Scalar quantity which is the magnitude of velocity.
Questions tagged [speed]
858 questions
314
votes
18 answers
Why does kinetic energy increase quadratically, not linearly, with speed?
As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $\frac{1}{2}mv^2$.
Why does this not increase linearly with speed? Why does it take so much more energy to go from $1\ \mathrm{m/s}$ to…
Generic Error
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96
votes
9 answers
Why is there a controversy on whether mass increases with speed?
Some people say that mass increases with speed while others say that the mass of an object is independent of its speed.
I understand how some (though not many) things in physics are a matter of interpretation based on one's definitions. But I can't…
User 17670
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85
votes
6 answers
If a mass moves close to the speed of light, does it turn into a black hole?
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.
From the show transcript:
Arunus Gidgowdusk…
shopsinc
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61
votes
3 answers
In a roller coaster, does the rear car have a higher acceleration/speed?
I am wondering about this question since I asked myself: why do people feel more weightless in the rear car of a roller-coaster than in the front car?
To feel the effect of weightlessness, you must accelerate at the acceleration of the gravity…
Citizen602214085
- 647
51
votes
4 answers
Can a tomato pierce a hole in a steel plate if only the tomato is travelling fast enough?
A tomato is travelling very fast towards a 1 cm thick steel plate.
Let's say this happened in a vacuum, so that the air resistance wouldn't rip the tomato apart before it even hit the steel plate.
Obviously the tomato would get destroyed too, but…
Fiksdal
- 656
50
votes
6 answers
The speed of sound is a single value yet the speed of atoms is distributed over many values. Does the sound wave front smear out?
Sound waves travel with constant speed, but air molecules that transfer action move with different speeds than the ones described by Maxwell distribution.
Why does the sound wave not smear out and dissipate quickly?
UPDATE. It is not just that…
Vashu
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45
votes
7 answers
Is it fair to judge this speedskating race by only 3 thousands of a second?
I'm reading this article:
Zbigniew Brodka of Poland won the Olympic men's 1,500 meters speed
skating title by just 0.003 seconds at the Adler Arena on Saturday.
Brodka clocked one minute, 45.006 seconds with Koen Verweij of
Netherlands crossing…
Kasper
- 551
45
votes
5 answers
Speed of light vs speed of electricity
If I arranged an experiment where light raced electricity what would be the results? Let's say a red laser is fired at the same time a switch is closed that applies 110 volts to a 12 gauge loop of copper wire with a meter at a distance of ten…
Lambda
- 4,753
41
votes
5 answers
Why doesn't Earth appear smudgy or blurred in space photographs due to its fast rotation?
Whenever we take a photograph of something moving at a considerably high speed, its image appears fuzzy/smudgy/distorted due to motion blur. Why doesn't this happen in case of Earth photographs from space, taking into account the fact that Earth…
user139580
36
votes
13 answers
Why don't we use rapidity instead of velocity?
In school we learn that we can add velocities together, and then later on we learn that it's not correct and that there is a speed limit. Why create all this confusion when we could just use rapidity to begin with?
Rapidity is defined as $w =…
Fax
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35
votes
3 answers
How does electricity propagate in a conductor?
On a systems level, I understand that as electrons are pushed into a wire, there is a net field and a net electron velocity. And I've read that the net electron drift is slow. But electricity travels through the wire, essentially at c, and I want…
Dov
- 1,035
30
votes
4 answers
Speed of sound at temperatures below 0 °C
How can the speed of sound be calculated for temperatures below 0 °C (down to -40 °C)?
Does the calculation $v=331\ \frac{m}{s} + 0.6 \frac{m}{s°C} \times T$ still hold (where T's unit is °C)?
user40343
- 269
30
votes
6 answers
Speed of electrons in a current-carrying metallic wire: does it even make sense?
Does it make sense to speak about the speed of electrons in a current-carrying wire (non perfect conductor)? If so, what is their speed?
Here are my thoughts:
On the Internet (Wikipedia, physicsforums, here on PSE, etc., and many other websites),…
30
votes
9 answers
Can a human size object move so fast that it ceases to be observable?
In many anime, comics, movies, etc, we see a lot of super human beings moving and fighting at such high speeds that a regular human being cannot see that they are fighting or moving pass by.
In particular, in the first battle between Goku and…
user1948847
- 435
29
votes
9 answers
What is the speed of the fastest moving body in our solar system?
On Wikipedia I saw that the average orbital speed of planet Earth around the Sun is a whopping $29 783\text{ m/s}$, and it made me wonder are there bodies (planets, meteorites, asteroids) that move faster?
My question is not about small photons or…
Willemien
- 381