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. 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...
  2. 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!
  3. idktbh1

    Achieving the speed of light at the end of a whip

    Is there a equation for the speed at the end of the whip. is there a consistent way to calculate how fast the tip of the whip will be including constant velocity and trajectory or is it chaotic. Im new to physics and had a thought that with enough legnth and power a wiphs end colud reach the...
  4. S

    Angular speed of rod shot by bullet

    1) Applying conservation of linear momentum: $$m.u = M.V + m.v$$ where ##V## is final linear speed of the rod $$V=\frac{m.u-m.v}{M}$$2) Applying formula of circular motion: $$V=\omega . r$$ $$\omega = \frac{\left(\frac{mu-mv}{M} \right)}{\frac{1}{2}L-x}$$ Is this correct?And can this be...
  5. L

    Improving the speed of induction charging

    If charging some smaller toy, phone, electric razor, toothbrush or the like with induction charging I assume that the speed (time it takes to charge an empty battery) will depend on 1. the battery to some extent, but for this sake let's omit that 2. the inductive charging it self 3. wall socket...
  6. S

    I Displacement vs time under a time varying speed limit

    The speed limits on a straight road are given by a known function g(x,t) where x is the location on the road and t is time. A car starts at x = 0 at time t= 0 and always drives at the speed limit. The location of the car is given by the (unknown) function s(t). Is there a differential...
  7. V

    Synchronous generator variable speed and constant excitation

    The question is regarding an electrically excited synchronous generator. Consider that this generator is not connected to the grid and thus not locked with the grid frequency. This generator is spinning at variable speed and gets constant and similar excitation current meaning that the...
  8. F

    I The Speed of Light: Comparing Photon and EM Wave Velocities

    Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?
  9. Traced

    Determine the speed and tension of keys swinging in a circular path

    a)Determine the slowest speed that the keys can swing and still maintain a circular path. Fnet = Fg + Ft Fc = Fg + Ft When Ft = 0, Fc = Fg So, Fc = mv^2 / r and Fg = mg mv^2/r = mg v = √gr v = √9.81 * 0.25 v = 1.56 m/s Therefore, the slowest speed that the keys can swing and still maintain...
  10. J

    Tension in a rope holding a mass under a helicopter rising at constant speed

    Helicopter lift weight of 500kg ,weight is connect with load cell to meassure tension. Neglect aerodynamic drag.. If helicopter accelarate up ,tension is rope is greater than 500kg. If helicpter accelarate down ,tension is rope is less than 500kg. If helicopter fly up with constant speed...
  11. Pyter

    I Speed of light not an invariant in GR

    Hi all, I need help understanding the light ray bending in the original GR 1916 paper, Die Grundlagen.... First of all, Einstein states the ##c## is not an invariant in GR. In fact, from (70) and (73), it stems that $$\gamma = \sqrt{ -\frac {g_{44}}{g_{22}} }, $$ where ##\gamma## is ##|c| <= 1##...
  12. P

    Finding angular speed geometrically

    So clearly the easiest way to relate the angular speed to the linear speed would be to start from ##\tan(θ) = x/h## and take a time derivative of both sides. However, it also shouldn't be difficult to find the angular speed geometrically. Using the diagram below one can see that: ##sin(dθ) =...
  13. Cosmic Lollipop

    I Speed of Light in Different Mediums: Exploring Maxwell Equations

    Based on Maxwell equations, we can determine the speed of light as: $$c=\frac{1}{\sqrt{\epsilon_{0}\mu_{0}}}$$ Where ##\epsilon_{0}## is the electric permittivity of vacuum and ##\mu_{0}## is the magnetic permittivity of vacuum. This notation makes me believe that in other mediums those...
  14. J

    I Understanding Relativity: How Moving Objects Experience Near Light Speed Travel

    Light speed is impossible for anything with mass as more and more energy is required with increasing velocity. But this is only to an observer in a different reference frame. To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?
  15. duchuy

    Wave propation -- Speed variation in different indices

    Hi, I have this question about the variation of wavelength and frequency as light travels to an environment with a different index. As we have learned in class, celerity can change as light enters a different environment, however frequency and wavelenght are independent and remain constant...
  16. F

    I Would circumnavigating the Universe allow one way measurement of light's speed?

    Suppose we're in a closed and non-expanding universe. You shine a beam of light in one direction such that it circumnavigates the universe and returns to its starting point from the other side. Now you put a detector at this starting point. Would this be a one way measurement of the speed of...
  17. A

    Speed for loss of of contact between a dumbbell and a table

    Attempted solution: Consider the instant when the normal force of the lower ball is zero. Conserving energy: $$\frac{mv^2}{2}+mgh_1=\frac{mv_1^2}{2}+\frac{mv_2^2}{2} + mgh_2$$ Applying the resulting torque to the upper ball where the rotation point is the lower ball. $$T=I.a =...
  18. person123

    I Computing the Speed of Traveling Waves in 2 Dimensions

    I have a 2-dimensionsal smooth function ##f(x,y,t)##. There may be multiple traveling waves across the domain. None of them are precisely traveling waves (the shape of the wave changes as it travels). Here is how one of these waves would look in 1-dimension: I want to find the speed of these...
  19. I

    A What is the packet speed in the discrete case?

