# Tagged Questions

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

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### Direction of static friction in inclined rolling motion

I can't seem to understand in which direction static friction faces for inclined plane motion with rolling motion. This considers rolling motion without slipping, how do i find the direction of the ...
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### What are our limitations on spinning a baseball sized 5 kg sphere to really fast speeds?

I know the limitation of the ball being able to hold itself together, lets assume that would not be an issue. If we were to apply a constant force over long enough time would anything short of the ...
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### Man walking on the rotating platform [closed]

The problem is: The solid cylindrical platform with mass $M$ and radius $R$ rotates with angular frequency $\omega$. On the edge of the platform there is a standing man with mass $m$. Then man ...
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### Four bar linkage moment transmission

The question is. I know the angular velocity of $d$ after applying dynamic mechanics and the constant angular velocity of $b$ of $2\pi\ \text{rad/s}$ and I know the input moment of the power source ...
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### Simple Rotating frame of reference

I have a car moving in straight line with certain time-varying acceleration and there is another car moving in a curvilinear way along some curved path with some time-varying acceleration. The ...
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### How do I handle elastic contacts in a simulation with friction [closed]

I'm trying to simulate a wheel as it hits the ground. Problem 1 Suppose a disc is dropped from a height. It has initial velocity of $-x,-y$ caused by throwing and gravity. It has no initial angular ...
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### Understanding the constraint of tension with a rigid body

I want to know why doesn't $T_1 = T_2$? I think its probably because of the Pulley but still aren't the Tensions Equal but opposite in direction.
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### Conservation of angular momentum during rolling

A disk having initial angular velocity $\omega$ is gently placed on a rough horizontal surface. What is the angular velocity of rotation when pure rolling starts? I've tried applying conservation ...
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### A sphere rolling down a plank - an error in calculating its energy

Consider the following situation: A sphere with radius $r$ rolls down a plank which forms an angle $\theta$ with the horizontal axis. The starting hight of the centre of mass is $h$. We assume that ...
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### Why does the pen does not move straight?

If i put a pen on a table in its horizontal position and then i try to move it horizontally by giving it a small push, so that it would fall off a table, i expect it to move horizontally but my pen ( ...
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### A problem regarding translational & rotational motion

Let a force $F$ be applied to a body of mass $m$ for time $dt$ where the line of action passes through the centre of mass. This means that the change in momentum will be $Fdt$, and the change in total ...
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### If I toss a coin, vertically, on the surface of Mars, will it land back in my hand?

When I toss a coin in Mars, is the planets atmosphere rare enough that I'd rotate with the planet (at its angular velocity), but not the coin?
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### What is the derivation of $r*F=Iα$? [duplicate]

Please give me the derivation of the formula r*F=Iα for extended body. where, F=applied force r=distance between point of rotation & point where force is applied I=moment of inertia of the body ...
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### Gyroscope precession

I have a system diagrammed and explained in the image below. Experimentally I believe the wheel will rotate around the pivot point where the cable is attached in a counter-clock motion if ...
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### Is a rotor balanced at constant angular velocity?

I have an asymmetrical rotating part. It vibrates its housing and emits audible noise. I need to add weights to ensure smooth rotation. However, I am constrained in which regions I can add ...
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### How is temperature and pressure related to when a tire rolls on the ground?

Actual observation: Here in Canada it is known that even when 'dry' snow is on the ground, and when a car drives over it that small amount of dry snow instantly becomes ice (NOT GOOD). Anyways as an ...
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### The precession component in the direction of spinning

Is the precession increasing the rotation around the axis of spinning? w = (nutation) i + (precession component) j + (another precession component + spinning) k or to rephrase, is the precession ...
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### Why isn't kinetic energy conserved in this rotational dynamics problem?

Consider a uniform rod which is spinning about an axis that goes through its centre, perpendicular to the rod itself. Two small rings are attached on the rod at equal distances from the centre. As the ...
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### The way planets rotate and revolve [duplicate]

Why is that the all planets move in the same plane?(correct me if I'm wrong) Why not some of them in anticlockwise direction and others in clockwise direction?
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### Parallel axis theorem: adding mass

I'm going through a worked physics problem and have a question about the parallel axis theorem regarding only adding mass while not changing is axis of rotation. Here is the problem: A physics ...
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### Angular momentum changes depending on orign

Consider the image below where we have two point masses $m_1$ and $m_2$ with different masses which are rotating around a fixed axis with angular velocity $\omega$. If the origin is placed on the axis ...
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### Change in acceleration due to gravity because of rotation of earth [duplicate]

The formula above is the equation for acceleration due to gravity when earth rotates. G is the original acceleration. Can someone explain how this formula came?
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### Is moment of inertia numerically additive?

NOTE: The following argument is being made for square only, not any general shape. We have this square plate: From Perpendicular Axis theorem, $$I_1 + I_2 = I_z \\ I_3 + I_4 = I_z$$ Also, ...
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### Why does this consider the center of mass instead of the end? (RIGID BODY MOTION) [closed]

A uniform rod of length L and mass M is free to rotate on a frictionless pin passing through one end. The rod is released from rest in the horizontal position. What is its angular speed when it ...
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### Confusion regarding torque and centre of mass

When a body slides down a rough incline plane, why does $mg \cos \theta$ and normal reaction produce no torque? And why does $mg\sin \theta$ produce torque?
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### Why centrifugal force results in a circularly rotating motion for a stationary object in the non-rotating frame

My doubt is the following: in a rotating frame of reference you have the centrifugal pseudo force. When you analyze and object in circular motion (due to a rope) in the rotating frame of reference, ...
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### Angular momentum of a rigid body about any points

Is angular momentum about all points same if the body is rigid and is rotating/translating/rolling with a constant velocity? Why? No external force is acting on the body.
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### What is the purpose of the directions for vector quantities such as angular velocity and torque?

