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. R

    What if the Earth would have spin around itself twice as it's speed?

    Summary: What if the Earth would have spin around itself twice as it's speed? What would have be the consequences? How would has it affect the climate and living things?
  2. jk22

    I Lorentz Boost & Galileo Speed: Exploring Relationship

    What about if the speed parameter in a Lorentz boost were in fact related nontrivially to a Galilean speed ? More formally ##L(v_L)=G(v)\circ F## where L is a Lorentz boost with Lorentz speed ##v_L##, G is a Galileo transformation with speed ##v## and ##F## is still an unknown linear...
  3. nineteen

    Does speed of light depend on the wavelength or frequency of it?

    I heard that the speed of light doesn't depend on the wavelength or frequency of it, how does it happen? I don't understand it because the ultimate equation is c = fλ (where c is the speed of velocity of light, f is the frequency and λ is the wavelength of it...).
  4. S

    Find the speed of a copper loop falling in a magnetic field

    Hi all, so I had this problem and on the exam and I got a solution but I had an mass-term in there which wasn't given. I used Farraday's Law of Induction to get the Voltage induced. Then I used ##rho* \frac{A}{4a} ## for the resistance and divided the Voltage by that to get the current. I then...
  5. K

    Calculating Speed Between Two Cars

    Hi, I found a programming problem that calculated the speed between two cars as below and I would like to know how the calculation is done. it is about determining the speed of two cars. The coordinate reference system (y,x) is attached to the ego vehicle (as shown in the attached figure). The...
  6. Chandrakanth_balusa

    How is the turbine speed controlled by Grid but not by itself?

    Hello People, Greetings! i am back again with a confusing question for which I thought I could get a correct answer from here. so my question is regarding the speed of the turbine. I learned that the speed of a turbine has to be maintained constant in order to maintain generating frequency...
  7. M

    How to get the real object speed?

    I am trying to get the real object speed at a current moment. The movement is circular. I have all the timings. This is the data, the speed is in milisecounds, and the numbers represent the revolutions: 1. -504ms 2. -720ms 3. -1044ms 4. -1332ms I know the average speed of the object, at...
  8. J

    MHB Differential equation - distance needed to achieve target speed

  9. S

    B Why is the speed of light limited?

    Summary: Why is the speed of light limited? Why does light have a maximum speed?
  10. N

    Engineering Calculating Efficiency and Speed of a DC Motor at Half-Load Torque

    First some basic figures which are very rounded as I'm interested in the approach to the problem rather than accuracy of the answer at this stage: The field current will be 500/220 = 2.2727 Amps, and so power is a fixed loss at 1136.36 Watts The armature current is 45 - 2.2727 = 42.72 Amps. At...
  11. S

    B What happens to speed and velocity in a rotating frame of reference?

    Let's say I can see a star in a distance of 1ly in front of me. I'm sitting on an office chair and begin to rotate with a speed of 1/s (1 rotation per second). Then I consider me as not rotating and everything else as rotating. Now I will see the star traveling a circular path with the...
  12. naji0044

    Inelastic collision problem: Bullet striking a wood block

    Hey everyone I'm struggling on the last part of this assignment. I need to find the total work done by the block and the bullet, when the collision happens. The informations is: mblock=0.3 kg mbullet=0.01 kg vg=700 m/s Height=0.72m The final speed after the collision is vf=22.6 m/s and the...
  13. Sophrosyne

    B Speed of light with quantum path integrals

    Richard Feynman formulated quantum path integrals to show that a single photon can theoretically travel infinitely many different paths from one point to another. The shortest path, minimizing the Lagrangian, is the one most often traveled. But certainly other paths can be taken. Using single...
  14. Plat

    Radiometer rotation speed as function of pressure

    I am experimenting with using a radiometer as an approximate indicator of pressure in my homemade high vacuum system, running a small turbo pump. I am interested in the relationship between pressure and vane rotation speed, with light intensity being constant. I have only been able to find...
  15. sadidrahman20

    Speed of falling object underwater

    Is there any formula for underwater falling sphere which can measure the velocity of that sphere at any time before it reaches its terminal velocity? A sample question is: I dropped a sphere into water with 0m/s initial velocity. what will be the velocity of that sphere after 3s? {the radius and...
  16. Hiero

    I Does a beginning to time imply absolute speed?

    I’m just imagining spacetime that is spatially infinite but has a beginning in time. Naively, there would be only one (class of) reference frame(s with the same velocity) in which time starts simultaneously everywhere. (This is a naive application of “relativity of simultaneity” from SR.) But...
  17. R

    Is the Speed of Light Really 3000 km/s?

