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. Merlin3189

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

    Someone asked me what a dime was (this is UK.) I, not knowing, nevertheless promptly replied, it must be 5 cents, because they called their 55mph speed limit the double dime. Then of course went to Google to check and found that it is 10 cents. How does double 10 become 55? So back to Google...
  2. 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...
  3. 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...
  4. TammyTsang

    Engineering Scaling of units for equations of motions

    In the picture, there is a problem where the t is in units of square root(l/g), and V in square root(gl) I am wondering 1. What it means when time is in units other than time? Does it mean that when solving I have to take time/squareroot(l/g) 2. How did they get square root(l/g). Thank you...
  5. Harini Krishna

    Calculating the speed of sound in water

    Hi,i want to calculate the speed of sound in water Containing Chlorine : 4mg/L in 1L distill Water Chloroamine : 4mg/L in 1L distill Water Florides : 0.5mg/L + in 1L distill Water Copper : 0.005mg/L + in 1L distill Water Lead : 1.3 mg/L in 1L distill Water Can you please help in...
  6. R

    Elliptical motion: An object is moving at a constant speed?

    I am confused why the acceleration doesn't point to the center of the ellipse or one of the focus, since it moves in circular motion. Shouldn't the acceleration be just in the radial direction
  7. omega_minus

    Surface speed of a strip over a deflection roll

    Hello, I am trying to solve a problem at work that at first I thought was easy but has proven trickier than I originally thought: If I have a strip of material traveling in a straight line and it is tangent to a roll turning below it, the roll would need to turn at the surface speed of the...
  8. NTL2009

    Electrical A speed limiter for a Universal Motor in an old Lionel Train

    I have a "262 Lionel Line" O gauge train, from the early 1930's. It has sentimental value, my Father-In-Law (RIP) got it as a gift when he was about age 10. I've set it up each Christmas, a short track around the tree. But it is difficult to control, the gearing is high (it goes fast), so...
  9. SlurryPumpPro

    Vertical Cantilever Slurry Pump Critical Speed

    Hello all, I am an engineer with a manufacturing company and I am working for calculating the critical speed for a vertical cantilever slurry pump. This pump design includes one ball bearing and one taper roller bearing within the bearing housing. Then the shaft extends vertically downward...
  10. J

    I Question on high speed astrophysics — when an object goes near of the speed of light

    I take the following example to explain my question : when a « black-hole » is attracting a planet there is a Force which is proportional to the 2 masses and inversely proportional to the distance between the 2 masses. When the planet moves towards the « black-hole » this attraction force...
  11. wicker558

    Object thrown at an angle: Initial velocity has to equal speed at t

    My solution is very sketchy, but we want the math right. However, I've came to 2 thoughts that helped me get to the solution, and those are: The only way that you can launch a ball at a velocity and have it be the same velocity two seconds later, is if it's already reached its maximum height...
  12. S

    I Speed Difference of Time on Poles vs Equator of Earth and Other Planets

    I had the wonder of whether, due to the speed difference, clocks ran at a different speed on the equator than on the poles. I then researched this, and found that, due to being closer to the centre of the Earth at the poles due to the bulging of the Earth due to spinning, the two essentially...
  13. anonymous99

    Does the speed of sound depend on the speed of the medium?

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  14. Saptarshi Sarkar

    I Speed of Light in Inertial Ref Frames: Photon Perspective

    According to the 2nd postulate of Special Relativity, speed of light in vacuum is the same in all inertial reference frames. If I take a beam of photons and see the other photons in the beam from a frame of reference of a single photon, do they look stationary or moving at the speed of light...
  15. cyclemark

    Transition time from one steady-state Speed to another for a cyclist

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  16. G

    I Speed of Light: Measuring Meters and Seconds

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  17. C

    Average speed needed to flip a vehicle

    Car A hits Car B's passenger side at intersection (T Bone). Car B flips over sideways in the direction of the driver's side. The intersection is basically level. Car A is an Audi 5, roughly 3,900 lbs. Car B is a Lexus NX, roughly 4,000 lbs. Car B had recently stopped and was traveling...
  18. T

    Adiabatic approximation in the derivation of the speed of sound

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  19. J

    B Can Info Travel FTL? Understanding Speed of Information

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  20. Athenian

    I Lorentz Boosts: Finding Speed, Coordinates & Rotation w/ Matrix Multiply

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  21. sigr

    B What Would Light Look Like If Train Moved at > Speed of Light?

