A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.
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 ...
15
votes
8answers
4k views
Why don't spinning tops fall over?
One topic which was covered in university, but which I never understood, is how a spinning top "magically" resists the force of gravity. The conservation of energy explanations make sense, but I don't ...
4
votes
4answers
3k views
Why does a ballerina speed up when she pulls in her arms?
My friend thinks it's because she has less air resistance but I'm not sure.
10
votes
6answers
3k views
How do you explain spinning tops to a nine year old?
Why don't spinning tops fall over? (The young scientist version)
My nine year old son asked me this very question when playing with his "Battle Strikers" set. Having studied Physics myself, I am very ...
7
votes
2answers
2k views
Why does Venus rotate the opposite direction as other planets?
Given: Law of Conservation of Angular Momentum.
Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based).
Surface same degree of aging all over.
Theoretical large ...
7
votes
5answers
2k views
What is the electric field generated by a spinning magnet?
Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
21
votes
1answer
1k views
Why does this object periodically turn itself?
See this video about 30 sec in. http://www.youtube.com/watch?v=dL6Pt1O_gSE
Is this a real effect? Why does it seem to turn periodically?
Can it be explained by classical mechanics alone?
Is there a ...
8
votes
1answer
293 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, ...
5
votes
1answer
270 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?
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 ...
11
votes
5answers
615 views
Why does the weighing balance restore when tilted and released
I'm talking about a Weighing Balance shown in the figure:
Press & Hold on onside of the horizontal beam and then release it. It makes some oscillations and comes back to equilibrium like shown ...
2
votes
3answers
22k 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
3answers
660 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 ...
1
vote
1answer
1k views
Would a light or a heavy ball roll fastest down a slope?
A small, light ball and a larger, heavier ball are released from the top of a slope.
Which will move further? which will come down faster?
0
votes
2answers
252 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. ...
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 ...
32
votes
10answers
4k views
Why does dust stick to rotating fan propeller?
Why does dust stick to rotating fan propeller?
Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers.
EDIT 1:
Thank you for the great explanations. I am ...
6
votes
7answers
1k views
Why does a car engine not do work if the wheels don't slip?
I saw this mind boggling result that if the tires don't slip then the work done by an engine to move a car is zero. Why is this true? Moreover, what does this truly mean?
Update: Sorry about not ...
2
votes
0answers
555 views
Forces and torques about the CENTER OF MASS of a physical pendulum
I'm currently stumped by the following situation. Say we've got a rectangular physical pendulum (think ruler with a hole-punch at one end).
It's trivial to analyze the motion of the pendulum with the ...
7
votes
2answers
225 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?
4
votes
1answer
151 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
5answers
257 views
Does the rotational speed of a planet consistently become faster and faster given that there are no conflicting events? [closed]
Does the rotational speed of a planet consistently become faster and faster given that there are no conflicting events?
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 ...
2
votes
2answers
214 views
What sustains a rigid body's rotation at its constant angular(rotational) speed?
Continuing from the following scenario from my previous question Centripetal force of a rotating rigid body? :
Consider someone pushing a roundabout in a playground. Initially the
roundabout is ...
2
votes
3answers
230 views
how to represent the effect of linking rigid-bodies together?
I have 2 rigid-bodies (b1,b2) if i linked one to the other (as if they are conjoined together) , how to represent b1 effect on b2 and b2 effect on b1
Is there any LAW that affect the ...
2
votes
3answers
479 views
How long for a frictionless top to fall over?
We've previously discussed why it is that spinning tops do not fall over, see: Why don't spinning tops fall over?
However, as the highest rated answer notes, the angular momentum of the spinning top ...
1
vote
2answers
61 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 = ...
1
vote
3answers
682 views
Centripetal force of a rotating rigid body?
Consider someone pushing a roundabout in a playground. Initially the
roundabout is stationary, but when it is pushed, it rotates with
increasing rotational speed.
The force of the push is ...
1
vote
1answer
221 views
Is this simulation following real physics?
I am trying to simulate a game in Box2D(Physics engine). The game that I am trying to simulate is very simple and can be found here: http://www.makaimedia.com/#/speartoss
What I want to know is that, ...
-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 ...
-1
votes
1answer
71 views
A mechanics question of finding maximum K.E [closed]
The question is in the diagram as well the diagram
