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. L

    I Time Dilation: Variables Beyond Speed & Gravity?

    Can time dilation vary without a gravity well or accelerating a viewer towards light speed? Are there other variables that affect the flow of time?
  2. nazmulhasanshipon

    Finding out the rotational speed of a mass

    **My Attempt:** Here, I considered for 2 kg mass the resultant motion is zero, which means it's acceleration (a) would be zero. So, if we consider the Tension force from the 1.5 kg mass to keep 2 kg mass from falling is L, then 2×9.8-L=ma=2× 0 ⇒ L=19.6 N. But where does the tension force from...
  3. S

    Speed and apparent weight of person in reverse bungee jumping

    a. To find the speed, I need to find the height where the cords first become loose, which is when the cord is 25 m long. $$h=30-\sqrt{25^2 - 5^2}$$ But the teacher's working is ##h=30-\sqrt{25^2 - 4^2}## How to get 4? b. My idea is to use Newton's 2nd law so I draw free body diagram of the...
  4. S

    Speed of train car when sand spilling out from it

    I have several confusion about this question: 1) Since the sand spilling out downwards, it means that there is change in momentum in vertical direction (before the sand spills and after the sand spills) but no change in momentum in horizontal direction so no net force in horizontal direction...
  5. H

    I How do forces change with speed?

    Hey dear physics community :) I ask myself what exactly happens to forces between materia when the materia hits near lightspeed. I know, that for an objective bystander watching let's say elon in his rocket with 99,99999% the speed of light, that the time goes slower, the mass of the rocket...
  6. David Lewis

    B Light Speed Traveling: Cosmic Background Radiation Effects

    If a spaceship travels near the speed of light, could the frequency of cosmic background radiation blue-shift?
  7. chwala

    Finding the relative extrema for a speed function using parametric curves

    I have no problem in following the literature on this, i find it pretty easy. My concern is on the derived function, i think the textbook is wrong, it ought to be, ##S^{'}(t)##=##\frac {4t} {\sqrt{1+4t^2}}=0## is this correct? if so then i guess i have to look for a different textbook to use...
  8. Huzaifa

    B Average Speed vs. Velocity: Explained

    Though average speed over a finite interval of time is greater or equal to the magnitude of the average velocity, Instantaneous speed at an instant is equal to the magnitude of the instantaneous velocity at that instant. Why so?
  9. D

    Average acceleration of a car changing speed and direction

    Nevermind I found what I had to do, thank you though.
  10. E

    Can you use speed and velocity interchangeably?

  11. E

    When an object is accelerating, does speed, velocity, or force change?

    qn3: why is statement 2 the only answer? isn't speed one of the factors in velocity too? then shouldn't statement 1 be true too?
  12. T

    B Explanation of light speed limit

    To an average person with high school math knowledge how would you explain in a few words why no object could travel faster than the speed of light ? Well it's because...
  13. J

    A Derek Muller and UCLA Prof. Alexander Kusenko -- Downwind Physics Wager

    Derek Muller is the Veritasium YouTube channel host and he had a wager against UCLA professor Alexander Kusenko on whether a unpowered car going downwind can go faster than the wind. Derek Muller said it can go faster than the wind, while Alexander Kusenko said it only seems that way because...
  14. M

    Max Speed of a Car: A Math Equation Analysis

    Summary:: Question: a car of mass 800kg, 3600N driving force experiences resistive forces of 120v Accelerates from rest. Form equation and show max speed is 30 ms^-1, assuming driving force does not change. my attempt at a solution -I can not see how to rectify it as I don’t think I did...
  15. F

    I Calculate Diameter of Quad Coil Array for Speed of Light Rotation

    I would like to know how to calculate, for any specified frequency in Hz, the required diameter of a 90 degree phased, quadrature coil array such that its generated EM field achieves rotation at the speed of light. Could someone please provide an example of the calculation using a specific...
  16. L

    What is the limiting factor for the maximum speed of stepper motors?

