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

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1answer
860 views

What is happening to rotational kinetic energy when moment of inertia is changed?

I know this question is asked here a lot, but I just had to ask this to finalise the concept. When a system lets say a rod of length $L$ and mass $M$ is rotating with angular speed $omega_1$ its ...
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2answers
1k views

Where does a spinning figure skater's energy go when she slows down?

Today in physics class we were talking about angular momentum and rotational kinetic energy. My teacher used the classic example of a figure skater spinning on ice - when she pulls her arms in, her ...
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3answers
531 views

Where does the kinetic energy go?

A uniform cylinder was placed on a frictionless bearing and set to rotate about its vertical axis. After a cylinder has reached a specific state of rotation it is heated without any mechanical support ...
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1answer
123 views

Energy of a simple pendulum like device

If there is a device in which a ball is attached to a pivot by a rod of length R and it is left to oscillate by dragging it to one side so that it is completely horizontal, at any point on its decent ...
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0answers
93 views

Terminal velocity for rotating system

Suppose we have a small bob/ball attached via rigid rod to a pivot in a viscous medium and then rotated by a constant torque, will the system finally(after considerable time) rotate with constant ...
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0answers
67 views

Total energy of a rotating body?

I've no problems with the first part. However, I'm struggling with the last part of the question. The first thing I did is to find the a new moment of inertia for the whole system including the ...
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1answer
1k views

How do I calculate the experimental and theoretical rotational inertia of a point mass?

I'm getting some weird results from a calculation I'm doing and quite honestly, I'm pretty sure it's my fault. I do have an apparatus involved for the experimental process for my lab but I don't think ...
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0answers
63 views

Two disks on frictionless surface, demonstration

Consider a system composed of two plane disks, which we will designate as $D$ and $d$. Their radii are $R$ and $r$ respectively $(R>r)$. The disk $d$ is fixed over the disk $D$ and a ...
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1answer
144 views

Heat engines and “Angular momentum” engines?

We know that the theory of heat engines is that, if you accept the second law of thermodynamics, $\Delta S > 0$ then you can define temperature using $\frac{1}{T} = \frac{\partial S}{\partial E}$ ...
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3answers
2k views

Force applied to wheel in pure rolling motion at contact point with road

Suppose a wheel with radius $R$ is resting on a non-inclined surface. A torque $\tau$ is applied to the wheel center. In an attempt to prevent wheel from spinning, the ground applies a static friction ...
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1answer
66 views

Ask an equation to solve the next location of an object?

It is a long time that I have not learnt physics. But now I really need an equation to solve a problem. I appreciate physicists give me a little hint. Here is a object on the desk without friction. ...
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1answer
198 views

Newton's second law for rotation

Can the second law of motion for rotation, $\vec{\tau}=I \vec{\alpha}$, be used for any axis? Is there any case that acceleration $\vec{\alpha}$ is not in the direction of applied torque ...
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2answers
647 views

Between a solid and a hollow cylinders of the same mass, which one has the greater rotational kinetic energy?

I know that rotational kinetic energy is $\frac{1}{2}I\omega^2$. Therefore, the rotational kinetic energy will depend on the moment of inertia. I came to the conclusion that since both have the same ...
0
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1answer
109 views

Centrifugal force in inertial reference frame [duplicate]

Take for example the case of a rod rotating about an axis passing through its centre of mass and perpendicular to it. It has a ring hung from one of its sides. The rotation of the rod causes the ring ...
0
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1answer
229 views

Centrifugal force acting on ring [closed]

Take for example the case of a rod rotating about an axis passing through its centre of mass and perpendicular to it. It has a ring hung from one of its sides. The rotation of the rod causes the ring ...
2
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2answers
212 views

Centripetal force in frame of reference of body moving In a circle

Suppose a body is moving in a circle about a fixed point. In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt? More generally, does a body only ...
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3answers
304 views

Moment of Inertia, why $r^2$and not $r$?

So my engineering mechanics book includes a brief discussion on area moments of inertia. Unfortunately, the ensuing chapter is predominately computational in nature. I don't have a thorough grasp of ...
2
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3answers
282 views

Ideally speaking, will a rolling disk with no slipping come to a stop because of friction from the ground?

Consider a rotating disk on a horizontal plane with static friction. The contact point of the disk with the plane has null instantaneous velocity. Assuming the center of the disk has fixed $v_0$ ...
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1answer
55 views

Force experienced on two particles in a rotating system?

I've a system of two particles of the same mass who rotate in a circle about the centre of mass of the two particles. Is the force experienced by the particles $F=MV^{2}/r$ or should I use ...
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2answers
107 views

Why doesn't Length of cycloid depend on speed of rolling?

