A tag for questions about the mechanical interactions of rotating objects, including torque and angular momentum.
3
votes
2answers
1k views
How to calculate cord tension in a vertical circle?
Mass m is connected to the end of a cord at length R above its rotational axis (the axis is parallel to the horizon, the position of the mass is perpendicular to the horizon). It is given an initial ...
0
votes
0answers
247 views
How does the Commutator and Brushes causes flip? [closed]
I'm trying to make a simple electric motor just for fun project. I've reading this article to understand how an electric motor works. So far, I have been able to make something like as seen in picture ...
2
votes
3answers
413 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 ...
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
3answers
138 views
Effect of surface treatment on fair dice
If I have a perfectly balanced and thus fair cubic die, then polish 3 adjacent faces (so that their coefficient of friction is effectively zero) and roughen the remaining faces (so that their ...
1
vote
2answers
380 views
In a circular pendulum, where does the equation $v=\sqrt{rg\tan{\alpha}}$ come from?
In a circular pendulum
the $v$ of the particle is $$v=\sqrt{gr\tan{\theta}}$$ where $r$ is the radius and $g$ is the gravity(positive sign), which is equal to ...
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 ...
0
votes
1answer
87 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 ...
0
votes
2answers
492 views
How to interpret this vertical circular motion problem?
A bucket of water is tied to a rope and swung in a vertical circle.
The distance from the bucket centre to the axis of rotation is
$2.08m$. Calculate the angular velocity (in $rad s^{-1}$) of ...
1
vote
1answer
263 views
What techniques can be used to analyze a rod rotating about the edge of a table?
A uniform rod of length $4x$ is rotating
about the edge $O$ of the table. (The rod does not fall off the table.) The centre of mass $G$ of the rod is distance $x$ away from $O$. The rod is making ...
0
votes
2answers
1k views
Rotational kinetic energy during vertical circular motion of a particle
Why is it not necessary to take into account rotational kinetic energy when using the Law of Conservation of Mechanical Energy to solve vertical circular motion problems? After all, the particle is ...
1
vote
2answers
179 views
Problem based on Rotational Motion #2
One fourth length of a Uniform Rod of mass $m$ and length $x$ is placed on rough horizontal surface and it is held stationary by means of a light thread as shown in the figure. The thread is brunt ...
2
votes
1answer
417 views
Normal force in a compound pendulum (physical pundulum) system?
Consider a compound pendulum pivoted about a fixed horizontal axis, illustrated by the force diagram on the right:
#
Okay, I can't figure out where the normal force on the pendlum should point ...
2
votes
2answers
203 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 ...
1
vote
3answers
652 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 ...
0
votes
1answer
128 views
Relationship between the “angle of the floor” and the angular velocity in a banked turn?
Wel, imagine that you're in a carousel, and the floor is, let's say at $\theta=0$ so it's totally horizontal, if $\theta=90$ the floor would be vertically.
The object put above the floordoesn't move ...
-3
votes
2answers
189 views
Which Axis of rotation?
Let us consider a body , A, of arbitrary geometrical shape of mass m1. Now let it be translating with a speed v. Let us consider a small point particle ,P, of mass m2. Suppose it strikes A with a ...
-1
votes
1answer
70 views
A mechanics question of finding maximum K.E [closed]
The question is in the diagram as well the diagram
1
vote
3answers
525 views
Work Done by Rockets in Orbital Motion
A weather satellite ($m_s = 4350$ kg) is in a stable circular orbit around the Earth ($m_E = 5.97 \cdot 10^{24}$ kg). It completes an orbit once every 2 and a half hours.
(I'm sure about these 2 ...
2
votes
0answers
234 views
Why does a coin falls faster when it's flipping as well?
From my experiments with measuring how fast a coin falls, I have consistently measured a faster falling rate for a coin that flips as it falls.
As an example, a coin dropping on its edge from height ...
4
votes
1answer
188 views
Does the Moon's orbital inclination follow the wobble in the Earth's rotation?
As the Earth wobbles during rotation, does the higher gravity at the equator tend to pull the moon toward an equatorial orbit even as the earth does that thousands of years wobble cycle? It would ...
3
votes
0answers
73 views
Videos of changing the orientation of an astronaut in space
Kane, Headrick and Yatteau describe in their paper "Experimental investigation of an astronaut maneuvering scheme" possible maneuvers to change the orientation in space without external torque. Is ...
1
vote
2answers
408 views
Moment of inertia of a coin
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 ...