Looking at just both angular velocity and torque, many other exhibit this same property. What is the purpose of and usefulness of having a direction that is in the Z-direction? (assuming that, in the ...
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### In case of pure rolling, does the formula $v=r\omega$ holds only for a small instant of time?

For example, on a rough track when a ball rolls and is under smooth rolling, friction continues to act in the lower portion and so, torque acts along the centre of the ball. As per the formula: ...
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### How far does the ball travel before pure rolling motion occurs? [closed]

A bowling ball is given an initial speed $V_o = 10 \text{m/s}$ on an alley such that it initially slides without rolling. The coefficient of friction between the ball and the alley is $\mu = 0.06$. ...
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### Expressing 3D orientation in alternative to Euler angles for 3D rigid body dynamics

I was unsure whether it would be best to post this in Physics, Maths, or other forums, so please say if this question is suited better elsewhere. I am trying trying to create a physics engine for a ...
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### How to find the angular acceleration (rotating reference frames)

If you chose the frame to rotate with $w_1$ instead of $w_2$, wouldn't the angular acceleration be $w_2 \times (w_1 + w_2)$? and it would have $+20 \jmath$ instead? Shouldn't the absolute ...
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### How fast will a ball travel if spun around with a string then released? [closed]

It may seem like a stupid question, but I was wondering: If a ball with a mass of 50 kg was rotated from a string with a radius of 10m and a velocity of 100 m/s how fast will it be travelling when ...
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### Considering rolling without slipping using axis at contact point with surface

For a ball rolling down a ramp without slipping, we can use $\Sigma \tau = I \alpha$ about two axes: an axis through the center of the ball, and an axis through the contact point with the surface. The ...
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### In golf, is there a rotational mechanical advantage of using a thicker grip?

I was asked to migrate this question to the physics exchange. Ok, so many professionals are now using the new SuperStroke golf grips. I am basically thinking about the concept of a bigger grip, but ...
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### How to calculate the equations of motion for a dipole in an electric field in three dimensions

I'm having trouble modelling a system, which I thought would be simple but has turned out to be quite complicated. The system is a charged sphere in an oscillating three dimensional electric field. ...
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### Is there any friction between wheel and road?

Human beings invented the wheel to get rid of the friction between the wheel and the road. But were we able to reduce it to zero? Is there any residual friction? This question is about only the ...
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### How does friction act in rolling motion? [duplicate]

In case of rolling, static friction at the lowermost point becomes zero once pure rolling starts. So how is that when we apply a constant force on the centre of mass of the spherical body (or any ...
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### Force required to rotate a rod around an axis [closed]

The problem I'm trying to solve is: How much force, momentarily applied to the rod, is required to rotate it around an axis by a given degree? Assuming there is friction applied at the axis (...
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### What point is the torque on a loop in a magnetic field taken about?

I know that the torque on a loop kept in uniform magnetic field is given by $$\vec{\tau}=\vec{M}\times\vec{B}$$ where $\vec{M}$ is magnetic moment. Now, can I take this torque about any point in ...
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### Moment of inertia of solid cube about body diagonal

How do I find the above mentioned moment of inertia? Steps I've tried: 1.) Triple integrations that proved to be to big. 2.) I noticed that the if we split a $2\times 2\times 2$ into individual ...
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### Accelerometer reading in a uniform circular motion

Question - there is an accelerometer kept in a car moving in a uniform circular motion. The accelerometer is kept in such a way that the z axis points downwards, x axis is pointing towards centre of ...
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### Why do throwing knives need to be balanced?

Indeed, unbalanced throwing knives are used, but I have read that balanced throwing knives are easier to use, because due to their geometric center being the same as their center of mass. Why would ...
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### Rotation of fluid filled cylinders [closed]

Given two cylinders of same mass , one completely made out of solid metal and the other a hollow one which has been filled with a viscous fluid say oil. Both have the same mass.Now if they are placed ...
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### Tangential Velocity of a Rotating Rod

I have the following question which has given rise to a doubt. Question: One end of a rod of uniform density is attached to the ceiling in such a way that the rod can swing about freely with no ...
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### How is angular momentum conserved

from a classical perspective, what is it about angular momentum fundamentally that means it has to be conserved? Surely if I have a rod about a fixed axis and a moving particle hits the end it will ...
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### Can a wheeled vehicle remain stationary on a water surface?

There are many field examples[1] of motorized, wheeled vehicles capable of staying on the surface of water. It appears that the requirement is sufficient "reverse-pressure" against the part of the ...
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### Where to hit the pool ball such that it satisfies the given condition? [closed]

The question Where to hit the ball in such a way that after covering a certain distance it rolls back? You are given a pool ball and you have to find where to hit it such that it rolls back after ...