Greatest Hits
25
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What is the difference between weight and mass?
What is the difference between the weight of an object and the mass of an object?
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170
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2
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Why is my dryer radioactive?
My Geiger counter measures a background radiation level in my home of 0.09–0.11 μSv/h.
When I stick it inside the dryer right after it finishes a cycle (while the clothes are still inside), it ...
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9
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2
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Example of situation with conduction, convection, and radiation
I am studying heat transfer and have learned there are three kinds of heat transfer: conduction, convection, and radiation. Some examples are:
Conduction:
Touching a stove and being burned
Ice ...
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15
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6
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How does pressure of a fluid change with area, according to the continuity equation and Bernoulli's equation?
In a nozzle, the exit velocity increases as per continuity equation $Av=const$
as given by Bernoulli equation (incompressible fluid). Pressure is inversely proportional to velocity, so we have lower ...
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34
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7
answers
252k
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What is the difference between diffraction and interference of light?
I know these two phenomena but I want to know a little deep explanation. What type of fringes are obtained in these phenomena?
20
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8
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Why do bulbs glow brighter when connected in parallel?
Consider a circuit powered by a battery. If light bulbs are attached in parallel, the current will be divided across all of them. But if the light bulbs are connected in series, the current will be ...
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75
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4
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How does water evaporate if it doesn't boil?
When the sun is out after a rain, I can see what appears to be steam rising off a wooden bridge nearby. I'm pretty sure this is water turning into a gas.
However, I thought water had to reach 100 ...
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18
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6
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Why does the comb attract the pieces of papers if they're neutral?
When we rub our hairs with a comb, and then try to attract small pieces of paper, they're attracted by the comb. The pieces of the paper were not electrified before they were attracted. Then they ...
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26
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5
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Why does a glass rod when rubbed with silk cloth aquire positive charge and not negative charge?
I have read many times in the topic of induction that a glass rod when rubbed against a silk cloth acquires a positive charge. Why does it acquire positive charge only, and not negative charge?
It ...
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198
votes
7
answers
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How do moving charges produce magnetic fields?
I'm tutoring high school students. I've always taught them that:
A charged particle moving without acceleration produces an electric as well as a magnetic field.
It produces an electric field ...
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49
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3
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Why do we use Root Mean Square (RMS) values when talking about AC voltage
What makes it a good idea to use RMS rather than peak values of current and voltage when we talk about or compute with AC signals.
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798
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24
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Cooling a cup of coffee with help of a spoon
During breakfast with my colleagues, a question popped into my head:
What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon?
A qualitative answer would be ...
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53
votes
1
answer
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How does this "simple" electric train work?
In this YouTube video, a dry cell battery, a wound copper wire and a few magnets (see image below) are being used to create what can be described as "train". It looks fascinating but how does this ...
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85
votes
9
answers
216k
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Why doesn't the Moon fall onto the Earth?
Why doesn't the Moon fall onto the Earth? For that matter, why doesn't anything rotating a larger body ever fall onto the larger body?
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26
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7
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233k
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Why do two bodies of different masses fall at the same rate (in the absence of air resistance)?
I'm far from being a physics expert and figured this would be a good place to ask a beginner question that has been confusing me for some time.
According to Galileo, two bodies of different masses, ...
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14
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6
answers
255k
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What is the difference between phase difference and path difference?
The path difference is the difference between the distances travelled by two waves meeting at a point. Given the path difference, how does one calculate the phase difference?
62
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17
answers
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How can momentum but not energy be conserved in an inelastic collision?
In inelastic collisions, kinetic energy changes, so the velocities of the objects also change.
So how is momentum conserved in inelastic collisions?
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80
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10
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Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?
How can light be called electromagnetic if it doesn't appear to be electric nor magnetic?
If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of ...
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27
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5
answers
230k
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How does the freezing temperature of water vary with respect to pressure?
I know when the pressure is reduced, the boiling temperature of water is reduced as well. But how does the pressure affect the freezing point of water?
In a low-pressure environment, is water's ...
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34
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7
answers
287k
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How exactly does time slow down near a black hole?
How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...
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6
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10
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285k
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What does the magnitude of the acceleration mean?
I am a little confused as to what the magnitude of acceleration is and what it means.
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36
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5
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Jumping into water
Two questions:
Assuming you dive head first or fall straight with your legs first, what is the maximal height you can jump into water from and not get hurt?
In other words, an H meter fall into water ...
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65
votes
8
answers
153k
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Why doesn't the frequency of light change during refraction?
When light passes from one medium to another its velocity and wavelength change. Why doesn't frequency change in this phenomenon?
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52
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6
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282k
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Why is AC more "dangerous" than DC?
After going through several forums, I became more confused whether it is DC or AC that is more dangerous. In my text book, it is written that the peak value of AC is greater than that of DC, which is ...
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207
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10
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If photons have no mass, how can they have momentum?
As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
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184
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21
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162k
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Given Newton's third law, why are things capable of moving?