0
votes
1answer
324 views
Calculation for force generated by a rotating rectangular blade
When trying to calculate the lift force generated by a simple rectangular blade, I've found the following equation: $$F = \omega^2 L^2 l\rho\sin^2\phi$$ in which $\omega$ is the angular velocity, $L$ ...
1
vote
2answers
438 views
Altering Venus rotational speed to match Earth's via weather manipulation
Venus rotates approximately 6.5 km an hour Earth rotates approximately 1650 km/h how fast could we speed up Venus's rotation via only weather manipulation ( maybe a giant fractal lens between Venus ...
1
vote
3answers
2k views
Factors affecting torque and RPM of a motor
I'm not a physics guy, not even basic concept of a DC motor is easy for me. My question is, how these parts of a motor affects it's RPM and Torque? I had my research a while ago so I filled out some, ...
2
votes
0answers
543 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 ...
-1
votes
1answer
95 views
How to find replacement function of a mass?
I wonder how I can find the replacement function of the center of the blue mass? The center of mass of the blue mass is $(0,0)$ and the blue mass is homogeneous. The masses do not move at t=0 in the ...
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votes
1answer
520 views
Moment of inertia of a sector about its center point?
What is the moment of inertia of a pizza slice that has a radius r, an angle (radians) of theta, and a height of h about the center point perpendicular to the cheese plane?
0
votes
1answer
159 views
What causes a gyroscope to eventually rotate/fall over?
Hey so I've just learned about angular velocity and momentum and how torque changes it.
Looking at a wheel spinning around an axis, with one end being held up by a rope, what causes the wheel to ...
1
vote
4answers
622 views
17 Joules of Energy From a Mouse Trap
Do you think it would be possible to get 17 joules out of a standard size mouse trap. By my math, it is a torsion coefficient of 3.45 or so out of the spring.
1
vote
0answers
238 views
How equivalent are heat energy and work energy in connection with a spinning flywheel?
Let's say we have two identical spinning flywheels, that have arbitrary geometry, and are made of copper.
Now we apply some heat energy at the center point of flywheel A, causing it to slow down a ...
4
votes
5answers
452 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?
0
votes
1answer
178 views
Angular velocity $\omega$ by $v$
We have two girls, with mass (M). They become close to each other in speed of V. The distance between them is 3L.
I was asked to calclute the Angular velocity (w) of the two girls. So I set the ...
2
votes
1answer
85 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 ...
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
3answers
223 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 ...
1
vote
1answer
225 views
Problem based on Rotational Motion [closed]
A spool of mass $\mathsf m$ and inner radius $\mathsf r$ and outer radius $\mathsf{2r}$, having moment of inertia $\Large\mathsf{\frac{mr^2}{2}}$ is made to roll without sliding on a rough ...
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
0answers
329 views
How to design a hand rotate dynamo [closed]
I want to make a dynamo which can produce electricity. My requirements for the dynamo are as follows.
I need to charge my laptop through this.
I want my laptop to run for 1hour with this energy ...
2
votes
1answer
106 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 ...
1
vote
2answers
214 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 ...
2
votes
1answer
187 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 $\frac{mL^2}{3}$:
...
2
votes
0answers
194 views
Levitation rotation speed involving laser acceleration, pyrolytic graphite and a vacuum
The experment would involve a small NIB magnet levitating between or on the diamagnetic material pyrolytic graphite, unlike other forms of levitation this doesn't require power to run such as ...
2
votes
1answer
180 views
What “I” should use in Rotational Energy formula $(I \omega^2)/2$
$\text{Rotational Energy} = \frac{1}{2} I \omega^2$. What $I$ should be used?
$I$ as a inertia tensor matrix = stepRotation * inverse moment of inertia * inverse stepRotation;
Or I as moment of ...
1
vote
0answers
184 views
How to model an accelerometer measurements on a car wheel?
I am working on kinematically modelling an accelerometer on a car wheel.
When working on the initial conditions, I am confused whether or not I should use the gravitational acceleration since there ...
19
votes
1answer
988 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 ...
2
votes
3answers
494 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 ...
0
votes
0answers
79 views
How long will the bar to travel 1.50 cm threaded rod? [closed]
A threaded rod with 12.0 turns / cm and 1.18 cm in diameter is mounted horizontally. I screwed a threaded rod with a hole to attach to the bar. The bar rotates at 237 revolutions / min. How long will ...
5
votes
1answer
855 views
Why are infinitesimal rotations commutative, whereas finite rotations are not?
Infinitesimal rotations commute and every finite rotation is the composition of infinitesimal rotations which should logically mean they also commute; but they don't. Why?
