# Tagged Questions

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

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### Rotation of rod [on hold]

A uniform rod of length "L" is free to move and rotate in gravity-free space . when an impulse is given to one end of the rod perpendicular to its length, its center of mass moves with velocity "V". ...
44 views

### Optimal axis of rotation of a baseball for lateral movement

Suppose a baseball travels at $90\;\mathrm{mph}$; what is the ideal axis of rotation for lateral movement of the ball? (two-seamer or cutter). So far I think that the following factors play a ...
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### What will be the new moment of inertia? [on hold]

Let assume we have a sphere of radius $R$, and we can simply calculate the moment of inertia if we consider the axis is going through the center O. Let's cut a sphere small radius $r$ from the sphere ...
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### What is the importance of torque direction? [duplicate]

I understand the right-hand rule and direction of the force on an arm, but what exactly is this 'direction' that results from the cross product of the vectors $T = r \times F$? On paper, what is this ...
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### How does this dumbbell rotate?

I am reading the book Spacecraft Attitude Determination and Control by J.R. Wertz, and on page 488 it says: My question is: how does the axis of rotation, $\hat{\omega}$, rotate? How does ...
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### Principal moment of inertia for a rotating body

My major is not in physics. I am reading the following paper: (my problem is simple and not related with any optimization) http://arxiv.org/abs/1410.2841 (p.5~p.6) Suppose The ...
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### Could a meteor cause the flood from Noah's time in the bible? [on hold]

In the Bible it was said before Noah's flood man lived to be around a thousand years old. Shortly after the flood the life span went to about 120 years. Today 80 years. Could a meteor have hit at ...
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### Simple Disk rolling with slipping on a horizontal plane

This is the first time I've encountered this scenario. I want to understand how to handle it. Since the disk is slipping, what would be the work done by the frictional force? What is the ...
24 views

### Stuck with 2D kinetics problem [closed]

I've been trying to solve this exercise for the last four hours and I'm totally stuck. The problems goes like this: Given the mechanism in the image, located in the vertical plane, the OB cord ...
32 views

### Rolling Coin (Old Physics Quals Exam) derivation help [duplicate]

I am trying to understand the derivation for the angular velocity of a rolling coin in the problem given on this website: http://functionspace.com/question/27/answer/121 How does one find angular ...
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### Moment of Inertia of a curved part of a graph [closed]

How to find the moment of inertia of a curve about a given axis. For example : How can we find the MI of the curved part of :$$\sin x\$$ EDIT: Assume the range from $$[0 ,2\pi ]$$ and the axis to ...
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### Dynamics of pendulum coupled to a gyroscope [closed]

We know the dynamics of a pendulum and we know the dynamics of a gyroscope. I put together the following I diagram below. The interesting thing as that the system has a beat frequency where the ...
61 views

### Will rotating magnets slow down one another when interacting?

Will rotating magnets slow down one another when interacting? Two discs are spinning. Both have separate magnets attached. Disc one has a diameter of $8~\mathrm{cm}$ and disc two has a diameter ...
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### Is it possible to throw a stick without giving it a rotational motion?

When walking with our dog I always throw wooden sticks for her. Then one time I wondered if I could throw the stick (normaly, so not like a spear) without imparting on it a rotational motion around ...
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### Does a counterweight eliminate recoil/counter-torque?

A long, long time ago, I remember reading some scifi book that involved catapults whose throwing arm had a counterweight at the other end, such that the arm was balanced. The counterweight was not ...
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### A ball rolling at the bottom of a spherical bowl [duplicate]

This ball oscillates at the bottom of the spherical bowl which has sufficient friction that it doesn't slip throughout the motion and rolls without slipping. At the extreme points of its motion (as ...
42 views

### Can we know the tension of the string that is connected to a pully?

If the pulley have mass, I learned that T1 and T2 isn't same. then what about the circled part of the string? can we calculate the tension with given T1 and T2?
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### how can we know the axle of an object's rotation? [duplicate]

For example, when we give a Force at an edge of an even stick, then we say that the stick rotates and the axle is the middle. But how can we say it will rotate at the middle? why can't the other part? ...
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### Does adding mass to the fan blades affect its max velocity?

If I replaced fan blades with congruent blades, the only difference being the second set has a higher density, would that affect the speed cap on the spin? I know it would affect acceleration, but I ...
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### How does mass affect slipping of a rolling object?

I experimented by rolling a cylinder down a tube and noticed that by increasing its mass, it's time taken to reach the bottom decreased. Found out that mass doesn't actually affect the time taken, ...
4k views

### Why is the vibration in my wire acting so oddly?

I was soldering a very thin wire today, and when I had one end firmly soldered, I accidentally bumped the wire diagonally with my tweezers. What I'd expect to happen is that the wire oscillates for a ...
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### How can you explain a paraboloid in a rotating liquid without centrifugal force or gravity?