    I've been struggling with the problem below for some time. It is not a homework. A simple bubble S is a spherical surface that expands with constant speed c. A vector bubble V also expands with the same constant speed c. There is a 3d vector associated with a V. If two S bubbles touch, they...
  20. seeker11

    Steam Speed from a Heat Ruptured 120 Gallon Water Tank?

    PROBLEM: A 120 gallon tank (26" diameter, 60" height) containing 40 Gallons of water is heated up until the tank ruptures from overpressure at 125 PSI (maximum tank rating). How to calculate the initial escaped steam velocity at the ruptured tank, assuming the gash is 2 feet long in the...
  21. F

    B Why do we know all photons have same speed as the speed of light?

    Why do we know the velocity of all photons are the same as the velocity of light?Can we deduce this or we must have experiment test?What is the experiment test?
  22. person123

    Optical Compact Device For Measuring Speed of Light

    Hi. This is an idea which I just happened to think of, and I was curious if it would be at all feasible. Here's a quick sketch I drew: The two curved mirrors should have a laser attached on one end and a video camera attached on the other. The laser would be tilted very slightly above...
  23. richengle

    What is the minimum gyroscope speed to yield force precession?

    Does anyone know what the minimum speed of a gyro to make it so a force at 0deg will yield a movement at 90deg... or even better, what the angle is wrt speed... see http://www.copters.com/aero/gyro.html for pictures. -thx, rich
  24. F

    Emptying a container and Efflux Speed

    Hello Forum, The speed of efflux ##v_{efflux}## of a liquid, say water, from an orifice in the lower part of a container depends on the vertical distance between the free surface of the fluid and the lower position of the orifice in the container itself. The faster the speed ##v_{efflux}## the...
  25. Haorong Wu

    I Is coordinate speed affected by gravitational waves?

    As in Bernard Schutz's A first course in general relativity, page 220, we suppose a gravitational wave travels in the z-direction with pure "+" polarization, so that the metric in the TT coordinate system is given by$$ds^2=-dt^2+[1+h_{+}(z-t)]dx^2+[1-h_{+}(t-z)]dy^2+dz^2 .$$ Suppose that two...
  26. A

    Why is my CPU not running at max published speed?

    I have a Intel(R) Xeon(R) E-2274G CPU @ 4.00GHz 4.01 GHz machine which has a published max speed of 5.0 GHz. However, when I run it under load on all 4 cores, it only gets up to about 4.5 GHz: Could someone explain to me why it's not ramping up to the full 5.0 GHz or what I could do to...
  27. T

    I Speed of Sound & Light: Is it True?

    Are the following statements true? --------------------------------------------------------------- If a speaker and a listener are stationary with respect to the air, a listener will measure the speed of sound at about 343 m/s. Since the speed of a sound wave in air is dependent only on the...
  28. J

    Work Problem: Spring and Friction, find final Speed

    First calculated non conservative work from friction using Ff=umg. Non conservative work was -8.82. Initial kinetic energy, 1/2mv^2, was 136.89. Change in potential energy, 1/2k(x)^2, was 8.1216. Ekf-Eki+Change Ep=Work NC Ekf=W NC+Eki-change Ep =-8.82+136.89-8.1216=119.9484 Ekf=1/2mv^2...
  29. richengle

    How does the speed of light change when it enters glass?

    its has been said that light slows down in glass, but some talk about phase velocity and group velocity... then there is the velocity of the photons, which some say is always c. What are those speeds [Vgroup, Vphase, Vphoton, Vlight] in glass, and what do they mean?
  30. D

    How does the body speed up metabolism?

    Hi everyone Could someone explain what happens in the body when metabolism speeds up? For example, does it produce more stomach acid? Or does it produce more concentrated stomach acid? Thanks
  31. Delta2

    Maxwell's equation in microscopic formulation and speed of EM-waves

    Starting from the microscopic form of Maxwell's equations and following standard mathematical procedure outlined in Inhomogeneous electromagnetic wave equation - Wikipedia we can have as end result the following equations: $$(\nabla^2-\frac{1}{c^2}\frac{\partial^2}{\partial...
  32. OscarF

    Calculate speed from elastic and inelastic collisions? (momentum)

    So to cut to the chase, I missed my class' lesson on momentum - have tried to catch up, quite successfully but am baffled about this question. I know the conservation of momentum etc. but after trying for ages it's just not happening this question so any help would be much appreciated, Oscar.
  33. H

    I Does photon self energy contribute to the speed of light?

