51 votes
Accepted

Why is it harder to hear someone underwater than on air?

Impedance mismatch. The impedance ratio or the admittance ratio (admittance = inverse of impedance) describes how much of a wave is reflected or transmitted at the boundary of two media depending on ...
  • 680
32 votes

When a cork is pulled out of a wine bottle, why does the inner end often expand more than the outer end?

It could simple be that the end of the cork that was in the bottle has greater moisture content so it has a tendency to expand. Wood typically expands when it absorbs water. If you waited long enough ...
  • 359
17 votes

Why is it harder to hear someone underwater than on air?

TL; DR: most of the sound (energy) is reflected from water surface. If we think about an actual setup of an experiment when we cannot hear somebody while underwater, the person that we want to hear is ...
  • 42.2k
16 votes

When a cork is pulled out of a wine bottle, why does the inner end often expand more than the outer end?

Wine corks are cylindrical before they go into the bottle. When you remove the cork then they may have different shapes. Not all corks have the tapered shape. Only some. Champagne corks The tapered ...
4 votes

When a cork is pulled out of a wine bottle, why does the inner end often expand more than the outer end?

Just to add another random guess (tho' at least for natural cork I agree it's largely moistoure): If the cork is long enough that the bottom extends past the narrow portion of the neck, then that ...
3 votes

If you moved your hand continuously through fire, would it feel hot?

I am answering because I can't comment Bob D (my reputation is below 50) As Bob D said for burn to occur the minimum intensity is 2.5 to 5 watt/cm² (in SI units 25000 to 50000 watt/m²) So reason one ...
2 votes
Accepted

Cup of Tea - Luxury Item?

To estimate that, https://en.wikipedia.org/wiki/Specific_heat_capacity tells you that the heat capacity of water is $4.184 \mathrm{~kJ}/(\mathrm{kg}\, \mathrm{K})$. Hence, to heat $1.5\mathrm{~l}$ of ...
  • 3,722
2 votes

Cup of Tea - Luxury Item?

My electricity costs roughly USD \$0.10 per kilowatt-hour. My electric stove has big burners at 2500 watts and small burners at one kilowatt. Making tea on the small burner requires substantially ...
  • 75.7k
2 votes

Where is energy for work performed by unpowered objects coming from?

The existing answers as of time of posting are incorrect as regards balloons, except in some cosmic sense tracking the energy of every particle through all of time. For example: We expend 286 kJ to ...
  • 9,815
2 votes

What's the reason that it requires less effort to walk against an escalator (to stay stationary) than walking up stairs?

The Galilean relativity principle is old physics but still more than correct enough for use in this type of situation. This states that the laws of physics are the same in in all inertial frames of ...
  • 580
2 votes

SHOCKED by tile floors in a condo

You have current flowing through the tiles somehow, probably from the mains through failed electrical insulation. This is not normal. It is dangerous. You need to call an electrician to sort this out.
  • 6,163
1 vote

When a cork is pulled out of a wine bottle, why does the inner end often expand more than the outer end?

I'm also not a cork expert, but maybe the answer is more straightforward. It can ofcourse be the case that the cork already was asymmetric from the start. The purpose of the cork is to seal the bottle,...
1 vote

Why do scissors perform so well (compared to knives) when cutting hair/paper, with much flater blade angle?

Presumably the phenomena follows a description like this: Step Scissors Knife 1 The operator presents the scissors to the hair, holding the hair slack so that the point to cut is stable The operator ...
1 vote

What is the angular velocity of a wheel as a function of time as it is spun up by a DC motor?

Imagine a generic motor torque curve that looks like this There are three distinct regions to this curve, that you might encounter with DC motors. We will attempt to devise equations for speed versus ...
1 vote
Accepted

If you moved your hand continuously through fire, would it feel hot?

Is this because your hand spends such little time in the fire that there isn't enough time for conduction to occur? Pain occurs when the skin temperature reaches the threshold of pain. The ...
  • 58.2k
1 vote
Accepted

Why do things have colour? (reflect certain wavelengths of light)

An object generally does reflect light in a multitude of wavelengths, just some more than others. In an apple, the color of the skin is determined by the balance of pigments it contains. In a red ...
1 vote

Eyes shut, can a passenger tell if they’re facing the front or rear of the train?

I once conducted such an experience, just out of curiosity, when I was traveling on a train. I went out into the corridor, closed my eyes, they twisted me several times and put me down. I couldn't ...
1 vote

Where is energy for work performed by unpowered objects coming from?

Both are examples of what can be thought of as potential energy. For the ballon, it cost energy to gather that helium and put it in a ballon. Just as it will cost energy to reel that ballon back down ...
  • 1,285
1 vote

Where is energy for work performed by unpowered objects coming from?

To fill a helium ballon you have to do work. either to extend the rubber or to expel the air from a container, then you can lift a weight say 10 m. and put it down there, to get the ballon down the 10 ...
  • 4,030

Only top scored, non community-wiki answers of a minimum length are eligible