A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.

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0answers
3k 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 ...
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5answers
7k views

Force applied to wheel in pure rolling motion at contact point with road [closed]

Suppose a wheel with radius $R$ is resting on a non-inclined surface. A torque $\tau$ is applied to the wheel center. In an attempt to prevent wheel from spinning, the ground applies a static friction ...
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3answers
2k 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 ...
2
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5answers
453 views

Is this expression for the kinetic energy of a spinning disk revolving about a second axis correct?

My question is motivated from a question from another user. You can see the configuration of the rotating system here: One disk/ring in double rotation and sum of energy. I am not interested in all ...
2
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4answers
172 views

Why does Torque exist?

Specifically, why does force increase with the moment arm? What about making a perpendicular force farther away from the axis of rotation increases it? How does the moment arm cause that?
2
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4answers
874 views

Why is moment of inertia dependent on $r^2$ and not on $r$ ? (physical reason)

Moment of inertia is the mass equivalent in rotational dynamics. I know , by mathematical arguments, moment of inertia of a particle is $$ I = \text{mass} \cdot r^2$$ . But what is the physical ...
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2answers
2k 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 $I$ ...
2
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3answers
142 views

How did planets start to spin and do all of them spin in same direction? [duplicate]

How did planets start to spin in the first place? Some say that gases which condense and form planets start spinning and hence planets acquire a spin. And some say that they start to spin to conserve ...
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2answers
315 views

Will the day/night cycle change when the poles melt?

I want to know what will happen to day and night if poles melt i think it will change but I don't know why? ^_^
2
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2answers
148 views

Conserved quantities in an angular system

Given the following question: A student initially stands on a circular platform that is free to rotate without friction about its center. The student jumps off tangentially, setting the platform ...
2
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1answer
518 views

During a turn, do the rear wheels necessarily trace out the same arcs as the front wheels?

When a vehicle makes a turn, the two front wheels trace out two arcs as shown in the figure below. The wheel facing towards the inside of the turn has a steering angle that is greater than that of the ...
2
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5answers
846 views

Forces acting on a point mass in a spinning rigid body

I have learned that all spinning objects will continue spinning even if no force is acting on it, and the tendency to do so is called moment of inertia. But I wonder about the fact that a single point ...
2
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1answer
135 views

Is it humanly possible to change Earth's axis?

Just what the title states with the qualification that the change must be affected without using other celestial bodies as mentioned in the Clarke/Baxter SF 'Sunstorm'. Obviously given the momentum ...
2
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3answers
1k views

Why do some satellites fall to Earth?

In another question How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?, one can read an affirmative answer. They how do you explain satellites ...
2
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2answers
44 views

Meaning of parallel axis theorem: why is the moment of inertia minimum if the axis passes through the CM?

From parallel axis theorem follows that, given a rigid body, the moment of inertia is minimum if calculated with respect to an axis passing through the center of mass. What is the physical meaning of ...
2
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2answers
100 views

Can torque relations actually be *derived* from Newton's Laws, or is it something extra?

For a long time I have wondered if there is a way to show that the rotational analogs of Newton's Laws are a direct consequence of just those laws, or are we adding more to them? I understand ...
2
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1answer
76 views

Why can we model a body to have a center of mass?

I accept that bodies have a center of mass. I also accept that if I were to suspend a body by only supporting it at its center of mass, then the body wouldn't rotate. In the scenario that we suspend a ...
2
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2answers
390 views

Why does locking the rear tires on a vehicle cause it to spin?

Handbrake Turns are often used in motor-sports and stunt driving to turn around tight corners. Essentially, the driver applies the handbrake as he enters the turn, in order to "kick out" the back end ...
2
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2answers
148 views

What motion can emerge in a still universe? [duplicate]

I consider a very large non homogeneous cloud of dust in isolation. No external gravitational interactions. It is initially completely still and start collapsing under the its own gravity. Can the ...
2
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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. ...
2
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2answers
680 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
4answers
620 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
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3answers
898 views

The rotating movement of an asteroid

I almost all movies where you could see an animation about an asteroid, they move in a very distinct way. I don't know how to explain better, but I think what we can see in the movies is that the ...
2
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3answers
677 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 ...
2
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3answers
193 views

Conservation of angular momentum in broken pendulum

I'm having troubles in understanding the correct way to use the conservation of angular momentum. Consider the pendulum in the picture with a neglegible mass rope and a slender rode of mass M and ...
2
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1answer
142 views

Change in acceleration due to gravity because of rotation of earth [duplicate]

The formula above is the equation for acceleration due to gravity when earth rotates. G is the original acceleration. Can someone explain how this formula came?
2
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2answers
544 views

In which direction does mud fly off a moving bike's tire & why?

