18
votes
Accepted
Do wind turbines spin at constant speed? If so, how?
While a large turbine will try to hold rotation constant by driving the pitch of the blades, this is not able to completely compensate for gusts and other rapid changes in speed. If such a system ...
- 41.8k
6
votes
Accepted
Why is the velocity of wheel double at the end than in the center?
When running laps on an elliptical running track as in the Olympic athelic disciplines, then imagine that you take the outermost path on the running track and that a friend of yours takes the ...
- 51.7k
3
votes
Why does a rolling ball slow down?
The ball slows down due to rolling resistance. That resistance equals the normal force times the coefficient of rolling resistance.
Rolling resistance occurs due to inelastic deformation of the ball ...
- 75.6k
3
votes
Why is the work done by a force on a rigid body zero if the displacement of point of application of force is zero?
The work done by a force refers to the displacement vector of the application point of that force: you cannot arbitrarily change it. So it is wrong to use the displacement vector of the center of mass....
- 75.5k
3
votes
If a massive sphere spins faster and faster, will a black hole appear?
Wikipedia gives the total gravitational mass-energy sum of a Kerr black hole as:
$$M_{total} = \sqrt{M_{\text{irr}}^2 + \frac{J^2c^2}{4M_{\text{irr}}^2G^2}},$$
where $M_{total}$ is the equivalent ...
- 7,400
2
votes
How does the Earth's orbit change as the Sun decreases in mass?
The best way to analyse this problem is consider the conservation of angular momentum for circular orbits. For a body of mass $m$ with speed $v$ and orbital radius $r$, the angular momentum is
$$ H = ...
- 1,724
2
votes
How to calculate linear momentum from angular momentum? (Newtonian)
To cut to the chase: the answer is that no, your equation is not correct.
To justify this answer, let me first rephrase what I think you're trying to do in different notation, that I find clearer.
You ...
- 52.1k
2
votes
The No Slip/Slip Condition for Rotating/Rotating and Translating Bodies
Is it correct to say that if $v≠ωr$, slip will occur
yes
where if $v<ωr$ the sphere will rotate "too fast"
yes
where if $v>ωr$ the sphere will slip?
yes
Does the direction of ...
- 9,856
2
votes
Why is the direction of friction inwards in a rotating disc even though there is no motion in the inward direction?
However, previously I had learnt that friction acts in a direction
opposite to the direction of motion
That applies to kinetic friction which acts opposite to relative motion that is underway between ...
- 75.6k
2
votes
Accepted
Rotating disc and current?
Preamble
The first text I posted was wrong.
There was a miscalculation and I forgot to include the electric charge at the rim around the disc in my calculations.
I apologise for that. A corrected ...
- 980
2
votes
Rotating disc and current?
Statements regarding emf and current like this one, $\dots$ it seems that there is no current even when an emf is generated (however small, a current must be present), are incorrect.
For example, a ...
- 98.6k
2
votes
Rotating disc and current?
This goes to a more general question: what constitutes presence of an electric current?
In cases like this it is often helpful to consider a parallel case.
Here is what I propose:
Create a setup with ...
- 22.1k
2
votes
Simulate the dynamics of a 3D discrete rigid body from "first principles"
You can do it, and it will be well defined. The key would be adding the constraints for a rigid body: the distances between masses never changes.
That being said, what you are doing is really ...
- 50.5k
2
votes
Why does a rolling ball slow down?
When a ball or other object is sitting still, the normal force does pass through the center of mass. You can see this must be so from your diagram. If it did not, there would be a torque and the ball ...
- 42.5k
2
votes
Physical Significance of Torque
If torque about O is non zero,there is rotational effect of Force and hence the body is rotating about O.
Maybe, maybe not. We can say that the angular momentum about that point is changing. ...
- 41.8k
2
votes
Changing the magnetic field of earth
In 2022, the total energy consumption of humanity was estimated at 24,398 terawatt-hours (about 24.398 terawatts continuously for the whole year), while the Earth's magnetic field's total energy is ...
- 1,409
1
vote
How to calculate tension in this case?
Your example is statically indeterminate, meaning one of the strings is redundant (i.e. not required for the bar to be in equilibrium).
This means the equations for static equilibrium alone are ...
- 75.6k
1
vote
How to calculate tension in this case?
If your three strings are non-parallel you should be able to solve for the forces independently. But if they are all vertical or there are many more, then then you have discovered a topic called &...
- 737
1
vote
Simulate the dynamics of a 3D discrete rigid body from "first principles"
As stated in @CortAmmon's answer, the problem can be resolved as you suggest, provided the correct number of independent rigidity constraints is added to the external forces.
I would not be negative ...
1
vote
The No Slip/Slip Condition for Rotating/Rotating and Translating Bodies
$\dots$ can this be extended to an accelerating system by taking the derivative with respect to time on both sides $\dots$ - yes
In the first three examples on the left, the friction force to the ...
- 98.6k
1
vote
Accepted
Why does a rolling ball slow down?
If the ball rolls without slipping while slowing down, there must be:
a net torque acting to reduce its rotation rate ($\tau = I \frac{d \omega}{d t}$) and
a net force acting to reduce its velocity ($...
- 1,724
1
vote
Why does a rolling ball slow down?
See in the figure center of mass has nothing to do with it, since this ball will slow down due to the rotational resistance and frictional forces applying on it (F) (I feel sorry for not putting a ...
1
vote
How to calculate the resulting force?
Edit: As @naturallyInconsistent pointed out in the comments I made a mistake in my first answer. The solution is actually very unintuitive. Here is the correct way to do the calculation.
I am going to ...
- 7,400
1
vote
Accepted
Why is the direction of friction inwards in a rotating disc even though there is no motion in the inward direction?
Since you confused about relative motion between the thing ($m$ at $R$) on the disc (spinning at $\omega$), let's solve it in a rotating frame so that every thing is at rest $(v=0)$, it's a statics ...
- 36.6k
1
vote
Why is the direction of friction inwards in a rotating disc even though there is no motion in the inward direction?
Friction acts opposite of the direction of relative motion if friction wasn't there.
In the absence of friction an object placed on a rotating disk (carefully to match the velocity of the object on ...
- 3,220
1
vote
Downward Acceleration of a Falling Chimney's Tip
The radial acceleration depends on the angular velocity $\omega$ by $a_r=r\omega^2$. Therefore, when the rod is first released from rest, $a_r=0$. The analysis is considering the instant in time when ...
- 58.1k
1
vote
Anthropogenic relocation of Earth's mass
When a large artificial reservoir in Canada began to fill with water some years ago, the redistribution of that mass was sufficient to alter the earth's rotation by an extremely tiny, but measurable ...
- 96.6k
1
vote
How to calculate linear momentum from angular momentum? (Newtonian)
It's better to try to coordinate free physics when dealing with a would-be fundamental relation. As we all know: physics doesn't care about our coordinates, so we should not need them.
The idea ...
- 36.6k
1
vote
Why is the work done by a force on a rigid body zero if the displacement of point of application of force is zero?
Why is the work done by a force on a rigid body is zero if the
displacement of point of application of force is zero?
Because in order for the force to do work on a body, the source of that force ...
- 75.6k
1
vote
Accepted
2D elastic collision between flat disk and rectangular block
And I don't really understand how angular momentum and energy can be
conserved in this system as it seems that they must be zero before the
collision and non-zero after!! Can anyone help??
The key ...
- 7,400
Only top scored, non community-wiki answers of a minimum length are eligible