We know that a point on the rim of a rolling body traces out a cycloid during one turn. Why is the length of the cycloid a constant = 8r. Shouldn't length of cycloid depend on speed of rolling body. ...
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4answers
2k views

Rotation of a slipping ladder

Imagine a ladder leaning against a wall. All surfaces are smooth. Hence the ladder will slip and fall. While falling it rotates because there are external torques acting on it. My question is about ...
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3answers
1k views

Why doesn't static friction decelerate a rolling body?

I know that static friction isn't the cause of deceleration of a rolling body. But if static friction is the only force in the horizontal direction, then shouldn't there be some acceleration produced ...
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1answer
114 views

Paradox of angular velocity

For a torque-free symmetric top, the Inertia tensor has an inverse $I^{-1}$, and $L=I\omega$. Which implies that $\omega=I^{-1}L$. But since $I, L$ are constants, $\vec\omega$ is a constant. However, ...
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1answer
191 views

Why is body frame angular velocity nonzero?

This question is relevant to Euler's angles and Euler's equations for a rigid body. Why aren't $\omega_1$, $\omega_2$ and $\omega_3 = 0$ in the body frame? How can we measure $\vec\omega$?
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6answers
2k views

Is there a way for an astronaut to rotate?

We know that if an imaginary astronaut is in the intergalactic (no external forces) and has an initial velocity zero, then he has is no way to change the position of his center of mass. The law of ...
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2answers
555 views

Angular momentum of a translating and rotating body

If a rod is rotating about one end, does it have pure rotation or do you also consider the translation of centre of mass when calculating its angular momentum? Also, how would one calculate the ...
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3answers
227 views

Will an object resting on a rotating platform move in a frictionless world?

Imagine that a pebble is placed on a uniformly rotating, frictionless disk. What will happen to this pebble? Will the disk slide under it and the pebble stay as is? Or will there be a centrifugal ...
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1answer
323 views

Will a free-falling rod (without drag) rotate?

When we consider a bicycle is turning on a flat plane, we know that there is friction, which provide centripetal force on the bicycle. And we know that the bicycle is no longer perpendicular to the ...
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0answers
246 views

Degeneracy in molecular rotation energy levels for asymmetric molecules

I was recently reading Atkins' Physical Chemistry, the topic of rotational energies of molecules. It states the degeneracies of spherical top, symmetric and linear molecules as being $(2J+1)^2$, ...
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3answers
195 views

Chinook Helicopter Torque

The Chinook Helicopter has 2 rotors to counteract the torque generated by spinning the blade. Theoretically, could you use a smaller "back" rotor that is farther away from the main rotor to achieve ...
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1answer
222 views

Artificial Gravity [closed]

Consider a structure that is in the shape as shown below rotating about an axis through its middle perpendicular to the long axis in order to provide artificial gravity. What would an ...
1
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1answer
497 views

Moment of Inertia and Rotational Dynamics? [closed]

I'm having problems with the intuition behind the Parallel axis theorem and the Perpendicular axis theorem. I'm self studying Mechanics for the British Curriculum but, the book I've is missing the ...
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1answer
232 views

Question regarding mass hanging from center edge of rotating disc

So, say you have a free to rotate disc, assuming no external torques, and you have a spool, radius 7.93 mm, attached to its centre. Say the spool has a string attached to a point on its edge and ...
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0answers
113 views

Torque on object [closed]

I would like to understand why the torque on white object is 0. I know gravitational forces are very low. But the torque must be at 0. I drawn all the study in first image. Blue color is a torus of ...
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1answer
509 views

Direction of friction in case of a rolling sphere

Suppose a ball rolls on a ground .obviously it is friction that helps to develop the required torque.Hence I find the direction of friction by pointing out the direction of rolling of the ball. But is ...
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4answers
339 views

How to end up with Newtons in the calculation of the lift force

In another StackExchange Physics question, @Rego had found the following equation to calculate the lift force generated by a simple rectangular blade. $$F = \omega^2 L^2 l\rho\sin^2\phi$$ in ...
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3answers
837 views

Why does a bicycle (without any support of stand) falls down being at rest, but not under motion? [duplicate]

I have always seen a bicycle not standing without any support, it either falls down to the right or to the left, may even to some other direction. But,the same two wheeler when under motion,moves ...
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2answers
167 views

How does the curve ball drag air around it?

In cricket or baseball there is a type of ball called the curve ball. This is the top spin of the ball.I read that due to spin the ball drags the air around it due to friction in the way shown ...
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0answers
91 views

Seeking help simplifying this EOM equation

I am working on solving the equation of motion for a particular system. It has been a long time since I've worked with matrix equations and need help in simplifying the following: ...