# Tagged Questions

Scalar quantity which is the magnitude of velocity.

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### How can time-travel be possible if speed is relative? [duplicate]

I have heard that time-travel is possible...relative to some observer. So, as I understand it, the following example would be accurate: There are two twins- TwinA and TwinB. Both have very accurate ...
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### Drift speed of electrons when the object is grounded

So I know that the drift speed of electrons is usually pretty slow. Let's say I have a charged sphere and I would ground it over a wire. How fast would the electrons leave the sphere? Would that drift ...
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### Distinguish between instantaneous speed and instantaneous velocity

I encountered a line in my text book of physics that: Average speed over a finite interval of time is greater or equal to the magnitude of the average velocity. But instantaneous speed at an ...
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### comparing torque and 0-60 speed in cars

Im looking a two cars, both are the same model but ones got a 2.0L turbo petrol engine and ones got a 3.0L turbo diesel engine. These are the full specs 2.0L Petrol 3.0L Diesel what im ...
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### What happens when the speed of a body is less than optimum speed?

Yesterday I learned about optimum speed. My teacher told that optimum speed is the speed with which one can take turns on banked road without wear and tear. $v_0=\sqrt{rg\tan\theta}$, $v_0$= ...
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### How fast can fire spread?

You can build a home-made flamethrower by using a spray and a lighter: (from this YouTube video) But this can be dangerous if the fire gets in the can and the can explodes. So I guess the speed at ...
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### Is there any significance to the negative sign on this speed?

A girl is riding a bicycle along a straight road at constant speed, and passes a friend standing at a bus stop (event #$1$). At a time of $60$ s later the friend catches a bus (event #$2$) If the ...
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### How could we define speed without time? [closed]

This is a bit of a brain twister but it's a very serious question. Please be precise in your answer. I do not believe time exists. I believe it is simply an illusion of our perception, the ...
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This stems from a riddle I read in a magazine perhaps 20 years ago so I apologise for the imprecise recollection. A dog that can run infinitely fast is placed on an infinitely large flat surface and ...
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### Where does energy go when performing a useless effort?

I went to school one day, so I thought I was able to get this simple one.. but it looks like I'm not anymore. :( One lonely little spaceship is resting into space. It has a small fuel capacity that ...
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### How does acceleration affect the orbit?

I want to know what roughly happens if we hit an orbiting body a little. And how is it possible that most of the objects have seemingly circular orbits around. How do they stabilize? I ask because it ...
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### How does the speed of electrons change around a circuit?

I have been thinking about ways of teaching electronics and I'm wondering if the following is true... For starters, when we talk about voltage as energy per unit charge, is this energy manifest ...
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### Rapids in a River

Can I expect to have bigger or smaller rapids in a river that has a water level that is higher than average? I am not sure which way this would go. With the lower water, one is closer to the rocks, ...
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### Speed of Water in a River

Does a river flow faster the higher the water level - or does it go at exactly the same speed? I was recently tubing in a river and the water seemed to be going faster with the water level being ...
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### How to calculate fuel consumption of car (mpg) from speed and accleration knowing mass, drag coeff and rolling resistance?

How can I calculate the current (instantaneous) mpg of my car if I know the speed and acceleration of the car? From reading various answers for the "car going level or up/down hill" question asked ...
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### -Thought Experiment- At superluminal speeds, to what degree would vision be impaired? [closed]

I was thinking about superluminal travel when thinking about advanced civilisations in the future. I was also thinking about the Doppler effect; so I decided to think about how light may be affected ...
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### Kinematics - Concept of average velocity [closed]

Average velocity, as I have heard, cannot be found simply by finding the average of two numbers. I have a question on average velocity, but am simply unable to proceed: A particle moving in a ...
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### Geostationary orbit and rotation speed of a planet

Assume for planets similar to Earth but with different rotation speeds (including zero), and we put a satellites at altitude of 35 786 km/hr (similar to the Geostationary orbit or earth), with speed ...
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### Effects of gravity on light [duplicate]

If gravity can bend light, why can't gravity slow light. At least momentarily? Wouldn't that give the illusion of the universe expansion speeding up?
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### Photon speed going from $0$ to $c$ initially? [duplicate]

I have read many questions which ask whether there can be photons at speed other than the speed of light and all of them are answered no! But when the photon is created for ex during electron ...