# Greatest Hits

540k views

### 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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79k views

### 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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708k views

### 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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502k views

### 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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252k views

### 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?
410k views

### 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 ...
332k views

### 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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176k views

### 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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256k views

### 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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235k views

### 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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219k views

### 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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259k views

### 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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118k views

### 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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216k views

### 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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233k views

### 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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255k views

### 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?
214k views

### 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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176k views

### 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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230k views

### 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 ...
287k views

### 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 ...
285k views

### 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.
426k views

### 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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153k views

### 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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282k views

### 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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263k views

### 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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162k views

### 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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274k views

### 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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223k views

### 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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200k views

### 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 ...
251k views

### 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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145k views

### 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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146k views

### Why do diamonds shine?

I have always wondered why diamonds shine. Can anyone tell me why?
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173k views

### 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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218k views

### 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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134k views

### 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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353k views

### 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 ...
257k views

### 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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1 vote
157k views

### 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 $u$ ...
168k views

### 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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100k views

### 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?
60k views

### 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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213k views

### 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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156k views

### Why is work a scalar and not a vector?

Work (in physics) is a scalar. Why is it not a vector?
183k views

### 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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160k views

### 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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145k views

### 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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108k views

### 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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103k views

### 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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161k views

### 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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