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. G

    B Speed of Light & Sound: Revolutionary Phenomenon?

    If sound (or any wave) was transmitted in a medium whose density was vanishingly low , would that wave propagate in the same way as em radiation in a vacuum? And vice versa does em radiation propagate in the same way as ,say sound so long as it is in a dense enough medium? In other words...
  2. U

    B The speed of light: Was Einstein's second postulate not so revolutionary?

    If the answer is yes then it seems that Einstein's second postulate was not so revolutionary.
  3. Drizy

    B Faster than speed of light due to time dilation?

    This probably has been asked before but i had a thought about the speed of light and time dilation. First off all i know the speed of light is constant and that it is the max speed anything can be but hear me out. So let's say a jet are traveling from point a to b in space with let's say 0,8c...
  4. Frouel

    How can I solve this physics problem step by step?

    I attended to many attemps and I not able to find out no dependencies in this layout. Does someone can show me how to solve this problem step by step? What we must take into considarion in solving such tasks?. How to start?. I got stuck. This task is from Polish Physics Olympics.
  5. notaphysicsmajor79

    Brain function & bipolar: fluid volume & speed of information in tubes

    This is a long post with limited amount of physics in it (but it is a physics question, so hopefully it is allowed). I am a scientist but trained as an ecologist (PhD) and despite my long interest in physics, my knowledge of it remains rather rudimentary. Apologies for that, in advance. I am...
  6. D

    Angular speed of a falling rod

    I have seen the solution and understand it. The solution defines θ to be the angle between the falling rod and the table. It then equates initial PE to the final (PE+KE) where the final PE=0 and the final KE to be (1/2) Iω2+ (1/2) ma2ω2 to finally obtain ω = √(3g/2a) But i would like to know...
  7. T

    A Integration of the Maxwell Speed Distribution

    Need some help on how to solve the integration formula for Maxwell speed distribution, here is the procedure on how to solve for the kinetic energy: Not familiar with the error function yet, but the result for the kinetic energy integration is...
  8. haushofer

    Rolling resistance when speed is zero

    Dear all, I have a very simple question. The rolling resistance (rr) of, say, a car, is defined as being proportional to the normal force/weight of an object. That seems reasonable: ##F_{rr} = C_{rr} \times F_N## The coefficient C_{rr} is a constant, depending on the materials. But now a...
  9. Sabertooth

    Expressing Elliptic Orbitals As Speed Functions.

    Hi everyone:) I have spend a couple of days trying to teach myself the math of orbital mechanics and have been able to generate a model of the orbital path of Haley's Comet, incorporating realistic distances and periods using Kepler's second law & ellipsoid functions. This is a GIF of the motion...
  10. docnet

    B Speed of Light & Expanding Universe: Does Hubble's Law Violate Rule?

    Hubbles law states the rate of recession of galaxies increases proportionally with distance, and the cosmological horizon is where distant galaxies recede away at the speed of light. Does this not violate the rule of faster than light speed travel?
  11. aspodkfpo

    Average speed with constant acceleration over a period of time

    Say that we have an instance where something falls down from a certain height with constant acceleration g. We know that the average speed with regards to the time period is less than (u+v)/2 since we spend less time at the higher speeds. How do we actually calculate the average speed over a...
  12. B

    Spring-mass system in circular motion has a maximum angular speed?

    and this is my solution for question (d), it may seems that $$R=(k)/(k-m\omega^2)R_0$$ so that $$\omega ≠ \omega_i =√(k/m)$$ but $$\omega_c <\sqrt{k/m}$$ is always true, ##\omega_i## corresponds to the limit case when ##F_max## is infinitely large Besides, I don't know other Physics prevents...
  13. Sagittarius A-Star

    B Explaining Galaxy Rotation Speed Discrepancies with GR & Gravitic Fields

    The speed of the ends of the galaxies is higher than what it should be. Current solution: This could be explained by hypothetical "dark matter", which was not found up to now, or by a MOND theory (MOdified Newtonian Dynamics). Can this be explained instead with rotational frame-dragging...
  14. S

    Must the limits on the propagation speed of waves refer to a media?

