# Tagged Questions

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

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### How I think about rolling [closed]

Below, I have written how I think about rolling. I wish to know if my idea as to how it works is correct. Consider a rigid ball on a plane which has considerable friction. If you push the ball ...
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### Question about friction in rolling without slipping?

I was under the impression that for a ball/cylinder to roll without slip, there must be a static friction force that opposes the direction of motion. Why, in this case, does the friction act in the ...
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### Force and Torque

Think of a uniform sphere. Sometimes if a force is applied to the sphere it does not only move but also spins. So there is a torque. But is it possible to calculate which part of the applied force is ...
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### Stacked motors thought experiment

first of all, I'm not a mathematician, just curious about something that came up to my mind and maybe you can help me out. Scenario: I have a motor that spins let's say at 20K revolutions per minutes....
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### Meaning of parallel axis theorem: why is the moment of inertia minimum if the axis passes through the CM?

From parallel axis theorem follows that, given a rigid body, the moment of inertia is minimum if calculated with respect to an axis passing through the center of mass. What is the physical meaning of ...
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### Cylinders rotating on a table

Please consult this video for context, its an interesting watch. https://www.youtube.com/watch?v=JuuYFt8bahE In this video Tadashi spins a tube on a table and it rotates around the tube axis and ...
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### Rotational work and forces such as static friction or ropes tension

I'm confused about the rotational work, defined as $W=\int_{\theta_1}^{\theta_2} \tau_z d \theta$ Where $\tau_z$ is the component of the torque parallel to the axis of rotation $z$. Consider a ...
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### Build rotational Hamiltonian based on Lagrangian of general form

I've been told that one could build rotational Hamiltonian based on Lagrangian of general form: $\mathcal{L} = \mathcal{L} (\vec{\Omega})$. By introducing Euler angles one could rewrite Lagrangian in ...
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### Force at axis of spinning wheel

This is a newbie question, but I don't quite understand the forces at work when an object is rotating. So I've read that i can use my right hand to determine the direction of the force at the axis ...
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### Does a central force have to be independent of angle?

When defining a central force, some sources, like Wikipedia, say that the magnitude of the force only depends on the distance $r$: In classical mechanics, a central force on an object is a force ...
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### Circular motion and car tire angle [closed]

I'm trying to figure out what angle to turn the front tires of my small robotic car (about a foot in length) so that it moves in a circle (on level ground) of radius r (will be accommodating circles ...
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### Does zero net force imply zero net torque?

I wonder, for a rigid body, if the vector sum of all external forces acting on it is equal to zero ($F_{\rm net} = 0$), does this imply that the vector sum of all external torques acting on it about ...
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### Why is Torque Generated from Friction?

Why does a rolling ball get torque from friction. I understand torque in relation to lever arms and the force being perpendicular to said arm. But what about wheels? There is no lever arm so how is ...
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### Does vehicle tire mass effect efficiency?

This question has an interesting origin: A tire salesman was recommending tires (aka tyres) for a highly fuel-efficient vehicle. He said the vehicle was light (compared to most production cars), and ...
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### What is Moment of Inertia of a cube about an axis passing through its centre and perpendicular to one of its faces?

Give a solid cube of uniform mass distribution and total mass $M$. I want Moment of Inertia of this cube about an axis passing through its centre and perpendicular to one of its faces where a is the ...
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### Problems with moment of inertia [closed]

I totaly confused with these two problems can someone give any hints, or just final formula i get tired with it. Find the velocity of (a) a cylinder and (b) a ring at the bottom of an inclined plane ...
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### Can torque relations actually be *derived* from Newton's Laws, or is it something extra?

