The speed tag has no wiki summary.
37
votes
14answers
7k views
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 $mv^2/2$.
Why does this not increase linearly with speed? Why does it take so much ...
10
votes
9answers
3k views
Accelerating particles to speeds infinitesimally close to the speed of light?
I'm in a freshmen level physics class now, so I don't know much, but something I heard today intrigued me. My TA was talking about how at the research facility he worked at, they were able to ...
9
votes
4answers
572 views
Why does the speed of light $c$ have the value it does?
Why does light have the speed it does? why is it not considerably faster or slower than it is? I can't imagine science, being what it is, not pursuing a rational scientific explanation for the speed ...
7
votes
2answers
394 views
Is it possible to travel at precisely the speed of sound?
I've been talking to a friend, and he said that it's impossible to travel at exactly the same speed as the speed of sound is. He argued that it's only possible to break through the sound barrier using ...
7
votes
1answer
259 views
Riddle about speed
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 ...
6
votes
5answers
8k views
How to calculate average speed
I recently encountered a puzzle where a person drove 120 miles at 40mph, then drove back the same 120 miles at 60mph. The average of the speeds is (40mph+60mph)/2 = 50mph, so the total trip time ...
6
votes
1answer
1k views
How does one derive the equation for the speed of sound?
In my acoustics books I see
$$c^2 = \frac{\mathrm{d}P}{\mathrm{d}\rho}$$
where $c$ is the speed of sound, $P$ is the pressure and $\rho$ is the density. Where does this equation come from? In my ...
6
votes
1answer
678 views
How fast would someone have to run to run over water?
I was thinking about Flash, the superhero, or the little boy in the Incredibles.
There is one Yahoo answer that doesn't answer a lot. Especially, I don't think surface tension would help a lot for a ...
5
votes
2answers
1k views
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 ...
5
votes
1answer
225 views
What is the speed of time
When we measure the speed of a moving element we do it with the help of a reference frame. Now if we need to measure the speed of time, is it possible? Is time really has speed?
Thanks in advance.
5
votes
5answers
1k views
Maximum speed of a rocket with a potential of relativistic speeds
Ultimately, the factor limiting the
maximum speed of a rocket is:
the amount of fuel it carries
the speed of ejection of the gases
the mass of the rocket
the length of the rocket
...
5
votes
1answer
708 views
How to compute the speed of sound in relativistic hydrodynamic?
In Weinberg: Gravitation and Cosmology chapter 2.10 (Relativistic Hydrodynamics) the speed of sound is derived as
$v_s^2 = \left(\frac{\partial p}{\partial \rho}\right)$
and the equation of state ...
4
votes
2answers
1k views
Help me get out of a speeding ticket
I was driving uphill from a complete stop for a distance of .4 miles estimated to take 1 minute in a navigation app. I was pulled over right after cresting the hill. The cop had me on radar going ...
4
votes
4answers
2k views
Is a signal traveling through fiber faster than a signal traveling through copper?
Does the transmission medium affect the speed of a signal? For instance does light traveling through a fiber cable get a bit from A => B faster than copper can transmit a bit the over the same ...
4
votes
4answers
2k views
How Felix Baumgartner has reached the speed of sound quickly
I have watched Felix Baumgartner freefall; but I wonder how Felix has reached the speed of sound quickly, in a matter of some seconds, then we had no idea of its speed?
Any explanation please.
4
votes
3answers
319 views
How fast do I need to go in order to avoid being seen by the Police?
I was driving down the road at roughly the speed of traffic. I saw a police officer parked on the side of the road, and also noticed that a Semi was traveling in the lane right next to him. This got ...
4
votes
3answers
143 views
If an electric car were to drive without having to stop, would the range be greatly affected by the speed at which the vehicle is moving?
Of course aerodynamics factors into this question, and the faster you are moving the more air you have to push out of your way, the more energy you use. But would the difference be only a small ...
4
votes
1answer
492 views
The speed of sound is proportional to the square root of absolute temperature. What happens at extremely high temperatures?
The speed cannot increase unboundedly of course, so what happens?
4
votes
2answers
217 views
Why does the wheel of a car appear to be moving in opposite direction?
When a car is about to stop, the speed of its wheel reduces. Why do we see or feel that the wheel of the car is moving in the opposite direction? This can also be observed of a fan at home. So why ...
3
votes
1answer
383 views
How does a particle of light reach the max speed of light? [duplicate]
Possible Duplicate:
How can a photon have no mass and still travel at the speed of light?
First of all I am not a professional physicist. I was curious as to how a particle of light can ...
3
votes
1answer
159 views
Does $\lambda\nu = c$ hold for all the waves in the universe?
Are all waves in the universe the same as electromagnetic waves?
Basically, my question arises from an equation I found in my chemistry textbook:
$$\lambda \nu ~=~ c.$$
This states that the ...
3
votes
2answers
117 views
How to relate speed of sound with relative humidity?
I am exploring the idea of measuring the humidity of a space using sound waves, however I am having trouble finding a mathematical relationship between the speed of sound and the humidity level.
...
3
votes
3answers
848 views
Does friction decrease as objects move faster against each other?
I was told that the faster two objects move against each other, the less the friction between them would be… compared to if they were moving slower. In physics class, we always use the coefficient of ...
3
votes
1answer
202 views
Local Mach number for an airfoil
I would like to know how to calculate the local Mach number on the upper surface of an airfoil given the ambient temperature, the local velocity on the airfoil surface, the freestream velocity, and ...
2
votes
3answers
528 views
Why would a fat skydiver fall first in free fall?
I was having one of those obnoxious conversations with a friend where he was arguing that a fat skydiver would reach the ground faster than a skinny skydiver. To me it seemed as obvious that the world ...
