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.

View More On Wikipedia.org
Recent contents Top users
  1. V

    Does the speed of sound change in metal when heated?

    Is the speed of sound different through the same piece of metal when it is heated compared to when at ambient temperature?
  2. opus

    Trying to get an average speed of 60 MPH

    Homework Statement I haven't taken a physics course yet, this is for independent study, so I don't have the strongest understanding quite yet. I was wondering if I could get some insight to this problem: Susie has planned a trip to a city 60 miles away. She wishes to have an average speed of...
  3. Domenico94

    Speed of fluid in a turbulent flow

    Suppose a fluid passes from having laminar flow, to having a turbulent flow (like when passing after an object). How do fluid speed and fluid density change after that?
  4. A

    Determining Speed given Horizontal and Vertical Distances

    Homework Statement A girl throws a ball high into the air, and at its highest point it lands on a roof, where it gets stuck. You estimate the ball landed at a height of 21.0m from the ground and a horizontal distance of about 20.0m from where it was thrown. What is the speed that the girl threw...
  5. vincent

    I Does a Photon Carry Velocity of Its Source?

    Earth is moving through space. If I throw a ball straight up, it falls down to me and is not left behind. This is obviously because it already carries the momentum of Earth's movement. But if I fire a photon straight up, does it also carry a velocity component in direction of Earth's movement...
  6. MeAndMyLucidLife

    Relative speed of two photons

    Homework Statement What is the speed of a photon with respect to another photon if: the two photons are going in the same direction. they are going in opposite direction? 2. The attempt at a solution I think the answer to the first question should be zero and to the second one be 2xC; C⇒speed...
  7. F

    I Variations in the speed of sound in a star

    I try to understand the following graphics with x-axis being the radius of a typical star : I would like to knwo if ##\delta c/c## (y-axis) represents the relative error between theoretical and experimental values or if it represents the fluctuations of speed of sound inside. If these are...
  8. K

    The speed of a metal wire on two rails with a magnetic field

    Homework Statement I tried to understand the problem b) and c).[/B] Homework Equations Faraday's law: ∇xE = - ∂B/∂t emf ε = Bdv Force : F =ma, Lorenz's force F=q(vxB) ==> ma = IdB Power : power of battery = εI, mechanical power of the wire = Fv The Attempt at a Solution I think I solved...
  9. P

    I Is there some prediction of the speed of rings?

    From NASA page: The inner parts of the rings move around Saturn faster than the outer parts, all in accordance with Kepler’s third law for small objects revolving about a massive, larger one. They orbit the planet with periods ranging from 5.8 hours for the inner edge of the C ring, to 14.3...
  10. B

    How to Calculate the Speed of a Sprocket?

    How to calculate the speed of sprocket connected to the turbine shaft of a cross flow turbine if I only have these given parameters: Net Head, Volume Flow Rate, Specific Weight of Water and Efficiency of Cross Flow Turbine? Does the Cross Flow Turbine, Turbine Shaft and Speed of Sprocket have...
  11. D

    Light Speed: Distance & Time Explained

    Hello forums, I'm kind of new nice to meet you guys and girls, Could someone please explain this to me? D=S * TTherefore Distance = Speed of light * TimeAnd then Distance = Speed of light + 7 * TimeWhat is Distance and what is time?
  12. Killtech

    I Constancy of the speed of light

    I am trying to get a better understanding about the constancy of the speed of light which is a well-established axiom of current day physics. for the start i want to understand how it is experimentally established and how these results are interpreted. My difficulty here is that this seems to be...
  13. M

    Angular speed calculation after an inelastic collision

    Homework Statement A disk [m=0.1 Kg; R=0.1 m] rotates about its center of mass [w=40 rad/s], on a smooth floor. A bar [m=0.1 Kg; lenght=R=0.1 m] moves on the floor with a speed Vb=4 m/s. At one point, the bar hits disk's edge in an inelastic collision, and they start rotating together. A)[Fixed...
  14. Luis Babboni

    I Understanding the Speed of Light in Ether: A Historical Perspective

    Hi people! May be this could be an historical question. Before Einstein, it is suppoused that speed of light in ether is always the same, I´m right? I mean, if observer O is at rest respect ether and observer O´ is moving respect ether and O´ send a photon to O, the speed at which that photon...
  15. B

    How to calculate the speed of a cross flow water turbine?

    How to calculate the speed of cross flow water turbine? Help, I'm struggling on our machine design.
  16. e2m2a

    Impact of speed on the coefficient of rolling friction

    I have conducted an experiment that measures the velocity of an object that is moving on a flat surface. The object rolls on four fixed caster wheels. The wheels are made of some kind of rigid material. Maybe nylon, maybe hard rubber. The total mass of the object, including the wheels, is...
  17. J

    I Traveling at Light Speed: Is a Photon Stationary?

