
I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy?
The ...
5
votes
1answer
114 views
Elementary derivation of the motion equations for an inverted pendulum on a cart
Consider a cart of mass $M$ constrained to move on the horizontal axis. A massless rod is attached to the midpoint of the cart, having a mass $m$ on its endpoint. See wikipedia for a picture and for a ...
1
vote
1answer
116 views
How do the energy eigenvalues of rotational degrees of freedom in statistical mechanics come about?
I want to understand the hierarchy different degrees of freedom of a mechanical system. Specifically, I want to understand which subsystems equibrilate faster and why. This question comes up:
Why ...
-1
votes
1answer
110 views
Can an electric motor force angular momentum not to be conserved in an isolated system?
An ice skater is in a spin, she pulls her arms in and she spins faster, she lets her arms extend outward and then she starts to slow down.
She will probably weigh on a weigh scale about the same ...
8
votes
1answer
282 views
Why does a cuboid spin stably around two axes but not the third?
Let $C$ be a cuboid (rectangular parallelepiped) with edges of lengths $a < b < c$.
Consider an axis that passes through the centers of two opposite faces of $C$. There are three such axes, ...
2
votes
3answers
573 views
Why do rolling disc (coin) move in circular path?
We have a coin that is rolled such that it's tilted at an small angle $ \theta $.
Question:: What turns around rolling disc so that it traces circular motion (spiral as it's speed decreses)?
...
0
votes
0answers
100 views
Assume a spool rolls down a slope with its body (not its ends) in contact with the slope
I was doing a little mental exercise and imagined the above scenario. I can intuitively see that in the described case, the smaller the diameter of the spool body, the larger the proportion of the ...
3
votes
1answer
234 views
Why do wheels appear to revolve opposite to the direction they are rotating?
When viewing cars that are driving along side of us, sometimes their wheels appear to be turning backwards even though they are traveling in the same direction as our car. Why do they look that way?
4
votes
1answer
207 views
How do I visualize the non-coaxial rotation of this device?
The picture below shows an isolated system with a fairly massive wheel at one end, attached via its axle to a long shaft, like a bike tire on a bike frame, but the bike frame is merely a low mass ...
2
votes
2answers
459 views
Meaning of angular velocity in a rotating system
When you study the motion of a rigid body you have $\vec\omega$, the vector associated to angular velocity. In the case you are using Euler angles and want a quick formula for the rotational kinetic ...
7
votes
3answers
339 views
Do the rings in Mass Effect's mass relays (2-axis gimbal) describe a stable rotation?
Just out of curiosity. In the game Mass Effect, devices called mass relays contain two rotating rings, one inside of the other. See http://www.youtube.com/watch?v=qPxw5QjxhIs for an example, best seen ...
1
vote
1answer
163 views
Determining axis of rotation from angular speeds about axes
I think my pure-math head is messing with me on the question below: my physics and CS friends both seemed to think it was a simple computational thing, and my program says the method works, but now ...
7
votes
1answer
336 views
Is this a quaternion representation of the equations of motion of General Relativity?
In The Quaternion Group and Modern Physics by P.R. Girard, the quaternion form of the general relativistic equation of motion is derived from
$du'/ds = (d a / d s ) u {a_c}^* + a u ( d {a_c}^* / ...