Given Newton's third law, why is there motion at all? Should not all forces even themselves out, so nothing moves at all?
When I push a table using my finger, the table applies the same force onto my ...
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44
votes
4
answers
274k
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Virtual vs Real image
I'm doing magnification and lens in class currently, and I really don't get why virtual and real images are called what they are.
A virtual image occurs the object is less than the focal length of ...
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2
votes
6
answers
223k
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Water pressure calculation for a volume of water at a given height
I'm looking to create a rain harvesting system. I have a 275 gallon IBC tote that is 48" x 40" x 46". I have an adapter for a 3/4 garden hose at the bottom of the IBC tote. I'm trying to figure out ...
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49
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8
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200k
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Difference between live and neutral wires
In domestic electrical circuits, there are 3 wires - live, earth and neutral. What is the difference between the live and neutral wires?
As there is AC supply, it means that there are no fixed ...
11
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3
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251k
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What is the sign of the work done on the system and by the system?
What is the sign of the work done on the system and by the system?
My chemistry book says when work is done on the system, it is positive. When work is done by the system, it is negative.
My physics ...
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12
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2
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145k
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How can a salt solution conduct electrical current?
How does a sodium chloride solution conduct electricity?
I know that sodium chloride dissociates into sodium and chloride ions in water, so when a voltage is applied, those ions can move. However, if ...
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13
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9
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What is a phase of a wave and a phase difference?
What is the meaning of the phase of a wave and phase difference? How do you visualize it?
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18
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5
answers
218k
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Which direction does air flow?
I remember learning this in high school, but have forgotten it, and can't seem to find it anywhere online.
Air travels from areas of high pressure to low pressure...correct? So if I have a cold room ...
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32
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4
answers
134k
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What is the difference between center of mass and center of gravity?
What is the difference between center of mass and center of gravity?
These terms seem to be used interchangeably.
Is there a difference between them for non-moving object on Earth, or moving objects ...
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53
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4
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353k
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On a hot day, when it's cooler outside than in; is it better to put a fan in an open window pointing inwards or outwards?
If it's really hot inside, but cooler outside; what is the best way to place a single fan to try and cool a room down?
I always assumed it would be better pointing inwards (and this thread suggests ...
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6
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4
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257k
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How to determine the direction of induced current flow?
There are three ways of inducing current in a loop/coil of wire as shown in my book. We can have a magnet approach a coil of wire, or a wire approaching a magnet. Both can be understood in the same ...
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4
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Rule sign for concave and convex lens?
I am just totally confused about the rule sign of convex and concave lenses.
The general formula: $$1/v-1/u=1/f.$$
Is okay but when solving problem sums sometimes $v$ becomes negative sometimes $u$ ...
6
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4
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168k
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Relation between Electric field and potential
I am unable to understand from this - sign comes. Which step I have done wrong?
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19
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8
answers
100k
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A simple derivation of the Centripetal Acceleration Formula?
Could someone show me a simple and intuitive derivation of the Centripetal Acceleration Formula $a=v^2/r$, preferably one that does not involve calculus or advanced trigonometry?
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5
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2
answers
60k
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What energy is transformed to heat when a candle is burned?
What energy is being transformed to heat when you burn a candle?
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26
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4
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213k
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Does the rotation of the earth dramatically affect airplane flight time?
Say I'm flying from Sydney, to Los Angeles (S2LA), back to Sydney (LA2S).
During S2LA, travelling with the rotation of the earth, would the flight time be longer than LA2S on account of Los Angeles ...
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12
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4
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Why is work a scalar and not a vector?
Work
(in
physics)
is
a
scalar. Why is it not a vector?
user29727
86
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14
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183k
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What is the difference between "kinematics" and "dynamics"?
I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics".
I first encountered this in Jackson's E&M book, where, in ...
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Why change in internal energy is zero in isothermal process?
In isothermal process $\Delta U =0$. But I am having trouble understanding it.
Say we have an ideal gas, and say my temperature is constant but I move the pressure, volume from $(P, V) \to (P-dP, V+...
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8
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What is the difference between stress and pressure?
What is the difference between stress and pressure? Are there any intuitive examples that explain the difference between the two? How about an example of when pressure and stress are not equal?
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What causes an electric shock - Current or Voltage?
Though voltage and current are two interdependent physical quantity, I would like to know what gives more "shock" to a person - Voltage or Current? In simple words, will it cause more "electric - ...
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20
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4
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103k
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Why does ice have a lower density than water?
Can someone explain me why is ice less dense than water?
As I know, all solids are usually denser than the liquids (correct me if I am wrong).
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4
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What exactly is the difference between radiation, conduction, and convection?
Okay, so everywhere I've read, I hear the main difference is the requirement of a medium. But for example, if you take the case of heat 'radiating' from a red-hot iron, isn't that actually convection ...
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Why do same/opposite electric charges repel/attract each other, respectively?
I know plus pushes another plus away, but why, really, do they do that? On the other hand, molecules of the same type are attracted to each other. I find that weird.
I do know some stuff about four ...
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