    If you were to travel alongside a train, as fast the train, to you the train would seem stationary. I read that if you were to travel along a photon of light, as fast as the speed of light, that photon would not seem stationary. Is this true? If so, why?
  18. shimun

    B Thank you for understanding. Have a great day!

    Hello Dear Ones. 1. How Scientists exactly calculated movement speed of solar system ? thank you very much.
  19. B

    Replacing speed with friction in formula

    Hi, first of all sorry... I know (close to) absolutely nothing about physics except from what I remember from school a looong time ago... :-). So, my question might sound a bit silly here. But thanks a lot for taking you time already! Homework Statement In retail business there is some...
  20. T

    Speed of sound in steel vs air

    I saw on a web page a video of tiny Agnes Scott College vs Princeton University from 1966 on GE College Bowl.Agnes Scott won (!), but one of the questions that both schools got wrong was asked by the moderator: The speed of sound in air is about 1,100 feet/sec. The speed of sound in steel is...
  21. D

    Calculating Minimum Speed for a Successful River Jump

    Homework Statement A stunt motorcyclist has to jump across a river 7.0 m wide from a ramp on one side with incline of 10 degrees. (a) What minimum speed the motorcycle must have to successfully land on the opposite side of the river? (b) How long will the motorcyclist be in the air? (Assume...
  22. kristjan

    Initial speed of the proton?

    Homework Statement The moving proton hits the second proton, which we consider to be stationary. In the moment of central strike gap between the protons is 10(-13)m. What was the initial speed of the moving proton? Proton mass is 1.67*10(-27)kg and charge 1.6*10(-19) C. Homework Equations...
  23. K

    Determining the Speed of a Rotating Disk with a Constant Force

    Homework Statement A solid uniform disk of mass 21.0 kg and radius 85.0 cm is at rest flat on a frictionless surface A string is wrapped around the rim of the disk and a constant force of 35.0 N is applied to the string. The string does not slip on the rim. 1- When the disk has moved a...
  24. R

    B Speed of Light Slowed for SuperBig Observer?

    How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this...
  25. R

    If light speed (not in a vacuum) was about human walking speed ....

    If light speed (not in a vacuum) was about human walking speed ... Did any sci fi writer try that?
  26. C

    I Measuring 1-Way Speed of Light: 15m Fibers & 1GHz Pulse Generator

    Drive two 15m long optical fibers extended in opposite directions with a 1GHz pulse generator. The recievers at the end of each fiber are now syncronised sources of 1GHz pulses. Connect them to pulse counters, one of which provides a signal whenever a pulse train is present. Connect this...
  27. I

    Why total time taken s₁/v₁ + s₂/v₂ + s₃/v₃ in average speed?

    Homework Statement As we know the Average speed = Total distance / Total time but why in the case below Distance traveled = s₁ + s₂ + s₃ and total time taken = s₁/v₁ + s₂/v₂ + s₃/v₃ How is it possible? so, altogether Vav = s₁ + s₂ + s₃ / ( s₁/v₁ + s₂/v₂ + s₃/v₃)Homework EquationsThe Attempt...
  28. Sjm_dynamo

    B Why is the speed of light a constant?

    Why is the speed of light a constant?
  29. Spinnor

    I Speed limit c in the multiverse

    Do we know enough of the workings of string theory to say what factors give rise to a large or small value of the velocity of propagation of massless fields for a given multiverse? Thanks!
  30. Sandeep T S

    Uncertainty and the speed of light?

    By Uncertainty Principle speed of a particle cannot be constant. then speed of a single photon not?
  31. H

    B What would happen if the speed of light were different?

    if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
  32. P

    Assumptions made when deriving the speed of light

    Homework Statement The problem I have been working with recently has been deriving the speed of light using maxwells equations, however in order to do this I must make two assumptions; there is no net charge or displacement currents in the space in which I am attemptin to derive the speed of...
  33. SamRoss

    I EFE: Deriving Speed of Gravity, Gravitational Waves?

    Newton's law of gravitation cannot be compatible with relativity because the gravity from a massive object applies a force to all other masses infinitely fast. General relativity is supposed to correct this flaw by setting a speed limit on how fast the effect of gravity can reach a distant...
  34. H

    Speed/acceleration calculations for an Electric motorcycle

    Good day all, This is my first post here, hope you all are able to help me out here. I am thinking about converting an old Suzuki GS500E from an internal combustion engine to an electric motor. At the moment I am having some difficulties with the physics side of things, I would like to...
  35. M

    What's the speed of the ball 1 second after it was kicked?