For a long time I have wondered if there is a way to show that the rotational analogs of Newton's Laws are a direct consequence of just those laws, or are we adding more to them? I understand ...
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### Velocity of a body in a pulley problem involving a spring fixed to the ground

So the picture shows the whole situation. My question is, how do you find the velocity of the object after it had descended 10 cm (0.1 m) ? Pulley's radius 0.2 m, moment of inertia 0.2 kg/m I have ...
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### Why does a force parallel to the axis of rotation do not produce torque? [closed]

One explanation can be that Torque is a cross-product (r x F). I want to know what is its physical interpretation.
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### Pendulum motion equation issue

The differential equation that gives the equation of motion of a pendulum where: $m$ is the mass $L$ is the distance between the pivot and the body's centre of mass $g$ is the acceleration due to ...
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### Rotation of join of two different rods when one is rotated [closed]

Two cylinders A and B made of same material and having same length of radii $r$ and $2r$ are soldered coaxially. The free end of A is clamped and the free end of B is twisted by an angle theta. The ...
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### Conditions to impose for accelerations in systems of rotating pulleys [closed]

I do not understand completely how do rotating pulleys with ropes (that do not slip) work. Besides the equations of motions, which are clear to me, some conditions on the accelerations are needed to ...
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### Does the centrifugal force affect stationary objects as seen from the inertial reference frame?

If an object is moving at the same speed as a rotating/accelerating frame of reference it's in contact with but in the opposite direction (making its displacement zero), would such an object be ...
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### What is the relationship between $W_1$ and $W_2$? [closed]

I have a model which schematic is in the linked picture. Blue $A$ and $B$ are rotational joints, black and red shapes are rigid elements. $I_1$ is moment of inertia of black and red elements combined ...
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### Difference between potential centrifugal energy and rotational/kinetic energy

I'm having troubles to understand the difference between kinetic energy and potential centrifugal energy in some situations. I make an example where I am confused. A object moves on a rod attached ...
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### Impulse and momentum on a system of three particles (equilateral triangle)

I found this exercise on a textbook which provided solely the description of what is happening and the answers with little explanation. The solution being provided, I am mostly interested in ...
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### If a disc is rolling on a surface, how do you verify that no slip occurs? [closed]

If I have the values for the coefficient of static friction (0.1) and all the other variables, how do I verify that no slip occurs?
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### In pure translation why the body does not exhibit rotation only and only about the CM? [closed]

Suppose I have a rod in pure translation as shown in the picture with some constant acceleration. The two forces $F_1$ and $F_2$ act as shown. So clearly due to the definition of pure translation ...
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### Does the rotational form of Newton's second law always hold?

Does the equation, $$\vec \tau=I\vec \alpha$$ (rotational form of the second law) always hold? If not, in what conditions does it hold true?
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### What kind of motion is caused by nonzero torques about two parallel axes?

Say on a rigid body there are non-zero torques about two parallel axes. About which axis does it acquire a angular acceleration? I'm looking for a mathematical explanation.
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### Rotating rigid rod in empty space [duplicate]

To begin, we assume there is a rigid rod in empty space which has no rotation with respect to the fixed stars. The frame of the rod is now an inertial reference frame, say $F$. 1) The rod now has a ...
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### Will future spaceships for very long distance rotate?

I have seen in the movie Interstellar and The Martian they use the model of spaceship which rotate around it's axis. This is the spaceship model from Interstellar. This is spaceship model from The ...
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### Why does Torque exist?

Specifically, why does force increase with the moment arm? What about making a perpendicular force farther away from the axis of rotation increases it? How does the moment arm cause that?
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### Cylinder on an accelerated rough plane [duplicate]

The following figure shows a cylinder resting on a rough horizontal rug that is pulled out from under it with acceleration $a$ perpendicular to the axis of the cylinder. What is the motion of the ...
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### Rotating movable pulley [closed]

I'm trying to understand how a movable pulley of mass $m$ and radius $r$, that rotates and doesn't slip actually works. Consider the picture, if a force is applied at the free end of the rope, are ...
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### Conservation of angular momentum in broken pendulum

I'm having troubles in understanding the correct way to use the conservation of angular momentum. Consider the pendulum in the picture with a neglegible mass rope and a slender rode of mass M and ...
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### Net force for circular motion

So today my physics teacher was telling us that the net force when discussing circular motion was actually the centripetal force. He didn't go further and I would like to know if there's any proof to ...
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### How to find speed of rotation of gears from kinetic energy of falling body?

A pulley is fitted to the first driven shaft of compund gear train. Last driven shaft of gear train(gear 8) is fitted to the 12V DC generator (6W).Now to the rope extension at the one end of the ...