2
votes
3answers
54 views
Wind effect on driving a ground-based vehicle
A friend of mine seems to think that wind affects the ground-speed of a ground-based vehicle in the same way airspeed affects an airborne aircraft. i.e. If faced with a 10mph headwind, your car isn't ...
2
votes
2answers
791 views
Speed of a particle in quantum mechanics: phase velocity vs. group velocity
Given that one usually defines two different velocities for a wave, these being the phase velocity and the group velocity, I was asking their meaning for the associated particle in quantum mechanics.
...
2
votes
1answer
807 views
Fuel usage at the same constant rpm at different gears
I've had a discussion with my father today, about the fuel usage of a vehicle at the same rpm, but a different gear.
He claims that the following situations have the same fuel usage:
...
2
votes
1answer
811 views
How fast do electrons move through a conductor?
If I apply $1 \text{ V}$ across a $1 \text{ }\Omega$ resistance, I'd get $1 \text{ A}$ flowing. $1 \text{ A}$ is defined as $1 \frac{\text{C}}{\text{s}}$, and $1 \text{ C}$ is equivalent to ...
2
votes
2answers
80 views
Does inertia increase with speed?
I have heard that when the speed of the object increase, the mass of the object also increase. (Why does an object with higher speed gain more (relativistic) mass?)
So inertia which is related to ...
2
votes
1answer
183 views
Where to find the current positions and velocities of the planets?
I've written a program which simulates the motions of planets and other bodies. I'd like to run it on our own solar system, but to do so I need to know the current positions (preferably in ...
2
votes
1answer
232 views
Laws of aerodynamics after breaking the sound barrier
my knowledge of physics is not very extensive, so I hope my question isn't too stupid.
I know that when (for instance) a plane breaks trough the sound barrier, the laws of the aerodynamics change.
...
2
votes
1answer
132 views
How to find speed when accelerating down a slanted wire
I saw this picture on one of my social media sites with the caption, "I'd do this in a heart beat! Who's with me!"
I was about to go balls to the walls and say, "I'm in! When and where??" But then ...
2
votes
2answers
428 views
How can I calculate the wave propagation speed in a copper wire?
First of all: I am a computer science student, so I don't have much knowledge of physics. So please keep your answers simple.
I recently learned something about circuit design and its problems ...
2
votes
1answer
173 views
Which speed can an electric scooter reach on a given slope?
Electric scooters are always "given" as "working with slopes up to xx%", but what does it mean?
Given motor torque and power, scooter+driver weight and wheels diameter, how can I determine at which ...
2
votes
2answers
528 views
Why does an object with higher speed gain more (relativistic) mass?
Today, in my high school physics class, we had an introductory class on electromagnetism. My teacher explained at some point that an object with a very high speed (he said it started to get somewhat ...
2
votes
2answers
122 views
Speed of Entropy change
If time in systems moving with different speed goes differently, does speed of entropy change differ in these systems?
(is "speed of entropy change" a valid term? can we compare them?)
2
votes
0answers
180 views
How dangerous is firing a bullet straight up in to the air? [closed]
Question's title says it all how dangerous are the bullets which are fired up straight in to the air? I often find myself in this discussion where there are people who argues that it is harmless and ...
1
vote
7answers
3k views
I need help with finding distance traveled
How do I find the distance traveled of an object if the speed is not constant?
1
vote
3answers
1k views
Is the same amount of electricity used for ceiling and table fans on different speeds?
We use ceiling and table fans in home which are can be set to low or high speeds using regulators. I want to ask that does it uses or consumes same amount of electricity at different speeds? Here, ...
1
vote
2answers
80 views
Currents and the Speed of Light
Why is it that currents don't flow at the speed of light, but rather significant ratios of the speed of light.
I don't have any formal reasoning as to why they would flow at the speed of light-I just ...
1
vote
2answers
101 views
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 ...
1
vote
1answer
206 views
What's the highest speed one can obtain safely at home? [closed]
Among common household appliances, things one can make from stuff in the garage or hardware store, and reasonably safe, e.g. within reach of hobbyists and high school kids entering a science fair, ...
1
vote
1answer
39 views
Gyro vs Accelero : Why is that assessment false?
Can we say that the Accelerometer's values are somewhat the integral of the Gyroscope's values ?
I think that because I know that the velocity is the derivative of the position with respect to time ...
1
vote
1answer
982 views
Which derivation of drift velocity is correct?
In the derivation of drift velocity I have seen two variations and want to know which one's correct.
$s=ut+\frac{at^2}{2}$
Assume that the drift velocity of any electron in any conductor is :
...
1
vote
2answers
368 views
Sign of Velocity for a Falling Object
I'm working on a homework problem in Mathematica. We have to graph the height and the velocity of a function given an initial height and initial velocity. However, when I do the graph for the velocity ...
1
vote
1answer
220 views
speed of light flashlights question
This might be a silly question but I take the risk because it's been puzzling me for quite some time :
If you have 2 flashlights, one facing North and one facing South, how fast are the photons (or ...
1
vote
2answers
168 views
Application of mean of speed distribution
Preface
The definition of average speed of an object is defined by the distance travelled over time:
$$v_{avg} = \frac{x_2 - x_1}{t_2 - t1}$$
The interpretation of speed is that when you multiply ...
1
vote
1answer
51 views
B-field and Magnetic forces, speed of a particle
Let's say I have a particle moving perpendicular to a uniform magnetic field of magnitude $x \ T$, and it moves in circle with a fixed radius. How do I find the speed of this electron? Initially I ...
1
vote
1answer
87 views
Weightlessness on Earth
Would the following work?
Imagine one would
1) create a straight 4000 km long tube (at ground level), following the curvature of the Earth, wide enough to hold a pod the size of an aircraft cabin;
...