    Problem Statement: I know that speed of light is 3000km per second Relevant Equations: Is it true? Can someone correct me if I 'm wrong,just interested thank-you
  18. andrea1313

    Terminal Speed of Vessel in Vacuum and Weightlessness

    In vacuum and weightlessness, at the bottom of a cylindrical vessel (a cup), there is a layer of solid substance of molar mass μ. This substance sublimes slowly (evaporates from the solid phase into gaseous phase) and pushes thereby the vessel to the opposite direction. Estimate the terminal...
  19. S

    Force on a copper loop entering into a magnetic Field B with speed v

    Hi, second problem in one evening, I'm sorry! But I'm also not quite sure if I did this one right. I had thought I need lenz's law but there is no current before entering the field so I just use the induced Voltage? My approach: ## V = \frac {B*A}{t} ## ## IR = \frac {B*A}{t} ## and ## A = v*t...
  20. S

    Spring is stretched, then attached to a mass, what is its speed at x?

    Dear all, I am back with another Spring problem. I have tried to use the insights I gained from your help last time: https://www.physicsforums.com/threads/find-v-x-of-a-mass-suspended-from-a-spring.972942/page-3#post-6190934 I figured I start with calculating the new equilibrium by ## x...
  21. BriggsFz

    Air Cannon Air Speed Calculations

    I am working on an Air Blast Cannon for work and am having trouble finding the proper formulas to calibrate it. This Cannon needs to have a barrel with about a 4 inch bore. It will only be used to generate high speed air blasts, so it will not have a projectile. Are there any formulas or online...
  22. pobro44

    Angular and orbital speed at perihelion

    Hello to all good people of physics forums. I just wanted to ask, whether the angular and linear (orbital) speed in perihelion of eliptical orbit are related the same way as in circular orbit (v = rw). If we take a look at the angular momentum (in polar coordinates) of reduced body moving in...
  23. V

    The speed of molecules and a machine that sorts them

    Is there any device that can sort the particles of some medium according to the speed at which they move in the medium or something like that? As far as I know, there's a machine that can sort particles by charge through a magnetic field. Is there such a thing for speeds and charged / uncharged...
  24. J

    Propagation speed of the electrostatic field

    I have a proton and an electron at a certain distance from it. The proton exerts an electrostatic force on the electron. I then neutralize the proton's charge by firing another electron at it from behind. How long does it take for the first electron to sense the change?
  25. O

    What is the final speed of a mass collision?

    I am stuck, please help. I did the conservation part but then i can't move on.
  26. T

    Speed usage in Lattice Boltzmann Method

    One thing that confuses me is the physical speed and sound speed. The lattice sound speed cs=1/sqrt{3} corresponds to the physical sound speed for isothermal flow (sqt{RT}). Why isn't the physical speed (e.g. inlet speed up of lid cavity) converted and use accoringly? $$c_p=\sqrt{RT}≈330m/s...
  27. G

    B Is the case for a Universal Speed Limit experimental or theoretical?

    ...Or even based on logic? I understand that it is expected that there might be a Universal Speed Limit and that this seems with extremely high probability to coincide with the speed of em transmission in a vacuum. This is borne out by experimentation and observation. Are there any other...
  28. S

    Speed of the outside of a wheel faster than the speed of light

    I'd like to know if anybody can tell me either; How large a wheel would have to be for the outer edge to travel faster than the speed of light if powered by a motor that's around now. We'll say the wheel is in outer space to eliminate drag and such
  29. S

    With what speed does the tip of a 25 meter high tree hit the ground?

    Hi, I am still working on the same exams problems, the second last one is giving me a bit of a headache after I found this https://physics.stackexchange.com/questions/2372/how-long-would-it-take-for-an-upright-rigid-body-to-fall-to-the-ground which contradicts my solution. Can I assume that when...
  30. G

    I What are the general parameters for the in-medium light group speed?

    An electromagnetic wave has a phase speed and a group speed. Or velocities, for that matter. In a medium, the phase speed of a wave is generally determined by the medium's permeability μ and permittivity ε. What are the general parameters that determine the group speed of a wave in a medium?
  31. Lifeboatboy

    Elementary Q: how to calculate speed of Mass B, upon impacting Mass A

  32. U

    B Train platform -- light speed and time quandry

    Summary: how can a train traveling at light speed, travel at two different speeds being the same train, depending upon whether you are on the train platform or in the train You say good bye to your friend at the train station. You get into train that will travel at just under light speed...
  33. G

    Automotive How much power is needed to haul a weight up an incline at a certain speed

    Good afternoon everyone, I need help with a certain calculation. How much power is needed to pull a 250lb load up a 45 degree incline at 5 mph. I am working on a project at school and would like to know what kind of motor we can use. Thank you very much!
  34. T

    Calculate the speed of the flywheel for my drone motor

    I have drone motor with propellers attached to the top of ring flywheels rim at the edge and the flywheel is 18 inches in diameter and weigh's 2kg. The drone motor with propellers rpm 5200 and thrust 900 grams. What rpm will the flywheel be rotating at.
  35. C

    At what wind speed will salt water "boil"?

