# Tag Info

## Hot answers tagged rotational-dynamics

### Can a Foucault pendulum really prove Earth is rotating?

The thing that Foucault did was not just to predict that a pendulum would undergo precession, but also to predict which way the precession would go, and how rapidly, depending on the latitude of the ...
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### Why is it possible to keep an object, say a book, horizontal while holding its corner?

Since contact forces act on the axis of rotation, they don't exert any torque. mg would exert a torque unless the book's center of gravity is directly above or below the "hinge". You are ...
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### 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 ...
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### Why is the vibration in my wire acting so oddly?

Your wire is not quite round (almost no wire is), and consequently it has a different vibration frequency along its principal axes1. You are exciting a mixture of the two modes of oscillation by ...
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### Why does a body always rotate about its center of mass?

You presumably already know that in the absence of external forces, the center of mass of any collection of particles moves at a constant velocity. This is true whether they are stuck together in ...
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### Why do bikes hardly ever skid while braking with the front wheel?

Because, when you brake, your weight is being shifted towards the front wheel. The inertia coupled with gravity puts your weight and that of the bike onto the front wheel. More weight ⇒ more pressure ⇒...
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### How can the centripetal force lead to objects flying apart?

In the lab frame of reference, you need to reverse the question - don't ask yourself what pulls the particles apart but what keeps them together. By Newton's laws, everything on which no force acts ...
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### Could pressing the brakes on a car in mid-air affect its pitch rotation?

It will affect pitch, but not the way it works in the game. In the game, braking while in the air stops your pitch axis rotation. In real life, it does something completely different. The wheels have ...
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### Does centrifugal force stiffen a rotating blade?

For simplicity, let's model the helicopter blade as a simple massless beam with a point mass at the end. When there is no gravity, the beam will be straight. We now introduce a force to the beam tip,...
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### Why does a liquid not rotate with the container?

Suppose your cup was full of a lump of stuff that had no friction. Then as you rotate your cup there is no force between the cup and the stuff, and so it would not rotate. In the case of coffee - the ...
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### Is angular momentum just a convenience?

I'm wondering whether angular momentum is just a convenience that I could hypothetically solve any mechanics problems without ever using the concept of angular momentum. If your criterion for ...
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### What will happen to a ball kept on a frictionless inclined plane?

...the torque exerted by $N$ is zero but the torque exerted by $mg$ is non-zero. This means the ball must roll... Actually, it means that the angular momentum about that axis must increase. That is ...
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### Principle of physics used in the lift of skateboard

The skateboard is able to lift off the ground because of the momentum imparted to it by the skateboarder pushing down on the kicktail. The skateboard acts as a lever around the rear wheels, so when ...
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### What causes a rotating object to rotate forever without external force—inertia, or something else?

Is it inertia that a rotating object will rotate forever without external force? Someone told me that this is not inertia [...] Well, sort of - it’s somewhat correct to say it is inertia, and ...
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### In car driving, why does wheel slipping cause loss of control?

Because friction is your method of steering! (- and of braking and accelerating.) As @MasonWheeler comments: This is such an important principle that there's a special name for it: in the specific ...
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### What makes a wheel spin?

Pushing a heavy block across the ground is going to be hard because as you say the frictional force between the block and ground is $F = \mu F_N = \mu mg$ and for a heavy block this is going to be a ...
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### Why is it easier to carry a person while spinning than not spinning?

You still need to provide an upwards force equal to their weight regardless of your motion. If you are spinning then your arms also need to provide a centripetal force component. So really spinning ...
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### Why are angles so weird?

Your concerns are valid, and for that reason, there is also an alternative way to view angular velocities, angular momentums, etc.: They are not vectors, but bivectors. A bivector is essentially an ...
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### Why is angular momentum a vector?

This answer elaborates on the answer-in-a-comment by Chiral Anomaly. (Incidentally, Stack Exchange specifically requests: "Avoid answering questions in comments") The precursor to the ...
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### Is Earth really flattened at the poles because of centrifugal force?

The description from an interial frame is that the bits of the Earth at the equator are moving faster than those near the poles, so they have more kinetic energy, and thus they are a bit farther away ...
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### What's the physics behind the jumps with seemingly non-conserved angular momentum?

To explain how orientation can change whilst angular momentum is conserved it is first best to look at a slightly simpler system - a cat in free fall! Here is a series of photographs taken in $1894$ ...
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### Principle of physics used in the lift of skateboard

Updated Answer based on comments by @ToddWilcox that actually know more than me about this. The answer by @NuclearWang is correct, but I wanted to illustrate the process a bit. At least the initial ...
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### What does the angular momentum vector really represent?

Angular momentum is the generator of rotations. Linear momentum is the generator of linear translations. This can be given a precise mathematical meaning, but intuitively it means something like the ...
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This has been tested, but with muons rather than apples. Muons spoil much faster than apples ($2.2\mathrm{\ \mu s}$ half life), but they don’t get squished by the enormous accelerations (\$10^{18}\...