    An example (I think) of creating a phenomena that appears to propagate faster than the speed of light would be to have a line of people holding flashlights and giving each person a schedule of when to blink his light. With proper schedule we could create the illusion that point of light is...
  15. A

    Light speed and refractive index

    Refractive index of a medium is defined as : n = c / v; v is speed of light in medium. I believe n is never measured directly as here is no way to directly verify c / v. So what I guess is that all refractive index values are experimentally measured using n = sin α₁ / sin α₂. But then there is...
  16. J

    B Speed of electrons in a 2-slit experiment

    In the 2-slit experiment using electrons, what speed do they travel at?
  17. A

    Did I Calculate the Speed of a Comet Correctly?

    Hi all, I just want someone to double-check my calculation. Using some typical values of a comet, if the comet has a speed of 300 km/sec when it is 100 million kilometers from the sun, how fast is the comet moving when it hits the radius of Jupiter from the sun (5 Au)? I think I did everything...
  18. greg_rack

    B Why is the speed of light a constant?

    Approaching for the first time to these "higher level" topics is mind-blowing, and indeed I cannot understand why is the speed of light a constant... why doesn't it vary relatively to the emitter state of motion? And isn't it affected by gravity(I know it is affected in the sense of a spacetime...
  19. M

    Can We Travel Close to the Speed of Light?

    Hello! My kid asks if this is theoretical idea is correct and I just don't know this stuff very well: It is impossible to travel at light speed but not impossible to travel just below. So the highest theoretical speed should be: "The distance light has traveled in one second" minus "One Planck...
  20. darthgoon

    B Dumb Q, let's discuss :: How would quantum communications work at the speed of light?

    I feel like the emerging idea of quantum communications; is that you can exchange data via the synchronous states of entangled particles, across any distance, in real-time. But, there's also this dilation in the physical world, at the speed of light, according to relativity. So- how would the...
  21. jamesdhurai

    Lifting height of a box in a truck going over a speed bump

    hey guys need to know the way to find the solution to the problem. Consider a truck which is traveled in a speed of 60km per hour. A package weighs 50 kg is placed inside the truck with side supports to arrest the movement in x and y direction. But the top movement (z axis) is not arrested...
  22. T

    Orbital speed variation as a planet orbits the Sun

    Summary:: At what distance from the Sun will the speed of the planet be equal to the average orbital speed? I'm not sure where to place this question, please move it in the right thread. [Mentor Note -- thread moved from the technical forums, so no Homework Template is shown] At what...
  23. e2m2a

    Does Rolling Resistance Change with Speed on Polyurethane Wheels?

    I have conducted an experiment which involves measurements of the velocities of a carriage moving in a straight line which has 4 polyurethane caster wheels attached to it. (The kind you can get a hardware store.) In one phase of the test the carriage is pushed at about .1 meters per second. In...
  24. I

    B Can an Object Exceed the Speed of Light?

    I was curious if the relative speed of an object can exceed the speed of light. Specifically, I am curious about the following thought experiment. I am not a physicists (and if I were asking the following would make me a poor one) and it has been 20 years since college physics. If a vessel is...
  25. G

    Why is the velocity (vector) the same value as the speed (scalar)?

  26. A

    How does electron speed affect electric power?

    I've got a quick question on the relationship between the kinetic energy of individual electrons and the total electrical energy they create. I have a radioisotope - a beta emitter - which produces electrons with around 1 MeV (1.60218×10-13 Joules / 4.45049×10-17 Watt-hours) of energy. I keep...
  27. Nomadyb

    Find EMF of ℰ on Wire: Speed v, Force, C, B, m, g, l & a

    a) the EMF of ℰ induced on The Wire, as a function of the speed 𝑣of the wire you can find it. b) flow induced in the wire𝑖, 𝐶, 𝐵, and are denominated in 𝑙 𝑎 (𝑎= the acceleration of the wire). c) find the magnetic force acting on the wire in𝐶,𝐵, 𝑙 and𝑎. d) 𝑎 acceleration, 𝑚, 𝑔, 𝑙, and are...
  28. GREENMONKEY

    I Why is the speed of photons different?

