What is Speed: Definition and 1000 Discussions

In everyday use and in kinematics, the speed (commonly referred to as v) of an object is the magnitude of the rate of change of its position with time or the magnitude of the change of its position per unit of time; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero.
Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph). For air and marine travel the knot is commonly used.
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in a vacuum c = 299792458 metres per second (approximately 1079000000 km/h or 671000000 mph). Matter cannot quite reach the speed of light, as this would require an infinite amount of energy. In relativity physics, the concept of rapidity replaces the classical idea of speed.

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  1. A

    I Relative Velocity of Particle ##p##: S vs S

    A particle ##p## is moving with a velocity ##u\hat i## with respect to S ... The velocity of ##p## with respect to S' is then ##-c \hat i##... Another part... Similarly taking the velocity of particle ##p## with respect to S' as ##-c\hat i## the velocity of ##p## with respect to S is then...
  2. r-swald

    Find the drift speed of of the electrons

    I've found I to be .167 using the potential and resistance. I also found the volume by multiplying the cross-sectional area by the length (?) and then dividing the # of conducting electrons into that to find packing density (n). To find drift speed, I would also need the area of the block as...
  3. J

    The hot chocolate effect to investigate the effects on speed of sound

    Summary:: The Hot chocolate effect is an investigation that extracts the essence about the effects on speed of sound. This is carried out in this sequence: A cup filled with liquid is continiously being tapped from the bottom. Meanwhile, a disturbance in the longitudinal sound waves is being...
  4. nomadreid

    I Lorentz boost -- speed or velocity?

    The Wikipedia article on Lorentz transformations is a bit confusing by its using speed and velocity almost interchangeably: of course γ (Gamma) stays the same, but (letting c=1) t'=γ(t-vx) , then if this is v⋅x, and x stays the same, then there would be a difference if something were going away...
  5. Shubh Goel

    B Time & Speed of Light: Not a Universal Constant

    Time as we perceive is not a universal constant. It depends on the speed of the bodies and the distance between them.
  6. P

    Calculating the speed using Potential graph

    My attempt, pictorially it looks like I am confused with the questions (a). Does the positron emerge from the field at x =0? There is no potential at x=0, so the positron will continue with the same speed hence its motion is not reversed. For the (b). The maximum volt is 200V, if i apply the...
  7. H

    Does Galilean relativity imply infinite propagation speed?

    Without assuming a universal speed that is constant in all inertial reference frames, is it a necessary consequence of Galilean symmetry that interactions are instantaneous? If this is the case how can we prove this?
  8. IamVector

    Exploring the Nature of Light and Relativity in Space Travel

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  9. halleff

    Signal Speed Below the Speed of Light for Circuit Abstraction

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  10. T

    B Accurate measurement of the speed of light

    [Mentors' note: Split from https://www.physicsforums.com/threads/is-there-a-thought-experiment-to-show-that-the-speed-of-light-is-constant.986641/] If I could provide you with a device to measure the speed of light, and the accuracy can reach 0.01 m/s, what would you do?
  11. S

    B Is there a thought experiment to show that the speed of light is constant?

    I know the amazing thought experiment by Albert Einstein with the two light clocks. (The observer at the train station has a light clock and the person in the train.) It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes. But this experiment...
  12. K

    Average speed and average velocity

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  13. S

    B What is the speed of a photon traveling along the sine function?

    On the image you can see a photon starting at point A at t=0. The photons travels along the sine function and arrives point C. I knot that this takes T=λ/c. But this is the time for a object traveling directly from the origin to point C and not along the sine wave! If the photon travels...
  14. danielhaish

    B Question about speed connection

    Lats say that i am moving on spaceship that moves 50% of the speed of light And this spaceship is on a planet that moves in speed of 50% of the speed of light . And we have one clock on the spaceship ,clock number one, and on the planet we another clock,clock number two. And on difference...
  15. danielhaish

    B Relativity & Computer Calculations: Speed Up 300x?

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  16. PickyBiker

    Understanding Force vs Speed for Arduino-Based Inverted Pendulum on Cart

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  17. J

    MHB Light Beam Speed on Shoreline from P - 16km

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  18. J

    Why is the most probable energy different from the speed?

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  19. M

    CPU Speed Test: How to Measure and Improve Your Processor's Performance

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  20. R

    I Exact historical explanation of deducing speed of light constancy

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  21. M

    Control Theory - Nyquist Plot - Speed of Response from Low Frequency Asymptote

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  22. Steven Ellet

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  23. B

    Speed of sound related to absolute temperature

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  24. seb7

    I Measuring One-Way Anisotropy of Light Speed

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  25. cianfa72

    I Speed of Light in Vacuum in Non-Inertial Frame

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  26. U

    Understanding the Minimum Speed to Keep Carriage on Tracks in a Loop

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  27. thaiqi

    I What is the meaning of constant speed of light?

    Which one does it mean: "phase velocity" or "group velocity" or "speed of the wave front"? In the postulate of constant speed of light .
  28. TaurusSteve

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  29. J

    I Question on the energy needed to go at a relativistic speed

    Hello, I consider to be in a relativistic area, where an object is moving very fast, seen from our Earth, at a speed v where v is less than the speed of light c. I have considered the following equations (relativity equations) : T = β * t M = β * m L = l / β β = 1 / (√(1 – v2/c2) )...
  30. TaurusSteve

    B Is E=mc² the Formula for the Speed of Light?