    Homework Statement A soccer ball is kicked with an initial horizontal velocity of 15m/s and an initial velocity of 16m/s. What's the speed of the ball 1 second after it was kicked? Homework Equations Initial velocity = sqrt([horiz. Velocity]^2 + [vertical Velocity]^2) Final velocity = initial...
  36. A

    Electron speed and acceleration in an electric field

    Hi everyone, I often work on a SEM, a type of microscope which is based on electron acceleration between an electron source and the sample you are working on. For this reason and since a few weeks I was wondering how an electron (in term of speed) behaves in a constant and linear electric field...
  37. José Ricardo

    Speed of the brain/ Speed of thought: How to determine them?

    I was playing a game of running (Need for Speed Underground) and was the second time that I thought how to determine the speed of the brain or the speed of the thought. I was running to more 200 km/h (you guys use mph, I'm from a country that uses km/h). And I crashing a lot with more 200 km/h...
  38. Dr. Courtney

    Direct Echo-Based Measurement of the Speed of Sound - Comments

    Greg Bernhardt submitted a new PF Insights post Direct Echo-Based Measurement of the Speed of Sound Continue reading the Original PF Insights Post.
  39. R

    B Measuring the speed of light from moving source of light

    The speed of light from a moving source of light is usually recognized indirectly, based on various explanations of phenomena. The speed of light is fairly simple directly measurable on the basis of autonomous and separate measurements of the frequency and wavelength of the light. Is a similar...
  40. R

    B Speed of light circling around a black hole

    Let us imagine a photon circling around a black hole, as the picture shows. The gravity of the black hole curves the movement path of the photon into the shape of a circle. From point 0, geometric points A and B appear simultaneously with the photon, each in its own direction. The points travel...
  41. Alexanddros81

    Determine the ground speed and the course of the plane

    Homework Statement 15.1 The airspeed of the plane is 200m/s, directed north. If the wind speed is 30m/s in the direction shown, determine the ground speed and the course (angle θ) of the plane. Homework Equations The Attempt at a Solution Is it so far correct the above? More questions...
  42. LukasMont

    What is the correct velocity of an electron in a bound orbit around a proton?

    Good evening! The question states: 9. Electron in bound orbit about proton. Suppose that an electron moves in a circular orbit about a proton at a distance of 2 X l0^-8 cm. Consider the proton to be at rest. (a) Solve for the velocity of the electron by equating the centrifugal and...
  43. S

    I C Speed, Gravitational Time Dilation & 4-Velocity

    Hi 1-)If an object's total velocity through space-time(four-velocity)is c, for example even we stand still we move with velocity c (through time) and if mass slows down time, can we say mass also increase our velocity in space? 2-) Is Four-velocity magnitude constant in General Relativity...
  44. V

    Speed of Sound Experiment: Hear the Results

    Imagine a car which has a speaker attached to the back. Speaker is blasting very high volume music towards the reverse direction of the car. Now the car starts accelerating and in a few seconds starts moving faster than the speed of sound. Will a listener be able to hear the sound now. Will...
  45. Sorcerer

    I Does infinite one-way speed of light violate p conservation?

    Suppose for the sake of argument someone said the outward speed of light is infinite and the return speed is c/2, creating a two-way speed of c. Wouldn't this violate the conservation of momentum? p = E/c. That means on the way out, the momentum of light would be zero, but on the way back it...
  46. john1800

    B What is the speed of the Earth traveling through the cosmos?

    what is the speed of Earth as it travels the cosmos,compared with the center of the universe.lets say the big bang starting point.?
  47. Ushitha Dissanayake

    How do I find the terminal speed of a body in water?

    Homework Statement A baseball has a terminal speed of 42 m/s in air (ρ = 1.2 kg/m^3). What would be its terminal speed in water (ρ = 1.0 x 10^3 kg/m^3)? A) 0.05 m/s B) 1.5 m/s C) 18 m/s D) 42 m/s E) 1200 m/s Homework Equations D=½Cρ*Av^2 where the magnitude of the drag force (D), relative...
  48. A

    Increasing the speed of a circular orbit

    What If the velocity of particle moving in a circular orbit has increased , would the particle be no longer in circular orbit or it would go in an orbit with bigger radius?
  49. E

    B Would planets fall out of orbit when gravity has a speed?

    As I understand it, Newton considered the influence of gravity between two objects to be instantaneous. Now it would appear that gravity has a speed limit. If the Earth is influenced by the sun from where it was 8 minutes ago, how does the Earth keep up with a traveling sun? Does relativity...
  50. Sanborn Chase

    B What is the Speed of Light in Dark Matter?

    Does anyone know the speed of light in dark matter?
Back
Top