    I am reading up on the special theory of relativity, and watched a video. In the video we have a train moving at ½ c towards a station, where an observer is waiting. The train's headlamp is on. How would the lamp's light appear to the observer? I was thinking that it¨'d appear to the observer...
  22. S

    Average speed of a probability density wave and wave packets

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  23. A

    Why Can a Sound Wave Not Travel Faster than the Average Molecule Speed?

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  24. V

    Does Fan Speed Remain Constant Under Varying Loads?

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  25. jisbon

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  26. A

    Kinematics and angular speed of a collision between a rod and puck

    initial total KE= (1/2)(0.6kg)(8m/s)^2 = 19.2J (0.6kg)(8m/s) = (0.6kg+1.8kg)(vf) vf= 2m/s final KE= (1/2)(0.6kg+1.8kg)(2m/s)^2 = 4.8J I tried to use linear speed=angular speed * radius : thus 2m/s= angular speed * (3.3m/2) angular speed= 1.2 rad/s Apparently that is wrong.
  27. Benjamin_harsh

    Calculating Relative Speed & Time to Meet: I Can't Find the Distance

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  28. zizzle

    Minimum initial speed to spin a particle around a disk (with gravity)

    For this problem, since the weight force on the "particle" (child) is not always aligned with the tangential circular path of the disks, I couldn't think of a way to use rotational kinematics equations. As such, I tried to solve the problem using work principles (namely, that the change in...
  29. F

    Opposites of Speed, Mass, Pressure & Momentum - Things I Wonder Of

    Also what do you think the opposite of mass is? What about pressure? Would you say the opposite of that is resistance? How about space? Is that opposition occupancy, or may be volume? What about momentum? Wtfluff is the opposite of that? If even. May be odd These are certainly not physics...
  30. D

    Calculating the average linear speed of all points in a rotating sphere

    Is there a way to calculate the average linear speed of all points in the volume of a sphere rotating on a single axis? Since points closer to the axis of rotation and the poles move slower than points further out, would the average speed be a simple function of r/2 and pi? It would seem that...
  31. Z

    Switching speed of ICs vs gate length

    Why is it that the smaller the gate length, the higher the max operating frequency becomes? In particular, I think I saw somewhere that a 6 um gate length, seems to limit ICs to 100kHz, where as 1 um is around 1MHz (all rough ball bark numbers, and can be persuaded otherwise). Was wondering if...
  32. S

    Engineering Calculate consumption, displacement and speed using given standards

    I calculated the value of kgf/cm^2 from given values. In the second table (Force exerted by the pneumatic cylinders) I see my value 5.6 exactly but how do I then read the table to get the diameter of the cylinder and then the rod? I then am stuck on how to calculate the total consumption...
  33. A

    The Nuclear Rocket that Could Reach 20% of the Speed of Light

    In 2009, the same society together with the Tau Zero Foundation announced Project Icarus, a similar spacecraft that could achieve 15% the speed of light. That year, a physicist called Friedwardt Winterberg announced a fusion spacecraft that could be used as a capacitor to produce proton...
  34. A

    Angular speed, acceleration, and angle of a Ferris wheel

    ω(10)=(1.3)∗(1.0−e^(−10/22) )= 0.475 rad/s 0.475 rad/s=0 +α(10second) α=0.0475 rad/s^2 ∫ω(t)=Θ =1.3t + 28.6e^(-t/22) | (t=10s, t=0) total angle by which the wheel rotates over this period of t=10 seconds = 2.55 rad Θ= 2(pi)(8m)= 1.3t + 28.6e^(-t/22) 0=1.3t + 28.6e^(-t/22) - 2(pi)(8m) t=34...
  35. posto002

    With what speed does the cart recoil?

    I started off with vf^2=vi^2+2ad and plugged in the final velocity, acceleration (9.8) and distance which is 2.1 to get: 0=vi^2+2(9.8)(2.1) I solved this to get: vi=6.42 I then plugged this into find the x-component of vi (I labeled this as simply x)...
  36. J

    B Red-Shifted Light: Speed Less Than C?