How does a rotating container with liquid in it cause a paraboloid to form? My physics teacher says that it is incorrect to use Centrifugal force and Gravity, but after reading though many forums on ...
173 views

### Why do rope walkers always carry a long stick with them?

I saw in the film The walk a guy who used to go from one high building to another via a rope. He carried with him this long stick. You´ll probably all know what I ...
2k views

### Does cutting of trees affect spin angular momentum of earth?

Cutting trees reduces earth's moment of inertia. So the spinning velocity of earth should be reduced day by day. Does it really happen?
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### Rotation of a rod due to a normal force

Suppose there's a rod suspended in space free from the effect of any force. Let a force F act at a distance x from one end. How can i prove that rotation will take place about an axis passing through ...
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### Predicting top speed of a flywheel [duplicate]

I connect a DC motor to a flywheel with a given moment of inertia. I then supply that motor with a constant voltage and current for an infinite time interval. What do I need to know about my motor to ...
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### Ball under the water jet

What is the name of the physical principle that show that a ball under a water jet (for example in a bathtub) rotates but does not move? I am not able to find any video of this
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### Cohesion Sliding Resistance meaning

What do we mean by "Cohesion Sliding Resistance"?? and how to calculate the value for the same for an iron object placed on a concrete surface for example? P.S.: All I know is this has the units of ...
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### Determination of nature frequency and differential equation of vibration of Hartnell governor?

I found this solution for the nature frequency but here it does not include the Ball weight and centrifugal force in the moment balance equation about the pivot (O), it is wrong answer...is not it? ...
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### What is rotational potential energy?

I don't understand the concept of rotational potential energy. During linear motion, when a force is applied, the work it does gets converted to kinetic energy and there is no change in the ...
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### Tension in a string with pulley and two objects at opposite ends

Tension is confusing me. According to my understanding of tension the tension in a string is uniform but while going through the solution of a problem I read that tge tension on the left part of the ...
49 views

### Verifying directions of forces during pure rolling?

Let us take a cylinder on a rough horizontal surface. The cylinder undergoes pure rolling A vertical downward force acts on the cylinder to produce a clockwise rotation. The point at which it acts ...
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### Expressing virtual force in terms of muscle contraction force

Two bodies connected with a hinge joint with axis in J. They also connected with a muscle at points M and M`. I’m trying to express virtual force Fv in terms of muscle force Fm acting at the point M ...
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### What is the degree of freedom of pure rolling motion?

A cylindrical body is pure rolling, what will be the degree of freedom for it? I am confused between 2 and 1 if slipping occurs than it has 2 but in pure rolling there's no slipping so what will it ...
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### Total torque about any line when resultant force is zero [closed]

While reading about rotational equilibrium I came across a statement which I was unable to understand: The torque can be taken about any line perpendicular to the plane of the forces. In general,...
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### A trio of spinning marbles

My little brother showed me this devised act in which he stuck three marbles in a triangular pattern When he spun any of those marbles, the triplet would first rotate normally, but moments later, ...
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### Why does a rigid body rotate and not simply translate when pushed with an instantaneous force?

Let's say we have a metal rod of consistent density sitting flat on a frictionless surface. I intuitively understand that if I push one of its ends away from me, (at a right angle to the length of the ...
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### Friction-free rolling/sliding on an inclined plane

Suppose a sphere is rolling down an inclined plane. There is no friction. The body will not roll and undergo just a translation. But why is this so? If we consider the axis to be along the point of ...
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### Physical pendulum dynamics confusion

I'm validating a Matlab Simulink SimMechanics model analytically. The model consists of a physical pendulum (uniform rod with length $l$ and width $w$) which is torque driven. I'm somewhat confused ...
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### Determining bond length of HCN using rotational constant (Physical Chemistry 2)

Context: 2nd year undergraduate student taking Physical Chemistry II. Question: Determine the HC and CN bond lengths in HCN from the rotational constants: B($^{1}H$$^{12}C$$^{14}N$) = 44.316 GHz B(\$...
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### What's behind the moment of inertia and other “body-global” properties of bodies?

I'm an electrical engineer currently doing some (computational) mechanics stuff. In introductory literature about mechanics, you can read plenty about the moment of inertia and how you use it in ...
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### Solving 9 variable Normal Reaction equations of Sudoku board [closed]

We have a rigid Square board of negligible mass, which has been divided into a grid of 9 small squares(like a Sudoku Board), at centre of each square a point mass is attached. The gravity on the board ...
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### Balancing an object with two loads

Suppose a bar (black) sitting on a suspended ball (blue) has two rigid loads of equal weight attached to it (red): Will the ball balance on the blue ball given even when the connection points of ...
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### Dampening a swinging mass

Suppose a bar (black) sitting on a suspended ball (blue) has two rigid loads attached to it (red, green): Because the bar sits on a ball, the bar can swing clockwise or anti-clockwise. Suppose the ...
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### The direction of frictional force in circular turning

Why does the frictional force in case of circular motion point towards the center even though the motion is tangential to the radius?
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### Accelerations of Cylinders

I got a box hanging from a rope that passes through an ideal pulley that attaches to the superior part of a Cilynder and makes it rotate. An exercise asks me two things: a) determine the magnitude of ...
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### Differential equation of motion of combined rigid body

I am reading the following paper: http://tinyurl.com/jv2pu2m I have a question about (1) and (2): To (1), I think the following link provides good explanation. Equation of Motion for Rigid ...
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### Problem of pure rolling motion

A suspension, shown schematically as a spring of spring constant k, is connected to a sleeve able to slide without friction along a horizontal guide, and to a wheel (that is, a cylinder of radius R ...