    I was discussing stepper motors with a friend. We touched upon different aspects. I showed how the coils in the motor have a limit to how fast they can be fully saturated with current, and how this time can be calculated by the aid of Kirchhoff's and Lentz's law. The theoretical speed calculated...
  17. sciencebum

    Maximizing Boat Speed: Exploring Human Power Ratios

    In competitive rowing, the more people in a boat, the faster it goes, but what is the ratio of additional human to speed, and what is the theoretical limit? If I could fabricate a boat where each additional person added 100kilos, 100watts of power, and 1 meter of length to the wetted surface...
  18. B

    What is the difference between rated speed and output speed in servo motors?

    Hi Everyone, What is the difference between "Rated speed nC(1/min)" and "output speed" in servo motors. At the same time While "rated motor speed" is 1405(1/min) in gearbox catalogue, Rated speed is 3000 (1/min) in servo motor catalogue for the same gearbox. If someone has a knowledge about...
  19. Barblorrane

    I Speed of EM & Mech Waves: Maxwell's Law Explained

    Based on Maxwell's Law, the speed of light can be defined by: $$c= \frac{1}{\sqrt{\epsilon_{0}\mu_{0}}}$$ Based on that, can we find a medium where a mechanical wave travels faster than a electromagnetic one? If so, how does that works?
  20. C

    Ejection speed and travel distance of a ball in a pressurized water pipe

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  21. I

    Polar Ice Caps and the Earth's Speed of Rotation

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  22. MikeandSuch

    Measuring the speed of light in a straight line

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  23. Justice Hunter

    B Thought Experiment : Try and get Alice to "0%" the Speed of light

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  24. Lucas123

    Engineering Solve Motor Coil Current at Startup & Full Speed

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  25. The Baron

    I One way speed of light measurement

    Hi so to those of you who don't know the problem here is a video that explains it very well. - I have an idea on how to measure it, can someone please tell me if, and why it is incorrect. Thank you! Okay so we will have a timer, and a lot of mirrors, set up in a circle, a light beam is fired...
  26. G

    Baseball Dad -- setting the speed of a pitching machine

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  27. R

    Variation of the Speed of Sound in metals under tension

    In a block of metal, each metal has a characteristic speed of sound. When metal is under tension, such as a guitar string, the speed rises as the tension increases. How does the speed vary (in a block say) as a function of tension along each of the three axes? I am assuming that transverse...
  28. J

    MHB Bird Flying Speed at 10m Altitude

    A bird is flying at an altitude of 10m above you, his angular position from your point of view is changing at 0.5 rad · s^-1. How fast is the bird flying when he is directly above you?
  29. Janos Meri

    B Relative speed of two oppositely directed light beams

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  30. C

    What speed v(t) enables constant power by solenoid pull?

    Assuming the position x = 0 if no air gap, max stroke of plunger is L. Bordery conditions: x(t=0) = L, x(t=T)= 0. Given magnetic force F(x) = kx(t)^-2, here k is constant. If uncontrolled, there is big crash noise afer solenoid actuated, so I wish it pull plunger at constant power. Help me find...
  31. J

    B Experimental evidence for effective mass increasing with speed

    What is the best recent experimental evidence for effective mass increase (momentum/velocity) with speed, with experimental details? Searching the web all I find is either very old ones (early 1900s). Or vague generalized statements like (Wikipedia) "Many additional experiments concerning the...
  32. snoopies622

    Confusion about the derivation of the speed of sound

    I've having trouble understanding a derivation of the speed of sound waves, which is actually similar to another derivation I found a couple days ago. Let's suppose the sound is moving through water in a long cylindrical horizontal pipe. The premises of the derivation are 1.) For a given...
  33. C

    What speed v(t) enables constant power by release compressed spring?

    We know linear spring force F = kx(t), k = spring constant, during any moment t of energy release. displacement x(t) = ʃv(t)dt the power p = F*V = kx(t)v(t)= kv(t)* ʃv(t)dt My question: Is there a mathematical function of special v(t) to make power p constant?
  34. J

    B Does Gravitational Time Dilation Affect the Speed of Light in Outer Space?