    During hurricane Lenny in 1999, I was anchored on my sailboat in a protected cove in the Virgin Islands. The winds were about 60 mph with gusts to an estimated 80 mph where we were. I observed that during the gusts, the salt water would "boil", that is, totally evaporate the surface into a cloud...
  36. P

    Extra speed necessary to orbit at 970 km

    So using conservation of energy where v0 = 7000 m/s $$ K_{i} + U_{i} = K_{f} + U_{f} $$ $$\frac{1}{2}mv^{2}_{0} - \frac{GMm}{R} = \frac{1}{2}mv^{2} - \frac{GMm}{r}$$ where R = the radius of the Earth and r = the distance from Earth's center plus the height its orbiting $$v =...
  37. D

    Finding Angular Speed and the Change in Kinetic Energy

    (4+.9)(Wf)=(4+.63)(.27) (4.9)(Wf)=1.2501 Incorrect
  38. H

    I Can an object fall into a black hole at faster that the speed of light?

    I am wondering if an object can fall into a black hole at faster that the speed of light. I have heard that the expansion of the universe can make distant galaxies appear to recede from one another at velocities faster than the speed of light.

 Intuitively, this makes sense to me. I am...
  39. ejantz

    A billiard ball moves at a speed of 4.00 m/s and collides elastically....

    I know this question has been asked before but I really need some help finishing this final piece of the puzzle.. I have attached an image of my work to show how far I have gotten towards the solution but my unfamiliarity with trigonometry equations has gotten my stuck. Please assist! thank you
  40. P

    Solving for c: Speed of Q2 in Q1 Reference Frame

    I've tried using this equation: Where: u' = 0.86c v = 0.3c u = Is the speed of Q2 as measured by an observer in the reference frame of Q1 = 0.922c Where have I gone wrong? Have I missed a negative symbol?
  41. A

    Most probable energy and speed for Maxwell-Boltzmann distribution

    I just recall the two expression for the Maxwell-Boltzmann distribution: $$ P(v)dv = \left( \dfrac{m}{2 \pi k T} \right)^{3/2} 4 \pi v^2 \exp \left(- \dfrac{mv^2}{kT} \right) dv \qquad P(E)dE = \left( \dfrac{4E}{\pi} \right)^{1/2} \dfrac{e^{-E/kT}}{\left( kT \right)^{3/2}} dE$$ The left...
  42. S

    Is Time Just Another Coordinate Axis That We Can Only See Moving Forward?

    Is it possible that time is just another coordinate axis? That it can be moved freely into backwards and forwards at any speed. And for some reason we as humans can only see it moving forward? and communicate at a definite speed in time. Maybe everything is already calculated and solved, and...
  43. G

    Will Headlights Work If My Car Travels Near the Speed of Light?

    If my car can travel the speed of light will the head lights still work?
  44. W

    What Would Happen if the Sun Suddenly Disappeared?

    I have a question I have been wondering about lately ,somewhat abstract . Hopefully my description will be adequate . Here it is … The Earth revolves around the sun because of the suns gravitational pull and Earth's inertia . It takes about 8 minutes for sunlight to reach the Earth . What would...
  45. GreatCucumber

    Several questions about the speed of light

    1. How did one came to the constant 299 792 458 m/s? 2. Why this particular number? Why not 521 334 992? 3. Why can't we travel faster than the speed of light?
  46. V

    How do you calculate wind speed using the Doppler effect?

    Problem Statement: The Doppler effect is routinely used to measure the speed of winds in storm systems. As the manager of a weather monitoring sta- tion in the Midwest, you are using a Doppler radar system that has a frequency of 625 MHz to bounce a radar pulse off of the raindrops in a...
  47. A

    Given RMS Speed, Find Temperature

    mass of N2 = 28 * 1.67e-27 kg not sure if this is the right one - just googled it triple covalent bond N2 radii = 54e-12 m I tried to first find the rms speed: v = x/t = 50 * 54e-12 / 3 m/s Then I solved for T (in K): [50 * 54e-12 / 3]^2 * [28 * 1.67e-27] / [3 * 1.38e-23] = 9.14e-22 K
  48. S

    Average speed of molecules in a Fermi gas

    My first most obvious attempt was to use the relation ##<\epsilon> = \frac{3}{5}\epsilon_F## and the formula for kinetic energy, but this doesn't give the right answer and I'm frankly not sure why that's the case. My other idea was to use the Fermi statistic ##f(\epsilon)## which in this case...
  49. Celso

    Required speed for two events to be simultaneous

    I'm doing some exercises about special relativity and one of them asks to find the speed in an arbitrary frame of reference (1) in such a way that it perceives two events at the same time that didn't happen simultaneously in other frame of reference(2). Is it correct to state that if the...
  50. Lars278

    I Sending a cryptographic key faster than the speed of light

    Quantum entanglement does not imply that you can send information faster than the speed of light since you cannot manipulate what your sending. You don't know what you have until you have measured it. But you do know that you have the corresponding photon at the other location in space no matter...
Back
Top