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

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0
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1answer
65 views

Will macroscopic object change its angular velocity after absorbing electron?

Consider macroscopic object (a ball for instance) which has angular momentum equals 0. Now single electron hits the ball and is absorbed by it. Let's assume it hits in direction perpendicular to ...
5
votes
2answers
369 views

How to derive relation for time derivative in a rotating reference frame

I am looking for an appropriate derivation of the $(\frac{d}{dt})_{\text{laboratory}} = (\frac{d}{dt})_{\text{rotating}} + \omega \times $ relationship that enables one to calculate all desired ...
0
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0answers
59 views

Gravitational Behavior of Rotating Body

Consider a stick with a (ball-shaped) weight at the end of it. Let's say this stick is in a state of rotation (it's floating in vacuum) and is also in a state of translational motion. Consider a ...
3
votes
3answers
276 views

Is a spinning object heavier than a non-spinning object?

Does spinning an object make it heavier? A real-world example: I was mowing the lawn in front of my house, a lawn that tends to have some steep inclines. I realized that the lawn-mower was easier ...
3
votes
5answers
1k views

Veritasium - Firing bullet in block - along center and away from center

This question is about this video on YouTube, in which a bullet is fired vertically into the center of a wooden block from below, sending the block up into the air. Next, a bullet is fired vertically ...
9
votes
3answers
515 views

How fast do you have to spin an egg to have it standing?

If you take a hardboiled egg and put it on a table and start to spin it, if you spin it fast enough it will start to spin in an upright position. What is the angular velocity needed for this ...
1
vote
2answers
370 views

Extracting acceleration vector from rotated aircraft

Suppose we have an aircraft with accelerometer measuring accelerations along each axis. It is mounted in a way so it is perpendicular to the plane in all axes (that should be obvious). We also have ...
0
votes
1answer
292 views

Why does a larger sprocket need more power to accelerate at the same rate?

Why does a large drive sprocket require more power to accelerate at the same rate as a smaller drive sprocket if the overall gear ratio is the same (with all other components such as the chain the ...
-1
votes
3answers
347 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?
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3answers
1k views

Time period of torsion oscillation

For the oscillation of a torsion pendulum (a mechanical motion), the time period is given by $T=2\pi\sqrt{\frac{I}{C}}$ which is a result of the angular acceleration ...
2
votes
0answers
37 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
621 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. ...
1
vote
1answer
140 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, ...
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votes
2answers
3k views

DC Motor Torque Constant

I am very new to DC motors and to stackexchange. Please correct me if anything I said does not make sense. For DC motors, the equation looks like this: $P = \tau\dot{\theta}$ where $P$ is power, ...
-1
votes
2answers
1k 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 ...
3
votes
1answer
315 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 ?
0
votes
0answers
33 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 ...
3
votes
3answers
2k 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, ...
2
votes
4answers
485 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 ...
-1
votes
1answer
254 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 ...
20
votes
3answers
716 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. ...
0
votes
1answer
79 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
237 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 ...
11
votes
2answers
1k 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 ...
2
votes
3answers
1k 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 ...
0
votes
1answer
72 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?
13
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3answers
1k views

Why does each celestial object spin on its own axis?

AFAIK all the celestial objects have a spin motion around its axis. What is the reason for this? If it must rotate by some theory, what decides it's direction and speed of rotation? Is there any ...
24
votes
4answers
1k views

Why does everything spin?

The origin of spin is some what a puzzle to me, everything spin from galaxies to planets to weather to electrons. Where has all the angular momentum come from? Why is it so natural? I was also ...
2
votes
1answer
358 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 ...
2
votes
0answers
99 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
127 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
87 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 ...
1
vote
1answer
639 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 ...
0
votes
1answer
678 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 ...
1
vote
1answer
376 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 ...
2
votes
2answers
176 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 = ...
0
votes
0answers
15 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 ...
14
votes
1answer
316 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 ...
0
votes
0answers
44 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 ?
1
vote
1answer
164 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
59 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) ...
5
votes
1answer
518 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
3answers
3k 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
82 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
438 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 ...
47
votes
8answers
10k 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 ...
4
votes
2answers
197 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 ...
0
votes
0answers
185 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. ...
2
votes
2answers
287 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 ...
8
votes
2answers
133 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. ...