    Hi Pf I read that in the light propagator there are loops of electrons. What would be the consequences if we could switch them off (or neglect them)? would it modify the speed of the photons?
  34. Byron Forbes

    Circuits and the Speed of Light (2)

    https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/circuits-and-the-speed-of-light/ Sorry, my other post must have been too brief. In another thread shut down by Dale, this very site (allaboutcircuits) was used by him to reference wave guides. This particular page is 100%...
  35. Byron Forbes

    Circuits and the Speed of Light

    https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/circuits-and-the-speed-of-light/Hmmmmm. :))
  36. Byron Forbes

    How Fast Does an Electrical Impulse Travel in a Copper Wire?

    The topic here is about the speed that the "electrical" signal (impulse) travels at in a copper wire. The speed is well known to be just under c. Often when this topic is spoken about, the term "EM field" is used and I don't understand why. We have electrons with a power supply that supplies...
  37. A

    Question about the speed of multi-core HP laptops

    Hi, I'm thinking about buying a fast laptop with 4 cores but I've never been quite clear just how fast they run. Here's the description of the CPU from HP's website: Does this mean that if I set up parallel processing on all cores, each core "can" run at (or near) 4.6 GHz or the combined sum...
  38. K

    I Is it feasible to measure one way speed of light this way?

    Theoretical experiment for measuring one way speed of light From 1905 to this day we have not experimentally measured the one way speed of light between a source to the detector only the roundtrip from the source to the detector and back again. We just assume that the speed of light is the...
  39. greg_rack

    Behavior of charged particles in a speed selector

    Considering the device above, which uses electric and magnetic fields placed properly to avoid charged moving particles with velocities different from the ratio ##\frac{E}{B}## to exit, getting deflected upwards or downwards. All that is easily demonstrable by equalling the forces acting on the...
  40. kolleamm

    Controlling motor speed digitally

    I'm trying to turn my milling machine into a CNC so I bought this motor driver on ebay. https://www.ebay.com/itm/DC-40A-DC-Motor-Speed-Control-Reversible-PWM-Controller-12V-24V-36V-48V-2000W/173177079359?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649 The only problem is I can't...
  41. adafus

    B Are time and light at the same speed?

    I'm probably am wrong in this but i guess you guys could light me in this one
  42. A

    I Verifying Speed of Light Varies by Direction

    Note, it possible to determine if the speed of light varies depending on direction, with the use of a single clock. Simply send a beam and reflect it back, but on the return trip introduce a medium to slow the beam. Three cases, T1, T2, T3 are total travel times. T1: assume light is the...
  43. Daveopg

    B Light Behaviour at High Speed: Explained

    First, I am way out of my field of understanding here so please keep it simple. I watched some videos on E=MC2 which led to how light reacts differently than matter at high speeds causing time to slow down when moving fast. My question is, if I was to shine a flashlight perpendicular (90...
  44. S

    I The Speed of Light: Can it Travel in Opposite Directions?

    Is it really possible for light to travel at different speed in opposite directions? This video seems to say that this is assumed as a fact (with of course Einstein being the one to make the assumption), and that since it is impossible to measure the speed of light in one direction, we really...
  45. R

    B Why is the speed of light constant?

    Visualize the following scenario. A star such as the sun, and a planet such as Earth are located in a solar system. A photon travels from sun (x0) to Earth (x1), and a satellite is in the midway observing the photon. This is relates to Einstein's special relativity. I ask this since it take...
  46. A

    Motion of a rocket with propellant exhaust gas speed = constant

    Now this is how I've tried to solve this $$ v_e = u0 \cdot ln \frac {M} {M- μ \cdot t} $$ After putting in the values I get this; $$ v_e = 200 * ln 0,36 $$ $$ v_e = 73,54 \frac m s $$ Now I'd say that this is the correct way to do it, but this part is confusing me "What is the speed of the...
  47. RandyD123

    B Measuring the One Way Speed of Light

    Does this video even make sense? And if so, is it right or wrong?
  48. F

    Spaceship is approaching the Earth with an unknown speed

    I started by finding the main events: Sending the first message Receipt the first message Sending the second message Receipt the second message Now, what we know is the time by ##S'## (comoving frame with the spaceship) ##T_1'## and ##T_2'## remaining to arrive to the Earth measured at...
  49. emzy168

    Calculating the speed of an airstream leaving a balloon

    I'm really not sure how to start this one!
  50. A

    B Kugelblitz at the speed of light misconception

    I have a question regarding Kugelblitz black holes. I know that they are purely theoretical, and I am perfectly fine with the matter-energy equivalence so I have no problem in assuming that concentrating a sufficient amount of energy in a certain radius might generate an event horizon. However...
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