    What is the equation/formula for the Speed Of Light? E=mc²? I thought it would be simple to find in a search.
  31. LawH

    B Energy, Mass, Speed of Light: Can We Reach It?

    Hello everyone! Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship...
  32. L

    Understanding Average Speed in Graph Analysis

    I get two different answers even the area of the graph. I think there is something wrong with the equation I constructed for average speed part. I know the method 1 is correct. But I'd like to know why the average speed cannot be used. And why I get two different areas for the graph Thank you
  33. R

    Relativistic speed accelerator for physics experiments in a capsule

    Is it technologically feasible today or in the near future, to accelerate in outer space a ~0.1 gram physics experiment lab, inside a cyclic accelerator and shoot it in a straight line at a constant speed of 5%-80% of the speed of light? That miniature capsule, must include all that is needed...
  34. R

    B Meaning of "Speed of Light is Constant": Explained

    Isn't the meaning of speed, a variable of distance divided by a variable of time? Therefor isn't the meaning of a constant speed of light, a constant distance divided by a constant time? If there is any truth in this saying and there probably isn't, then what is the meaning of light constant...
  35. R

    B Constant speed between 2 objects close to the speed of light

    If i am moving away from an object at a certain constant speed close to the speed of light, is that object also moving away from me at the same constant speed?
  36. R

    B Moving Faster Than Light: Are There Symmetrical Rules?

    Would it be correct to say, that we are moving away from stars at the edge of the universe, at the same rate that these stars are moving away from us? I am relating to stars that are moving in relation to us, at a speed that is faster than the speed of light. Is the symmetry that maintains that...
  37. A

    I Why is the speed of light absolute?

    To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?
  38. N

    Does speed affect energy consumption over a certain distance?

    Hi! I just watched a video about cardio training, where it is claimed that the only thing that matters is the distance covered, and not the speed. "Four miles burns more calories than three miles." So, walking four miles burns more calories than sprinting three miles. (Click the link and watch...
  39. B

    I Everyone Measures Light in a Vacuum at the Same Speed

    I ran into something on a non-technical message board that is familiar to me - a poster, clearly without any scientific knowledge, saying that Relativity is wrong. Unfortunately, the point he used as a battering ram is one to which I don't know the answer. He suggested that the light...
  40. H

    I Measuring the Speed of Light: How Did We Do It?

    I understand that the meter is defined from the speed of light (distance light travels in 1/299792458 of a second). But how did man measure this exact distance to this level of precision? With any apparatus, isn't there an unknown amount of bottleneck somewhere?
  41. Curious_Student

    Silly question -- Perpendicular forces have no effect on speed?

    If route perpendicullar forces supose to have no affect, why isn't this the case when somebody tries to cross a river? the motion is perpendicullar when water speeds aprox. at the same direction.. what am i missing?
  42. BiggestAfrica

    Determining speed at an angle using an energy balance

    I first found the height of the ball after it's passed the 45 degree angle by doing 4.12*sin(45) = 2.9133, and plugged in the rest of the variables (masses cancel) .5(m)(vo^2) = (.5)(m)(vf^2) + mgh (17.5^2)(.5) = (.5)(vf^2)(9.8*2.9133) vf = 15.7845, however this is incorrect I don't understand...
  43. Fra Ser Mur Chie

    What is the relationship between wave speed and frequency?

    Frequency nU = W/2pi = 287.6/6.28 = 45.796 = 45.80 Speed C = nU * I = 45.80 * 186.9 = 8560 m/s
  44. V

    Modeling a wingsuit base jumper's speed

    Fgravity = m*g Ffriction = k*A*v^2 Ftotal = Fgravity - Ffriction a = Ftotal/m v = v+a*dt if t<7 then Fgravity>Ffriction if 7<t<67 then v =55,88 (meters per second) if t>67 then A = 45 (square meter) if t>67 then Fgravity<Ffriction the t defines the seconds the first 7 seconds are the free fall...
  45. shadedvertex

    Flaw in alternative approach to determine ideal gas speed distribution

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  46. C

    I One Way Speed of Light: Is it Constant?

    Is there any reason to believe that the one way speed of light is constant? Not isotropic, but constant in the sense of being independent of position.
  47. greg_rack

    How to link frequency and speed? Doppler effect

    I had many attempts on trying to solve this one, but I got always stuck in the problem-solving part: how do I manage to find the source-speed from the Doppler formula, in an analytical way, and then reach to the result-formula? Anyway, I'm pretty sure the only formula needed to solve this...
  48. Merlin3189

    Is Double Dime Really Code for a 55mph Speed Limit?

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  49. M

    Propagation speed of Coulomb fields

    Moderator's note: This thread is a spin-off of https://www.physicsforums.com/threads/what-are-the-most-important-open-questions-in-classical-physics.983671/ R. de Sangro, G. Finocchiaro, P. Patteri, M. Piccolo, G. Pizzella, "Measuring propagation speed of Coulomb fields", Eur. Phys. J. C 75...
  50. greg_rack

    Speed of sound in an ideal gas

    First of all I thought it was necessary to calculate the temperature(the only data missing for the formula) using the ideal gas equation(since I've already been given 'p' and 'V'), and plug it in the 'v' formula, but the problem immediately occurred when i tried to find out the number of...
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