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164 votes

Can ants walk on liquid mercury?

Needing any excuse to break away from the work I was doing, I immediately assumed the task of answering this question. Yes, ants can walk on mercury with no trouble at all. I bet it was even kind of ...
147 votes

When I walk down the stairs where does my potential energy go?

Make it simple. If a mass of your weight fell down the height of three flights of stairs through the air, when it landed where would the kinetic energy accumulated by falling go? moving the earth ...
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90 votes
Accepted

How does light combine to make new colours?

Color perception is entirely a biological (and psychological) response. The combination of red and green light looks indistinguishable, to human eyes, from certain yellow wavelengths of light, but ...
87 votes
Accepted

Stood in front of microwave with the door open

The very first thing you should do is stop using your oven and have it checked out by an authorized repair service. If in fact the oven was operating with the door open, there was a failure of the ...
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85 votes
Accepted

Does physics explain why the laws and behaviors observed in biology are as they are?

To get more insight into a question like this, you might like to ponder the relationship between logic gates and programming languages in the case of computers. This is a lot simpler than the physics—...
78 votes
Accepted

Could I survive at (or near) absolute zero with a very, very, very thick sweater?

A super-thick sweater probably isn't the way to go - you may be better off wrapping yourself in aluminum foil. The body loses heat through a handful of mechanisms: During conduction, your body ...
  • 55.6k
74 votes
Accepted

Why can we distinguish different pitches in a chord but not different hues of light?

This is because of the physiological differences in the functioning of the cochlea (for hearing) and the retina (for color perception). The cochlea separates out a single channel of complex audio ...
74 votes

How can we discern so many different simultaneous sounds, when we can only hear one frequency at a time?

But that still means the ear is only moving at one frequency at any given time No, it doesn't mean that at all. It means the eardrum is moving with a waveform that is a superposition of all the ...
  • 23.9k
65 votes
Accepted

How do photons get into the eyes?

Yes - we are surrounded by a "sea of photons". An individual object that reflects light (let's assume a Lambertian reflector - something that reflects incident photons in all directions) sends some ...
  • 117k
62 votes
Accepted

Can ants walk on liquid mercury?

Whether or not a small animal/insect can walk on a liquid is determined much more by surface tension than by density. To see why this is consider a dense liquid without any surface tension. You would ...
53 votes
Accepted

When I walk down the stairs where does my potential energy go?

The heat is predominantly generated in your muscles. More direct conversion of potential energy to heat is when a person is sliding down a pole to get to a lower floor quickly. With sufficient ...
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49 votes
Accepted

How can ultrasound hurt human ears if it is above audible range?

Ultrasound is used to break kidney stones and "burn" brain tumors. So no surprise that it can damage body parts, including inner ear. The effects of ultrasound on human body are multiple. ...
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48 votes

Is it safe to keep uranium ore in my house?

With a half-life of 4 billion years, uranium is only very weakly radioactive. In fact, since uranium is a heavy metal, its chemical toxicity is actually more of a danger than its radioactivity. If you ...
  • 2,355
45 votes

Why can we distinguish different pitches in a chord but not different hues of light?

Our sensory organs for light and sound work quite differently on a physiological level. The eardrum directly reacts to pressure waves while the photoreceptors on the retina are only senstive to a ...
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43 votes

Why aren't 100% UV blocked sunglasses safe to view an eclipse with?

You are correct that almost always it is the UV content of sunlight and not its power that is the main hazard in staring at the Sun. The lighting during a total eclipse is one of those situations ...
43 votes

Why can't the human eye focus to make blurry photos/video clear?

The human eye focusing is resolving all the possible detail it can from a scene that is sharp and not distorted. The details of exactly how your brain forms an image from what your eye does is ...
42 votes
Accepted

How is it possible for other animals to have better night vision than humans, who can detect individual photons?

That research shows that humans can detect single photons, not that we're particularly good at it. Averaging across subjects’ responses and ratings from a total of 30,767 trials, 2,420 single-...
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42 votes
Accepted

Why do different letters sound different?

You don't sing a single pitch - you sing a frequency and its harmonics. Using a simple spectrum analyzer, this is me "singing" the letter A and M, alternately (AMAMA, actually): The letter "A" is ...
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41 votes
Accepted

If we feel it's hotter when humidity increases, then why do we feel it's colder when inside water?

You feel cold when heat is flowing from you to the surroundings, your body tries to burn more energy to keep up your temperature, so you shiver. Water conducts heat much more effectively than air (...
41 votes

Why can't the human eye focus to make blurry photos/video clear?

Here's an explanation using geometric optics. I'll replace the human eye, which has a quasirandom scattering of light sensors on a curved surface, with a digital camera that has a regular grid of ...
  • 19.4k
40 votes
Accepted

Does pure yellow exist in variations we can't discern?

Our ability to separate different colors from each others depends crucially on how many different receptors we have for colored light. Humans have three different receptors for light, which means ...
38 votes

What happen if all the carbon-14 atoms in a person body decays at once?

There is about 1 radioactive $^{14}$C atom for every $10^{12}$ $^{12}$C atoms. With a half-life of 5730 years, this means there are usually about 0.2 decays per gramme of carbon per second. Carbon is ...
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37 votes
Accepted

Is it safe to keep uranium ore in my house?

Assuming the measurement is 1µSv per hour, looking at XKCD's handy radiation chart suggests that it is equivalent to eating 10 bananas or having an arm X-ray. If you were staying around the ore it ...
36 votes
Accepted

Why aren't 100% UV blocked sunglasses safe to view an eclipse with?

The damage to your eyes comes from the total energy from the visible and near - infrared region even when you wear a 100% UV blocked sunglasses. When you look at the sun in normal days, the visible ...
  • 724
35 votes

Does physics explain why the laws and behaviors observed in biology are as they are?

Emergence Physics does not determine every facet of biology! Biology is a great example of what is called emergence - the phenomena where the composite of many simple entities has properties that the ...
  • 42.3k
33 votes

Does pure yellow exist in variations we can't discern?

Mikael Fremling's answer is excellent, but here is just a little more detail: The light that hits your eye is a mixture of many different pure wave lengths, all at different intensities. The red ...
  • 12.7k
32 votes
Accepted

Why can our bodies bleed?

The Systolic and Diastolic blood pressure are measured relative to the atmospheric pressure, hence they are "stronger" than atmospheric pressure. The blood pressure measured (Systolic and ...
31 votes

Humans have an average energy budget of $100$ Watts, but the power radiated from the body is $1000$ Watts?

You are forgetting that you also absorb radiation from the environment. The formula you want is $$P_\text{net} = \epsilon A\sigma\left(T_\text{skin}^4 - T_\text{env}^4\right)$$ You can find more ...
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31 votes
Accepted

What determines whether colors you can't see are visible or not?

Color is a double valued variable.For physics there is a one to one correspondence between frequency of light and the color assigned to visible frequencies. As far as the spectrum of colors (rainbow) ...
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