If a bike moves through a muddy area, mud gets on its tires. Then the mud flies off from the tires. Which forces are acting on it? In which direction does it fly off? On my physics test, I wrote ...
2
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2answers
31 views

A hypothetical question on mechanics

Being located in a tropical region, I am quite acquainted with the Ceiling fan. I have a question about it. If the top, that is, the axle (I'm not sure of the terminology: I mean the part which is ...
2
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2answers
114 views

Could galactic rotation be similar to an irrotational vortex?

Could black holes' near light speed rotation cause galaxies to move like an irrotational vortices?
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4answers
686 views

I find a problem in the law of angular momentum conservation

Consider a system of spring mass as shown in the figure.entire loop is free but only one nail is there at the point A... Initially the mass is at rest and then released. Assume that the spring is ...
2
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2answers
1k views

Angular momentum of a translating and rotating body

If a rod is rotating about one end, does it have pure rotation or do you also consider the translation of centre of mass when calculating its angular momentum? Also, how would one calculate the ...
2
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2answers
344 views

How does the curve ball drag air around it?

In cricket or baseball there is a type of ball called the curve ball. This is the top spin of the ball.I read that due to spin the ball drags the air around it due to friction in the way shown ...
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3answers
3k views

Moment of inertia of a planet

Is there a good way to directly measure a moment of inertia of the Earth, or say, other planet?
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1answer
6k 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
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1answer
315 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 ...
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4answers
4k 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 ...
2
votes
1answer
659 views

How to know the time a disc takes to stop from rotating and the numbers of revolutions

Assuming I rotate a disk, I want to know how long it takes to completely stop, and the number of revolutions it made since I removed my fingers off the disk. Lets say a DVD I rotate with my fingers. ...
2
votes
1answer
181 views

When will the Earth stop rotating?

I looked up leap second in Wikipedia. It is a second added (usually) to clocks to keep them in sync with the atomic clock. It said that 25 leap seconds were added in the last 43 years and that none ...
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2answers
86 views

Why does the period/frequency of a fan slow down significantly when I taped a piece of rubber band to it?

All of this was done with a standing fan set horizontally on a table. During an experiment, I had to tape a piece of rubber band to one of the standing fan's blade and measured the period of the fan. ...
2
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2answers
118 views

Clarify excerpt from Feynman Lectures on rotations in three dimensions

In The Feynman Lectures, vol. I, chapter 20 Feynman discusses rotations in three dimensions and explains angular velocities may be added as vectors; in particular he says: What about angular ...
2
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1answer
757 views

If I bend a rod, will its moment of inertia change?

In the first picture, there is a homogeneous metal rod of length $2L$ and mass $M$. If it rotates around a normal axis passing by $O$ (which is the center of gravity), then its moment of inertia is: ...
2
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1answer
220 views

Velocity of hitting the ground when falling forward

A friend of mine fell while he was snowboarding which led to a discussion. He mentioned that, in general, if your center of mass is traveling at a certain velocity, and you fall forward, your head ...
2
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3answers
526 views

Ideally speaking, will a rolling disk with no slipping come to a stop because of friction from the ground?

Consider a rotating disk on a horizontal plane with static friction. The contact point of the disk with the plane has null instantaneous velocity. Assuming the center of the disk has fixed $v_0$ ...
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3answers
5k views

Cylinder rolling down an inclined plane

I've been trying to solve this problem: A cylinder is rolling down an inclined plane (angle between plane and horizon α). Coefficient of friction is µ. What is the translational and angular speed of ...
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4answers
2k views

rope wrapped around a pole

I would like to solve this question without using conservation of angular momentum(because of some reason I'll elaborate later). So imagine that we have a pole with radius $r$ and a ball attached to ...
2
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1answer
8k 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. ...
2
votes
1answer
206 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 ...
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2answers
2k views

Moment of inertia of a coin [closed]

I have a a coin infinitely thin, rotating along the diameter. How to derive the formula for it's moment of inertia passing through the diameter. I was suggested to use the surface density and ...
2
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1answer
104 views

Synchronising the Earth's rotation via mass redistribution

How much material would have to be moved per year from mountain-tops to valleys in order to keep the Earth's rotation synchronised with UTC, thus removing the need for leap seconds to be periodically ...
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2answers
370 views

Why do balls in a spinning ellipsoid move to the minor axis plane?

There is a question concerning the Physics of a small child's tall that has been bothering me for some time now. I have investigated this to a small degree, but I have not been able to find a ...