    Due to gravitational time dilation, the speed of light in outer space will be higher than on Earth. Do astronomers use the corrected value?
  35. The Baron

    I A problem with how the speed of light connects to special relativity

    [Mentors' note: This question was orginally assumed a train traveling at the speed of light. Further discussion shows that this assumption is not required for the question; any relativistic velocity is enought to raise the question. The post has been edited, changes in boldface, to reflect...
  36. E

    Speed of sound in a relativistic fluid

    Let us consider the co-moving observer ##\mathscr{C}## for whom ##E = \epsilon## and ##\mathbf{\vec{V}} = \mathbf{\vec{0}}##. Doing the perturbation stuff to the first of the relevant equations gives$$\partial_t \delta \epsilon + \boldsymbol{\nabla} \cdot ([\epsilon + p] \delta \mathbf{\vec{V}})...
  37. TheGreatDeadOne

    Speed of a hanging rope sliding on a nail (using energy conservation)

    I solved this problem easily using Newton's second law, but I had problems trying to use mechanical energy conservation to solve it. How I solved using Newton's second law: ##\text{(part of the rope that is on the left)}\, m_1=x\rho g,\, \text{(part of the rope that is on the right)}\...
  38. CallMeDirac

    Speed of processors, does it have a max?

    I had heard that computer processors are reaching the speed of light. Is this true, and if it is how do we combat this cap?
  39. I

    B Time Dilation: Away or Toward Observer?

    I thought the answer was no. I.e. the traveler's proper time always passes more slowly than a stationary observer's clock regardless of the traveler's direction with respect to the observer. I was watching Brian Greene's NOVA episode on time, however, and, at 23:15, he has a demonstration...
  40. guyvsdcsniper

    Does coefficient of kinetic friction depend on speed?

    In this part of the lab we pushed a block on a flat table and let it slide until it stopped. So it is decelerating with no force being applied to it while moving. In this case acceleration is negative. The only force acting on it is kinetic friction. Therefore I have come up with the following...
  41. C

    Max tip speed of a spinning cable

    A company called SpinLaunch claims it can get something to 2200 m/s by spinning it up on a carbon fiber composite arm. I'm trying to figure out the limit of that approach. How fast can you go with existing materials? I tried to work it out for a constant with cable with no payload (result...
  42. L

    Uncertainty of Speed of Sound in water, using Mackenzie's Equation

    I need to calculate the overall uncertainty of the value I have obtained for the speed of sound in water, using Mackenzies equation... I am not sure in what way to combine the above uncertainties. Any help would be greatly appreciated! Lloyd
  43. A

    MHB Finding Distance with Constant Speed: Solving for m and b

    John is walking at a constant speed in front of a motion sensor. After 1 s, she is 2.5m from the sensor, 2 s later, she is 4 m from the sensor. a) Find an equation of in the form d=mt+b b) Determine the slope and d intercept and explain what they mean c) How far will John be from the sensor 5s...
  44. A

    I Isotropy of the speed of light

    It has been put to me that a simple spectroscope could in theory demonstrate the isotropy of the speed of light . By using a frequency standard (laser comb or Th Lamp for example) with the spectroscope in various orientations the lack of shift of the spectral lines would prove its isotropic via...
  45. B

    Speed Transmitting Between Motor Gearbox and Sprocket

    Chain length: 1400mm. Pitch of the chain:12,7 mm Diameter: Ø55 For z=12 Diameter: Ø71 For z=16 Diameter: Ø87 For z=20 Hello friends, Can someone who has knowledge of how to calculate the speed transferred to the sprocket wheel can help in such systems with various gear numbers (z = 20, 16)...
  46. G

    B Light Speed Invariance: Experiments, Difficulties & Clarification

    Let me clarify my question, is there any experiment directly proved the invariance of light speed to observers? Let's not get to the argument of equivalence between source and observer. SR was based on the postulate that the light speed is constant and independent of both the motions of source...
  47. K

    B Speed of Light: Questions Answered

    Yess! speed of light it's driving me crazy. I know it might sound too noob and silly but please enlighten me, First of all how does we concluded that speed of light is constant for every observer in all possible inertial frames of reference. Next thing how come a photon doesn't experience...
  48. H

    Where did I go Wrong? Solving for Particle Speed with Mass & Charge

    F= 9*10^9 (5*10^-9)^2 / (0.05)^2 a=F/m = F/(3*10^-6) v^2 = u^2 + 2aS = 0 + 2a*0.03 v = 60 Where did I do wrong? Thanks!
  49. idktbh1

    Achieving the speed of light at the end of a whip

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  50. S

    Angular speed of rod shot by bullet

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