Tagged Questions
1
vote
1answer
55 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?
...
0
votes
1answer
76 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
1answer
112 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
41 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 ...
4
votes
2answers
91 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
183 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, ...
2
votes
2answers
424 views
What causes precession or nutation in a spinning object?
What causes precession in a spinning object?
What causes nutation in a spinning object?
What causes a top, gyroscope, and the earth to wobble?
Just because it's a simple question, I'm not ...
0
votes
0answers
65 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 ...
1
vote
3answers
567 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
101 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
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 ...
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
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 ...
2
votes
3answers
562 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
1answer
85 views
Period of an Object in Periodic Motion
My attempt (if it matters):
The initial period is given by $T_X = \frac{2\pi X}{v}$ for some $v$.
The new period is given by $T_Y = \frac{2\pi Y}{v}$ for the same $v$.
$Y = \frac{X}{2}$, so ...
6
votes
3answers
852 views
Conservation of angular momentum for a rigid body rotating about a fixed point
Picture a rigid body such as a sledge hammer. Imagine that the base of the handle is attached to a fixed point such that it can rotate but not translate. I give the hammer a good push to get it ...
4
votes
5answers
451 views
What happens to angular momentum when matter is converted to energy?
Let's say a spinning star radiates mass-energy only from it's pole regions. How does the loss of mass-energy effect the angular momentum of the star?
2
votes
0answers
147 views
Why do control moment gyroscopes exhibit “torque amplification”?
There are a number of articles that describe the benefits of using control moment gyroscopes (CMGs) over reaction wheels in inertial navigation applications. One of the primary benefits of using a CMG ...
2
votes
1answer
186 views
What happens to a rotating rod that breaks in two?
I know that the approximation for the moment of inertia of an infinitely thin rod of mass $m$ and length $L$ spinning around an axis perpendicular to its own axis at its center is $\frac{mL^2}{3}$:
...
5
votes
3answers
399 views
confused about ball rolling and rotational energy
I was solving a practice test problem and it was just a conservation of energy problem
where a spherical ball is falls from a height h to the ground such that
$$mgh = \frac{1}{2}mv^2 + ...
3
votes
4answers
300 views
N particles, will there be any rotation after a period of time or everything will collapse
This is in context of classical Newtonian physics.
Consider a system of n different point mass particles. Initially all are spread around on one plane. No particle possess any velocity to begin with. ...
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 ...
2
votes
3answers
470 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 ...
3
votes
3answers
796 views
Could life survive a pole shift caused by an asteroid collision?
Could life on earth survive a large pole shift caused by an asteroid collision?
I became aware that there are people who believe that the earth's pole suddenly shifts. That is, its rotational ...
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.
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 ...
