814 votes

Can I compute the mass of a coin based on the sound of its fall?

So, I decided to try it out. I used Audacity to record ~5 seconds of sound that resulted when I dropped a penny, nickel, dime, and quarter onto my table, each 10 times. I then computed the power ...
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  • 13k
322 votes

Can I compute the mass of a coin based on the sound of its fall?

If you have the dimensions and material of an object, you can compute both the mass and the normal vibration modes. Just the mass is not enough - a large paper "coin" will have a different fundamental ...
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  • 116k
318 votes
Accepted

Why do ballpoint pens write better on pages that have pages below them?

I'd say the culprit is the contact area between the two surfaces relative to the deformation. When there are other pieces of paper below it, all the paper is able to deform when you push down; ...
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  • 14.8k
269 votes
Accepted

Does Earth really have two high-tide bulges on opposite sides?

There is no tidal bulge. This was one of Newton's few mistakes. Newton did get the tidal forcing function correct, but the response to that forcing in the oceans: completely wrong. Newton's ...
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  • 39.2k
250 votes
Accepted

Strange ice found in my garden

Congratulations, you found an inverted pyramid ice spike, sometimes called an ice vase! The Bally-Dorsey model of how it happens is that first the surface of the water freezes, sealing off the water ...
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235 votes
Accepted

Why don't fluorescent lights produce shadows?

To complement Floris's answer, here's a quick animation showing how the shadow changes with the size of the light source. In this animation, I've forced the intensity of the light to vary inversely ...
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  • 1,787
229 votes
Accepted

Why do phones land face down?

A physicist working at Motorola actually did this experiment as part of a promotional push for shatter-proof screens. This same physicist had previously written a paper on the same question, applied ...
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  • 1,529
222 votes

Why does ice cream get harder when colder?

A couple of decades ago I was peripherally involved with some research on the properties of ice cream being done by the company Walls in the UK. The work was on relating the consistency of the ice ...
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210 votes
Accepted

Why does a mirror split my laser beam?

You are getting reflections from the front (glass surface) and back (mirrored) surface, including (multiple) internal reflections: It should be obvious from this diagram that the spots will be ...
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  • 116k
199 votes

How do towels stay on hooks?

Since this is PhysicsSE, I am happy with an answer based purely on theoretical analysis of the forces involved. Oh boy, time to spend way too much time on a response. Lets assume the simple model ...
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  • 3,639
183 votes
Accepted

When separating an Oreo cookie, why does the cream stick to just one side only?

The "stuff" sticks to itself better than it sticks to the cookie. Now if you pull the cookies apart, you create a region of local stress, and one of the two interfaces will begin to unstick. At that ...
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  • 116k
175 votes

Which is stronger, a rope without knots or a rope with knots?

The fibers in a rope strand are 'layed' stretched out straight, then twisted. Multiple strands are combined, by twisting, to make the rope. When a rope is pulled taut, all the fibers have similar ...
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  • 8,925
171 votes

Why can't we see images reflected on a piece of paper?

Because the real situation looks a lot more like this: Your pen is (presumably) not made of mirror-like polished metal, but rather of something like wood or plastic that reflects light diffusely. ...
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170 votes
Accepted

Why can I touch aluminum foil in the oven and not get burned?

You get burned because energy is transferred from the hot object to your hand until they are both at the same temperature. The more energy transferred, the more damage done to you. Aluminium, like ...
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163 votes
Accepted

Why are the wet patches on these floor tiles circular?

Both thawing and evaporation involve heat exchange between the stone tile, the water sitting atop the stone tile, any water that's been absorbed by the stone tile, and the air around. The basic reason ...
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  • 70.1k
161 votes

Why does a lot of water vapour come suddenly after the heat source of boiling water is removed?

What you are seeing is not actually vapor - vapor is invisible. The mist seen above boiling water, commonly but inaccurately called vapor, is actually made of tiny droplets of liquid water, formed ...
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  • 1,381
157 votes
Accepted

Why does paper cut through things so well?

Paper, especially when freshly cut, might appear to have smooth edges, but in reality, its edges are serrated (i.e. having a jagged edge), making it more like a saw than a smooth blade. This enables ...
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138 votes

Why are the windows of bridges of ships always inclined?

Look at CandiedOrange's answer This answer was accepted, but CandiedOrange has the right answer. See this document page 21: The second way in which reflection can interfer e with controller’s ...
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  • 42.6k
138 votes
Accepted

Why doesn't water actually perfectly wet glass?

In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract ...
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136 votes

Do the weights of two liquids not add when mixed?

Of course, by common sense, if you put together two objects with masses $m_1$ and $m_2$, and nothing comes out, then you end up with mass $m_1 + m_2$. Weights are a little more complicated because ...
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  • 94.9k
135 votes
Accepted

How does this baby rattle work?

As you said, it's probably not magnetism if the balls are free to rotate; there is no reason they wouldn't just flip over and stick together, north to south. You can test this by buying some of those ...
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  • 94.9k
131 votes
Accepted

Why is oil a better lubricant than water?

Your derivation is composed of correct statements and indeed, if something is known to act as a lubricant, we want the viscosity to be as low as possible because the friction will be reduced in this ...
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128 votes

How is 6W equivalent to 40W, as claimed by adverts for LED light bulbs?

A 40W incandescent light bulb has a luminous efficiency of 1.9%. That means only 1.9%, or 0.76W, of the energy consumed by the bulb ends up as visible light. LED bulbs have an efficiency of around 10%...
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126 votes
Accepted

Why does water stop boiling immediately after turning off the heat?

In large part because under normal circumstances water doesn't get hotter than boiling - at that point it becomes steam, as you know. You can add heat and boil it away faster, but the water can only ...
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  • 1,016
125 votes

Why can I touch aluminum foil in the oven and not get burned?

While it's true that the difference in specific heat capacity is to your advantage, its effect is really dwarfed by the mass difference. Typical aluminium foil is 0.016 mm thick and weighs 0.043 kg/m²,...
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124 votes
Accepted

How do towels stay on hooks?

There is some contribution from the friction of the various surfaces, but the main factor is the balancing of weight. It's important to note that the hook is set slightly away from the wall, which ...
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  • 2,578
121 votes
Accepted

Why does my wooden door disperse light into a rainbow color spectrum?

You have created a rather poor pinhole camera (camera obscura). You can see an "image" of the sky, a green space (trees) and even a reddish brown blur that is your driveway. This is not ...
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  • 116k
112 votes

Why can we see the moon when it is between the Earth and the Sun?

The premise of this question is wrong. If the moon is in between the earth and the sun (as shown on your diagram), and you can see the moon, then it is day, not night: If on the other hand, you are ...
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  • 674
112 votes

Why don't we use infrared light to heat food?

We do use (near) infrared radiation to heat food – whenever we toast food or grill (UK)/broil (US) by beaming infrared downwards on to food! The point is that the infrared is strongly absorbed by the ...
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