A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.
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3answers
201 views
Does the Earth spin because its surface has less mass than its core as lighter things travel faster in space?
Would heavy elements from space hitting the Earth slow down the earth's rotation as the difference between the masses of the core and surface would be less?
2
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0answers
12 views
Computing Latitude Given Quiescent Gyroscope Data
Suppose I place a gyroscope in a theoretically perfectly quiescent, closed room. Let its output be given as a vector ${\bf v} = (v_x, v_y, v_z)$ indicating rate of rotation around three orthogonal ...
1
vote
1answer
44 views
Torque on a Box
I think I'm missing something with torques. I seem to have gotten myself confused.
I have a box that's centered at ( 0 , 0 , 0 ) with length ( $x$ dimension ) = 1 , width ( $y$ dimension ) = 0.25, ...
0
votes
1answer
37 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.
...
0
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1answer
34 views
Conservation of Angular Momentum, as related to a flywheel
Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts.
For a given flywheel (thin-walled ...
0
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0answers
31 views
Why do you spin slower when you extend yourself horizontally? [duplicate]
When you are ice-skating, or spinning in an office chair, or freely spinning at all, why do you spin slower if you move parts of your body away from the centre, and relatively quicker when you have ...
0
votes
1answer
95 views
how does international space station change its orientation?
In what way does the rotating gyroscope help in changing the orientation of the international space station in space ?
Is there any other way to change the the orientation of ISS ?
3
votes
3answers
163 views
Relationship between torque and angular momentum
I want to know how to deduce the equation $\vec{\tau}=\vec{\omega} \times \vec{L}$, where
$\vec{\tau}$ is the moment of force (also known as torque),
$\vec{L}$ is the angular momentum,
...
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votes
1answer
189 views
A Rolling Quarter [closed]
A U.S quarter is rolling on the floor without slipping in such a way that it describes a circular path of radius $R=4 \text{cm}$. The plane of the coin is tilted at an angle of $\theta=45^{∘}$ with ...
0
votes
1answer
42 views
Conservation of energy of a rotating body [duplicate]
The famous example of acrobats shrinking their bodies to increase their rotation speed is well known. Where does the energy to increase the speed of their rotation comes from?
1
vote
1answer
66 views
What is the friction between cylinder and wall (ground)?
A hollow cylinder (radius $R$) is rolling against the wall at angular speed $\omega$. The coefficient of friction between the cylinder and the wall(ground) is $\mu$. After how many rotations the ...
1
vote
3answers
103 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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votes
2answers
119 views
Center of a mass of a hemisphere [closed]
How can I show that position vector of the center of a mass of a hemisphere is $(0,0,\frac{3a}{8})$ where $a$ is radius of a hemisphere, $x$ and $y$ axis are laying on the base and $z$-axis is ...
0
votes
1answer
55 views
Why do cosmic bodies revolve? [duplicate]
Why do cosmic bodies such as planets, stars, satellites revolve?
What made them to revolve after the formation of universe?
20
votes
3answers
460 views
A rope attaches the Moon to the Earth. What happens?
Consider the Earth (mass $M$, radius $R$, rotating about its own axis at $\Omega$) and the moon (mass $m$, radius $r$, with axial rotation equal to $\omega_m$), whose centre of masses are $d$ apart. ...
2
votes
1answer
84 views
Does the Magnitude of the Drag Coefficient on a Rectangular Prism vary with Rotation?
I have a question about the drag coefficient in the drag equation.
Let's say I have a rectangular prism oriented such that, looking down on it, the long side is parallel to the y-axis. Moving forward ...
1
vote
0answers
38 views
Angle of rotation of an ellipsoid in a linear shear flow field
I am modeling the motion of an ellipsoid in a linear shear flow field. The ellipsoid is rotating about its shortest semi-principal axis which I have designated the $z$-axis in the body-fixed frame, ...
2
votes
1answer
81 views
Mechanics of a rolling drum
I have no clue on how to approach this. The professor only discussed centripetal acceleration and angular velocity (As in $2πr\over T$ $= ωr$). Does the acceleration along the axis of the drum act ...
1
vote
0answers
55 views
Why does angular velocity lies in the axis passing through the center of the circumference?
I understand that it can't be placed anywhere on the radius because it doesn't vary with it ( and so of course it doens't make sense to place it anywhere else on the plane), but why do we place it ...
6
votes
0answers
80 views
rotational oblateness
I am trying to compute the amount of oblateness that is caused by planetary rotation. I picture the force of gravity added to the centrifugal force caused by the rotation of the planet as follows:
...
3
votes
2answers
72 views
Direction of angular velocity
Angular velocity is the rate of angular displacement about an axis. Its direction is determined by right hand rule.
According to right hand rule, if you hold the axis with your right hand and rotate ...
1
vote
1answer
39 views
Component of angular velocity along an axis inclined at $\theta$
If an arbitrary rigid body rotates with angular velocity $\omega_0$ about some axis, can it be said that the body will rotate with an angular velocity $\omega_0 \cos(\theta)$ about an axis which is at ...
0
votes
0answers
12 views
a spinning disk asymmetrically weighted [duplicate]
When I was browsing through YouTube I found this vidéo
http://www.youtube.com/watch?v=h0SZZTBQmEs
Can any body explain what causes this.
I know it's not friction so I need an answer that is deep in ...
2
votes
4answers
352 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 ...
14
votes
1answer
244 views
What is the evidence for the super-rotation of Earth's inner core?
What are the geophysical observations that support (or contradict) the hypothesis that the Earth's inner core rotates at a faster rate than the Earth's mantle?
Summary of Answers:
1) Studies of ...
3
votes
0answers
80 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
...
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0answers
36 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 ?
11
votes
2answers
324 views
Stability of rotation of a rectangular prism
I've noticed something curious about the rotation of a rectangular prism. If I take a box with height $\neq$ width $\neq$ depth and flip it into the air around different axes of rotation, some motions ...
1
vote
1answer
59 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
38 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
165 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
31 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
96 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
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0answers
60 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.
...
0
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1answer
53 views
Kinetic rotational energy of a bar hooked to a coil
I have solved an exercise and I'd like to know if my proceeding about finding kinetic energy is correct or not, because this is the first time that I "meet" a situation like this.
"A bar has mass $M$ ...
3
votes
2answers
175 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 ...
46
votes
8answers
6k 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
74 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 ...
7
votes
2answers
101 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. ...
8
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2answers
295 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?
2
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2answers
87 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
75 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
49 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
98 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
365 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
371 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
174 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
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1answer
128 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
96 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 ...
9
votes
0answers
223 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 ...
