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114 votes
Accepted

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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47 votes
Accepted

When a bus goes around a corner, does the person sitting at the back travel further distance than the person sitting at the front?

The center of the front axle must travel along a larger arc than the center of the rear axle or any point between those points in order for the vehicle to avoid whatever obstacle the vehicle is ...
g s's user avatar
  • 13.9k
45 votes

Could one perform a test to determine whether water was warmed by a microwave rather than over a flame?

No, and this is one of the important facts about thermodynamics: Once a system has reached equilibrium, its "thermal history" (how it got heated in the first place) has been erased and ...
niels nielsen's user avatar
39 votes
Accepted

Why do disk brakes generate "more stopping power" than rim brakes?

For both rim brakes and disk brakes the limiting factor is the friction between the tyres and the road i.e. both types of brake can apply enough force to make the wheels skid. So it is untrue that ...
John Rennie's user avatar
38 votes

Would a giant ball on earth roll towards the poles?

The brief answer is no. The Earth is that oblate shape due to gravity and centripetal forces reaching an equilibrium, where essentially the gravitational potential is the same everywhere (at sea level)...
KDP's user avatar
  • 5,321
37 votes
Accepted

How does a violin produce notes, microscopically?

Of course, the bow string does not know when to attach and release a violin string, it is the choice of materials which dictates this. When viewed under a microscope it is possible to see the tiny ...
Farcher's user avatar
  • 97.2k
37 votes
Accepted

Why do the pieces of breaking objects scatter?

Generally, when an object collides with the floor there will be some sort of asymmetrical deformation of the object before it ultimately fails. This deformation puts a rather large amount of strain on ...
Matt Hanson's user avatar
  • 3,162
35 votes

Why doesn't the windshield fog up where my kid drew on it with her fingers?

An oily surface is poorly-wetted by water molecules whereas a glass surface is well-wetted by water molecules. This is because glass consists of silicon, calcium, sodium and aluminum oxides. This ...
niels nielsen's user avatar
34 votes
Accepted

Does the fingernail test for detecting two-way mirrors really work?

A standard mirror consists of a glass pane with a reflective backing. The reflection that you see comes from the reflective backing, not the glass. The principle of this test is that if you see a gap ...
gandalf61's user avatar
  • 55.8k
31 votes

Why is rock or metal often cold to the touch but wood or plastic is not?

What we perceive as an object being "hot/ cold to the touch" is related to the rate at which heat is transferred from the object to your hand. In the case of touching an object with your ...
Nic Christopher's user avatar
30 votes

When a bus goes around a corner, does the person sitting at the back travel further distance than the person sitting at the front?

The opposite is true, the person with the longest track is always the driver. The point is that all busses that I know of have the front wheels steered. The remaining axle(s) simply follow the front ...
cmaster - reinstate monica's user avatar
28 votes

Why is an arrow pointing through a glass of water only flipped vertically but not horizontally?

Because it is a cylinder, which is a lens, along one axis, and homogeneous along the other. :) As in @fectin's comment: if it were a sphere, rather than a cylinder, then it would flip through the ...
paul garrett's user avatar
28 votes

Why do water particles appear to be exiting a stream of water from a faucet, instead of the stream being held together by surface tension?

Did your faucet nozzle contain a bubbler (which mixes air into the water and produces a white, frothy stream down to the bottom of the sink)? Most kitchen faucets do, and when the bubbles closest to ...
niels nielsen's user avatar
27 votes
Accepted

How does the mirror know what’s behind the paper?

Perhaps a diagram would help? Excuse my poor artistry: Clearly, you can see that there is a trajectory a ray can take which starts at the top of the egg, hits the mirror, and then intersects your eye....
Riley Scott Jacob's user avatar
26 votes

How does a violin produce notes, microscopically?

We don't hear a discontinuous noise from a violin; rather we hear smooth continuous notes. You're taking that for granted. You can produce a discontinuous noise from a violin quite easily, and all ...
DKNguyen's user avatar
  • 9,319
26 votes

Could one perform a test to determine whether water was warmed by a microwave rather than over a flame?

