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90 votes
Accepted

Why don't helicopters use reaction wheels to counter the main rotor?

You're talking about a device (in helicopters the tail fan imparting horizontal thrust) that counteracts the torque imparted on the main rotor (and therefore on the helicopter) by the surrounding air ...
Selene Routley's user avatar
73 votes
Accepted

Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?

You are right. Perfectly rigid bodies are an idealization, like point particles or massless frictionless pulleys. They do not exist. But they are useful. Plenty of objects exist that are so rigid that ...
mmesser314's user avatar
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41 votes
Accepted

Why certain rotations are unstable? (Euler Equations)

There is another nice way of seeing this mathematically. It is not too hard to show that in the body frame, there are two conserved quantities: the square of the angular momentum vector $$ L^2 = L_1^...
Michael Seifert's user avatar
38 votes

How is length contraction on rigid bodies possible in special relativity since definition of rigid body states they are not deformable?

Buzz's answer is correct in that there's no such thing as a perfectly rigid body in relativity. But even more importantly for your question, a body in uniform motion does not feel any kind of ...
Ricky Tensor's user avatar
  • 2,040
32 votes

Spinning gyroscope loses weight?

You are substantially overstating the precision of your measurements. The readings changed rapidly so they had to be read by stepping through videos of the experiment. What you mean is that you ...
Dale's user avatar
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30 votes
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Does a rotating rod have both translational and rotational kinetic energy?

It depends what you consider to be the "pivot" about which the rotational kinetic energy is calculated here: If you choose the pivot as the end of the rod that is physically held in place, ...
Uyttendaele's user avatar
25 votes
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Does rigid body rotation contradict Newton's first law?

First of all, a more precise statement is that "an object will move with constant velocity if there is no external force acting upon it". This is the same as saying the object moves in a ...
Puk's user avatar
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23 votes
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Will a pure rolling cylinder stop on a rough surface?

As Yashas Samaga said, it will not stop on a smooth, but frictional surface. It will stop however on an actual rough surface (as it does in reality – e.g. a steel marble rolling on a rough stone ...
leftaroundabout's user avatar
23 votes
Accepted

How can friction do no work in case of pure rolling?

In a scenario of pure rolling of a rigid wheel on a flat plane, you don't need any friction. Once the wheel is rolling, it will continue to do so, even if the friction coefficient becomes zero. If ...
rnva's user avatar
  • 1,201
23 votes

When we say a rigid body is a system of particles, what exactly are 'particles' here?

TL;DR Never forget that we are talking about models here. We know that they cover only part of reality, but they are good enough and much easier to handle than "the full story". A model need ...
Ralf Kleberhoff's user avatar
23 votes

How does the parallel axis theorem explain the opening of a door?

How does the parallel axis theorem explain the opening of a door? - It doesn't. The moment of inertia of a door does not depend on where you push it. The torque that you apply to a door depends on the ...
Farcher's user avatar
  • 96.8k
22 votes

Does a rotating rod have both translational and rotational kinetic energy?

You have to understand that concepts like rotational kinetic energy are just shortcuts to solve problems efficiently. In Classical Mechanics, we start by defining concepts like kinetic energy on point ...
Noumeno's user avatar
  • 4,529
19 votes

Motivation for the definition of angular momentum?

Angular momentum is a conserved quantity$^1$. This is an experimentally confirmed reality, and the equations you talk about naturally follow from this fact since one of them defines this conserved ...
joseph h's user avatar
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19 votes
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Why a moving car dips to front when brakes are applied?

Take the centre of mass as the pivot point. Weight passes through the centre of gravity, which is practically the same position as the centre of mass, and so contribute nothing to the angular ...
naturallyInconsistent's user avatar
18 votes
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How can different points on a rigid body move with different speeds but also be relatively at rest?

Having a non-zero relative velocity is fine as long as the distance between the points isn't changing. This certainly holds for a rotating rigid body. As another example, take a ball on a string and ...
BioPhysicist's user avatar
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18 votes
Accepted

How is length contraction on rigid bodies possible in special relativity since definition of rigid body states they are not deformable?

Perfectly rigid bodies are not possible in relativity, although this is not directly related the Lorentz contraction mentioned in the question. One immediate consequences of relativity is that no ...
Buzz's user avatar
  • 16.2k
17 votes

Will a pure rolling cylinder stop on a rough surface?

Assumptions made in this answer: By rough surface, you meant a flat surface which has friction. The cylinder/sphere/disc/etc. are ideal; they do not deform. This is my reasonable guess; I am aware ...
Yashas's user avatar
  • 7,183
17 votes

Does a rotating rod have both translational and rotational kinetic energy?

Since the rod's center of mass is changing, does this mean that it also has translational kinetic energy? Yes. You have both translational kinetic energy and rotational kinetic energy. The ...
Bob D's user avatar
  • 72.5k
15 votes

Why certain rotations are unstable? (Euler Equations)

There's an alternative to @MichaelSeifert method which uses angular momentum and moments of inertia: it is to deal with the vector $\vec\omega$ directly as we are interested in the evolution of this ...
ZeroTheHero's user avatar
  • 45.6k
14 votes

How can friction do no work in case of pure rolling?

The pure rolling here is under a constant external force on the object so friction does act. As an example consider an object rolling, without slipping, down an inclined plane. There will be a ...
Farcher's user avatar
  • 96.8k
14 votes

Why do we talk about inertia tensor?

A rank 2 tensor is something that relates two vectors. In this case, the MMOI tensor relates the rotational velocity vector to the angular momentum vector. Given a solid whose internal particles are ...
John Alexiou's user avatar
  • 38.6k
13 votes
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Rigid body rotation apparent energy paradox

The problem here is that no force can be perfectly impulsive because then the distance moved while the force was being applied would be zero and therefore the work done would be zero. If we assume ...
John Rennie's user avatar
13 votes

Objects falling from table

If the centre of gravity of the object is vertically above the edge of the table then the object is in equilibrium. However, this equilibrium position is unstable (like a pencil balanced on its point) ...
gandalf61's user avatar
  • 55.4k
13 votes

Spinning gyroscope loses weight?

One thought as to why you might see this: Is your gyroscope perfectly balanced? If not, it will generate up and down forces as it spins. At one instant, it will press down harder on the scale and be ...
mmesser314's user avatar
  • 39.5k
13 votes

Spinning gyroscope loses weight?

A lot of people have been misled by this apparent effect, notably Eric Laithwaite in his later years1. I suggest that you need to be able to collect six-axis data at the point of contact, ...
Mark Morgan Lloyd's user avatar
12 votes

Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?

Not quite. You are correct that rigid bodies do not exist, but this is not the reason. Your assumptions imply that for a velocity to go from a positive value to a negative value, it must go through ...
Cort Ammon's user avatar
  • 49.7k
12 votes
Accepted

When we say a rigid body is a system of particles, what exactly are 'particles' here?

In macroscopic mechanics (in other words, "usual" mechanics), a "particule" is a mesoscopic system. The mesoscopic scale is an intermediary scale between microscopic and ...
Miyase's user avatar
  • 6,170
11 votes

How can different points on a rigid body move with different speeds but also be relatively at rest?

If the velocity of point B relative to point A is always at right angles to the line AB joining them, then the distance does not change.
mike stone's user avatar
  • 53.7k
11 votes
Accepted

Why does a smooth rolling ball roll indefinitely despite there being static friction?

Real rolling objects on a horizontal plane (i.e., for $θ=0$) do not slow down due to static or dynamic friction but aerodynamic drag and rolling friction (caused by deformation of the rolling object). ...
Wrzlprmft's user avatar
  • 6,295

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