    The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source. If so then the original quantity of...
  37. U

    B Train traveling near light speed

    A train is traveling around the Earth at just under light speed. Light would circle the Earth around 7 times per second so let's say this train cricles the Earth 6 times per second. There is a physical ticker on the track of the train that records revolutions. Each time the train makes one...
  38. Haorong Wu

    The speed of a spaceship flying through dust

    The given solution is: However, I could not agree this solution, and my solotion is: So which one is correct? it seems a exponential decay would be more convincing. Thanks!
  39. NoctusPartem

    How can I calculate the smallest speed of a proton in a nucleus?

    My approach: Assuming non-relativistic velocity: $$p = mv$$ $$mvL = \frac{\bar{h}}{2}$$ $$v = \frac{\bar{h}}{2mL} = \frac{h}{4\pi mL}$$ $$v = (6.626*10^-34) / (4pi * 1.67*10^-27 * 4.3*10^-15)$$ $$v = 7.34*10^6 = .0245c$$ This answer is incorrect. What have I done wrong? --- Edit: My...
  40. matternat968

    B Relative Nature of Speed: Einstein and Orbits

    At the risk of sounding stupid, this question has always perplexed me. Einstein theorized that mass can not travel faster than the speed of light. I don't really understand it, I assume it has something to do with mass just being energy. Anyway imagine two objects in orbit of something with...
  41. Frigus

    How can the pressure be more in liquid flowing with less speed?

    Before I have read Bernoulli's theorem I believed that pressure is more in liquid flowing with more speed but today I have read Bernoulli's theorem where this theorem says that the pressure is more in liquid flowing with lesser velocity please tell me how can this explained,with equation I...
  42. jk22

    I Photon Speed Addition: Can it Be FTL?

    It is well known that relativistic speed addition is a non-commutative group (gyrogroup). The addition of two orthogonal speeds is given by : ##\vec{u}\oplus_\perp\vec{v}=\vec{u}+\sqrt{1-u^2/c^2}\vec{v}## Hence if ##u## is describing a photon with speed c then we can add whatever ##v## it...
  43. EEristavi

    Water Pressure and 2 dimensional motion

    $$ H = \frac { V^2 - V_0^2 sin \Theta} {-2g} $$ $$ H = \frac {V_0^2 sin \Theta} {2g} $$ So, I need to calculate ## V_0 ## I'm thinking about pressure. $$ P = \rho g \Delta h $$ $$ \Delta h = h - L sin \Theta $$ $$ F_A = P S_A $$ $$ F_A = P S_B $$ Dead End here...
  44. J

    I Speed of Light Paradox: Neutron Star Rotation Rate

    Sorry if this is a stupid question but I couldn't find an answer anywhere. According to 2 scientific papers, the neutron star PSR J1748-2446ad has a rotation rate of 716Hz, which equates to a linear surface speed of 0.24c. What if this star was originally rotating, let's say, 5 times (or more)...
  45. A

    Current: the speed of charges vs the number of charges past a point

    Current is determined as the amount of charge moving past a certain point of reference (like the point in a wire with current meter clamps around it) in 1 second. This got me thinking , so in a classical resistive load situation with Ohm's law there would be two ways to increase the amount of...
  46. G

    How is the speed of light derived in Maxwell's equations?

    I know we end up with ##c=\sqrt{\frac{1}{μ _0.ε _0}}## The reason I would like a bit of help is that I understand that the value of c as deduced from Maxwell's equations is independent of any frame of reference. I can see that this is the case from the above equation involving the...
  47. ColdheartedGod

    Why does the speed of light change in different media?

    In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down? Macroscopically speaking, the speed of light does slow down. What about in the...
  48. Clay Gillespie

    Thought Experiment about light speed

    If we’re looking through a telescope at a craft we launched from Earth that is now passing Mars and send a radio signal to our craft telling it to turn on one of its lights on and it takes thirteen minutes for the radio waves to get from Earth to our craft how long will it take before we see the...
  49. M

    What happens if I speed up an Apollo group asteroid?

    Okay, having devised the antimatter bomb, I'm moving along to the concept of using Apollo group asteroids as freight trains for the inner solar system, but my knowledge of orbital mechanics is zilch. For example, (343158) 2009 HC82 appears to have a max speed of about 56 km/s at perihelion...
  50. G

    Pneumatic cylinder and working speed

    we have a pick & place application so please give the solution for that data cylinder stoke length 200mm, travel distance 200mm ,what is the picking time?
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