    Hi, I read the Feynman's book about the quantum electrodynamics and I realized, that he was talking about the different speed of photons. I know, that the light travel's "slower" in a material, but he is also talking about the different speed of photons. I read on the web, that some photons...
  29. C

    Trig type question -- Speed of sound in water and in air

    unsure
  30. A

    Engineering Speed of a DC generator is proportional to the generated voltage?

    I'm confused as I'm aware speed is directly proportional to the EMF, but is it proportional to the voltage as well? If the speed of a DC generator is halved and everything else is remaining constant the EMF will have so will the output voltage half
  31. ttpp1124

    Solving for the speed of volume

    Is this correct?
  32. S

    Maximum possible speed of explosion fragments

    Let say the object explodes to three identical parts A, B, and C and initially the mass moves to the right before explosion. To obtain maximum possible speed (I assume C is the one with maximum speed), I imagine C should move to the right and A and B both should move to the left. Is this...
  33. S

    Force to make 2 blocks move relative to each other at constant speed

    Let: Smaller block = m1 = 1 kg Large block = m2 = 2 kg Coefficient of friction between the two blocks = μ1 = 0.2 Coefficient of friction between larger block and floor = μ2 = 0.3 Tension connecting two blocks through two pulleys = T Angle between tension and horizontal = θ = 37o Friction between...
  34. K

    Physics: Decreasing Speed Parabola Equation

  35. theudster

    Effects of hypersonic speed on the behaviour of normal shocks

    I am learning about hypersonic flow now and there seems to be a lot of discussion about the effects on oblique shock as the Mach tends to be very large. I was wondering what happens to the normal shock as the Mach number tends to be high and I can't seem to find anything about it. Is it because...
  36. F

    Earth's satellite and its first space speed

    Hello All :) I am a student of mathematics, but I have only one semester of physics in college. I can't solve one with homework. Will there be anyone wise who can solve this? The task is as follows: Calculate: at what height the artificial satellite must move (orbit height): geostationary...
  37. therealist

    How do I find the frequency, speed and direction of a wave

    Hey I am trying to learn how waves move in time and I am not sure how to solve the following question. Can someone please guide me through it.
  38. M

    How Quickly Can a Conical Spring Expand from Compressed to Free Length?

    I require a small conical spring to open from its compressed pancake height of its wire diameter to a free length of 11mm > 0.2 seconds (20 milliseconds) The large end outer diameter = 7.80mm. The small end inner diameter must be greater than or equal to 2.75mm. The conical's spring's will be...
  39. C

    Finding the angular speed of a hinged rod without using torque or acceleration.

    The centre of mass of the rod would be at the middle of the rod i.e. at l/2=[50*10^(-2)]/2 The force responsible for torque will be acting downwards = mg The Torque = mg*l/2*sin(30) =mg*l/4 We know that Torque=I*alpha Hence alpha = mg*l/(4*I) Moment of inertia of rod about the end= ml^2/12 +...
  40. A

    Speed and rest energy of a particle collision

    I honestly have no clue where to start, any help would be great.
  41. J

    I The Speed of Light -- Why can an object not travel faster than the speed of light?

    I apologize if this question is in the wrong forum section - but I could not find a proper place for what is a basic question or perhaps a nonsensical one. Is the reason why an object cannot travel faster than the speed of light because the object itself is ultimately made of waves? Is the...
  42. 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...
  43. 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...
  44. 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...
  45. 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...
  46. 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.
  47. 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...
  48. 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?
  49. IamVector

    Exploring the Nature of Light and Relativity in Space Travel

    so I was recently studying some ray optics and then suddenly a weird question came across my mind I just don't know if it's the correct thread to ask but let's continue anyway So the question is : suppose there is a spaceship traveling at light speed and someone fire a laser inside it what...
  50. halleff

    Signal Speed Below the Speed of Light for Circuit Abstraction

    I've read a couple of other topics on the Physics Forums about the lumped circuit abstraction requiring that signal timescales (i.e. the period of the highest frequency signal component) be much longer than the propagation delay of the signals though the circuit and that the wavelength should be...
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