127 votes

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 ...
rob's user avatar
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115 votes
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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 ...
AlphaLife's user avatar
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90 votes
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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 ...
Selene Routley's user avatar
85 votes
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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 ...
Floris's user avatar
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76 votes
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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 ...
mike stone's user avatar
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65 votes
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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 ⇒...
Alex Doe's user avatar
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59 votes
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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 ...
ACuriousMind's user avatar
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59 votes
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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 ...
Polynomial's user avatar
56 votes

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,...
DeltaLima's user avatar
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54 votes
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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 ...
Ponder Stibbons's user avatar
53 votes

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 ...
Dale's user avatar
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51 votes
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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 ...
Dale's user avatar
  • 97.8k
51 votes

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 ...
Nuclear Hoagie's user avatar
47 votes

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 ...
GenlyAi's user avatar
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44 votes
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Why can we calculate moment of inertia, but not inertia?

Classically, the inertia of something is just its mass. If you want an analogous equation, just integrate the mass density $\rho$ of the object over the volume of the object: $$m=\iiint \text dm=\...
BioPhysicist's user avatar
42 votes

Why does a force not do any work if it's perpendicular to the motion?

As explained by SchrodingersCat, mathematically work is proportional to the scalar product of force and line element. Therefore any forces acting perpendicular to the path do not contribute to the ...
user1583209's user avatar
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42 votes
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Why is it easier to carry a person while spinning than not spinning?

Normally, lifting someone up while stationary requires significant activation of the anterior deltoid (a relatively small muscle). When spinning, that force translates to back muscles, which are ...
Greg Schmit's user avatar
41 votes
Accepted

If I toss a coin, vertically, on the surface of Mars, will it land back in my hand?

It depends on where on Mars you toss the coin, and how high you toss it. In a rotating frame of reference, an object in motion appears to be affected by a pair of fictitious forces - the centrifugal ...
Floris's user avatar
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41 votes
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Why certain rotations are unstable? (Euler Equations)

There is another nice way of seeing this mathematically. It is not too hard to show that in the body frame, there are two conserved quantities: the square of the angular momentum vector $$ L^2 = L_1^...
Michael Seifert's user avatar
40 votes
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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 ...
Steeven's user avatar
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40 votes

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 ...
John Rennie's user avatar
40 votes

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 ...
BioPhysicist's user avatar
40 votes

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 ...
Vercassivelaunos's user avatar
38 votes
Accepted

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 ...
Cleonis's user avatar
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38 votes

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 ...
Javier's user avatar
  • 28k
34 votes

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 $1894$ ...
Farcher's user avatar
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33 votes

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 ...
John Alexiou's user avatar
33 votes
Accepted

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 ...
Jbag1212's user avatar
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33 votes
Accepted

Does special relativity hold for rotating reference frames?

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}\...
Dale's user avatar
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