I agree with niels nielsen's answer at the spherical cow level, but I can see two factors that will be different in the real world. Whether they can be measured or not I do not know. Both come down ...
Loren Pechtel's user avatar
24 votes
Accepted

Why is rock or metal often cold to the touch but wood or plastic is not?

In a room at normal room temperature, certain materials, such as metal, glass, ceramic, or rock, will feel cold to the touch, but others, such as wood or plastic, do not so much. When you touch ...
Bob D's user avatar
  • 72.8k
24 votes

Why do disk brakes generate "more stopping power" than rim brakes?

First off, disk brakes do not offer "better stopping power", that claim of the article is incorrect. However, it doesn't matter. Both disk and rim brakes are able to provide enough stopping ...
Draco-S's user avatar
  • 409
24 votes

On a tinted (reflective) window, why do I need to look from up close to see inside?

Things are called a one-way mirror when the amount of reflected light overwhelms the amount of transmitted light. When you put your head up to the glass (and maybe put your hands up surrounding your ...
BowlOfRed's user avatar
  • 40.6k
23 votes

Does the fingernail test for detecting two-way mirrors really work?

The fingernail test simply measures the distance between the surface of the mirror and its reflective layer. It tells you nothing about the reflective layer itself. Neither does it tell you whether ...
cmaster - reinstate monica's user avatar
22 votes

Why do headphone speakers sound 'tinny' when far from the ear?

The speakers in a headphone couple poorly to 3-dimensional space full of air- that is, there is a severe impedance mismatch between the itsy bitsy speaker cone in the earcup and the air that is more ...
niels nielsen's user avatar
21 votes

Why do the pieces of breaking objects scatter?

What you describe is the shift from rigid body dynamics to fluid dynamics. Let's take your fragile object falling on earth to the grandest scale. A planetary collision. The planets don't bounce or ...
AlphaLife's user avatar
  • 12.3k
20 votes
Accepted

Why can you hammer a hole into an underwater ceramic cup?

First some background. Glass and ceramics are stiff, but not totally rigid. Thin sheets are surprisingly flexible. To break a sheet of glass along a straight line, you scratch a line along the surface....
mmesser314's user avatar
20 votes

Why does microwave take more time to heat more food?

The microwave transfers a certain amount of energy per time. This is known as the power $P$, and is what you set when choosing how many watts you want: $1\mathrm{W} = 1\mathrm{J}\cdot \mathrm{s}^{-1}$....
paulina's user avatar
  • 1,305
19 votes

Why do disk brakes generate "more stopping power" than rim brakes?

Most answers here focus on braking force. On the other hand, the Question is about the power and we are at Physics.SE so this is what happens with the braking power: Braking generates heat. A steep ...
fraxinus's user avatar
  • 7,916
18 votes

Rotating a Pot of Boiling Water on a Stove

When you rotate the pot, you are jiggling the pot and stirring up the boundary layer between the pot surface and the bulk water. Shaking & stirring the water increases the chances of temporarily ...
niels nielsen's user avatar
17 votes

Can I get burned with warm air?

Not really. In a more precise language, what you are describing is that temperature is an ensemble property of particles, and individual particles can store more energy (in case of a gas: mostly be ...
Wrzlprmft's user avatar
  • 6,294
16 votes
Accepted

How does a Magic Mirror work?

This is commonly called a "one way mirror", which really isn't a good name. It is really two way. It is a partially reflective, partially transmissive mirror that works the same either ...
mmesser314's user avatar
16 votes

How does the mirror know what’s behind the paper?

The other answers are quite right, of course, but I thought it might be helpful to know the comic has a second page in which Aki Hayakawa explains how mirrors work. This prompts a rather... crude ...
Sturrum's user avatar
  • 459
15 votes
Accepted

Why a car stops slower when you press the brake pedal fully rather than progressively?

I assume you are referring to a car without ABS brakes. If the wheels lock, then the wheels skid, and the friction force exerted by the ground on the wheels slowing the car down changes from static ...
Bob D's user avatar
  • 72.8k

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