The time rate of change of the position of an object
Questions tagged [velocity]
2337 questions
78
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14 answers
Does throwing a penny at a train stop the train?
If I stand in front of a train and throw a penny at it, the penny will bounce back at me.
For the penny to reverse its direction, at some point its velocity must go to zero. This is the point it hits the train. Two objects in contact have the same…
Whose
- 767
66
votes
10 answers
Why does a free-falling body experience no force despite accelerating?
Note: For the purposes of my question, when I refer to free fall assume it takes place in a vacuum.
From my (admittedly weak) understanding of the equivalence principle, falling in a gravitational field is physically indistinguishable from floating…
AdamJames
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64
votes
7 answers
If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?
Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm traveling faster than the speed of light?…
ed209
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58
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7 answers
Why isn't the Euler-Lagrange equation trivial?
The Euler-Lagrange equation gives the equations of motion of a system with Lagrangian $L$. Let $q^\alpha$ represent the generalized coordinates of a configuration manifold, $t$ represent time. The Lagrangian is a function of the state of a particle,…
Trevor Kafka
- 1,903
55
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13 answers
Is there an intuitive explanation for why Lorentz force is perpendicular to a particle's velocity and the magnetic field?
The Lorentz force on a charged particle is perpendicular to the particle's velocity and the magnetic field it's moving through. This is obvious from the equation:
$$ \mathbf{F} = q\mathbf{v} \times \mathbf{B} $$
Is there an intuitive explanation for…
Stephen Jennings
- 667
51
votes
13 answers
Why doesn't a braking car move backwards?
The net force on an object is equal to the mass times the acceleration, $F = ma$
When I brake on a (moving) car, the net force is negative, therefore causing the resulting acceleration to also be negative. This all makes sense, but if the…
Harnoor Lal
- 647
41
votes
4 answers
Can water falling from a tap follow a spiral path?
The faucet design depicted below is driving me crazy. The water falling from the tap appears to follow a spiral path. No one seems to agree whether it is physically possible for the water to spin in such a way. Is it possible? Please explain your…
Laure Joumier
- 429
38
votes
3 answers
Is it possible to estimate the speed of a passing vehicle using a musical ear and the doppler effect?
I've found a number of questions that concern the Doppler effect, but none that seem to address my question.
I have a background in music. People with a musical ear can generally tell the ratio between two frequencies (as a musical interval). For…
M_M
- 491
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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36
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14 answers
Why does the (relativistic) mass of an object increase when its speed approaches that of light?
I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy.
However, the equation breaks down when the electron velocity approaches the speed of light as mass…
Kit
- 1,483
34
votes
7 answers
The usage of chain rule in physics
I often see in physics that, we say that we can multiply infinitesimals to use chain rule. For example,
$$ \frac{dv}{dt} = \frac{dv}{dx} \cdot v(t)$$
But, what bothers me about this is that it raises some serious existence questions for me; when we…
Clemens Bartholdy
- 8,309
34
votes
5 answers
Why don't things get destroyed by gas molecules flying around?
Gas molecules go at an insane velocity, and though they are miniscule, yet there is a LOT of them. Of course, because of all these molecules hurtling around, there is air pressure; yet if you envision a lot of bullets flying around, they don't…
HyperLuminal
- 1,978
33
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1 answer
Can one determine the speed of the rain from the shape of the rainbow?
I was watching the rainbow today and started thinking about the effects of the rain falling in different directions.
The idea I had was that normally we model rain drops as small spheres, and this gives a characterizing angle for the perceived…
Mikael Fremling
- 3,967
32
votes
7 answers
How can kinetic energy be proportional to the square of velocity, when velocity is relative?
Let's start with kinetic energy (from los Wikipedias)
The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having…
geelen
- 421
31
votes
6 answers
(Almost) double light speed
Let's say we have $2$ particles facing each other and each traveling (almost) at speed of light.
Let's say I'm sitting on #$1$ particle so in my point of view #$2$ particle's speed is (almost) $c+c=2c$, double light speed? Please say why I am…
Templar
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