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. Narasoma

    I Gravity Wave Speed: Deriving Constancy from Maxwell Eqs.

    We can derive the constancy of the speed of light from Maxwell equations. My questions are: 1. Why it is then need to postulate it when we can obtain it from Maxwell equations? 2. It is stated in many books that gravity wave also propagates with the same speed, c. How do we conclude that? Is...
  2. F

    I Is the concept of a one-way speed of light meaningful and measurable?

    We can measure the two-way speed of light, but not, apparently, the one-way speed. Light could travel at c in every direction or c/2 in one direction and instantaneously in the other. Nature does not provide us with a way of determining the one-way speed. I can follow some of the basic...
  3. K

    I Why do we still say “the speed of light” instead of “c”?

    I get it, it sounds cool. But it is a very misleading and sometimes confusing way to represent c. The speed of light is not constant. When I say that, I’m talking about the speed of LIGHT. Not c. Cherenkov radiation is a result of particles moving through some material (usually water) faster...
  4. L

    I Do objects gain mass while approaching light speed?

    I watched a fermilab video claiming objects don't actually gain mass as they approach light speed. Is that true? What keeps things of mass from reaching the speed of light or beyond? I assume matter doesn't accumulate higgs-bosons while in motion?
  5. A

    I Speed of Light: Spaceship & Gravity Law Impact

    A ship that moves at a speed very close to light, its relative mass increases and may be comparable to the mass of the planets or the sun, in this case it can change the orbit of the planets and the sun (according to the law of gravity of two bodies) Is this analysis correct?
  6. H

    A Calculate the group velocity in EIT (famous paper: light speed 17m/s)

    hello everyone! Recently,i'm reading a paper about slow light,that's really a famous work published in Nature.[Light speed reduction to 17 metrespersecond in an ultracold atomicgas]. But I'm trouble with some calculation about the velocity of slow light.here are below: i try to use the...
  7. LarryS

    I Speed of individual photons in a vacuum?

    Is there experimental evidence that confirms that the speed of individual photons in a vacuum never varies, even slightly? Thanks in advance.
  8. P

    I Does the Light's One/Two-Way Speed Distinction Mean Anything?

    There is a trivial metaphysical freedom, that emerges from the fact that people living in a world must use their own measuring devices, that are part of that world. E.g. people living in a flatland must use their own rulers to measure distance. If one half of their world is widened, they...
  9. J

    Max linear speed of propeller tip in water

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  10. N

    How to Implement 25us Delays in PIC16F84A PWM Code?

    Hello how are you :D I have some problems with my code, which is to control the speed of a DC motor by pwm with the pic16f84A. I already have the combinations and the diagram of the signals, but I still need to implement the part in bold. How would the delays be implemented so that they have...
  11. vytrx

    Finding speed of a falling brick using distance and the acceleration of gravity

    Since the brick has fallen 30m and the acceleration of gravity is 10m/s^2 the brick would have fallen 3 seconds speed of brick would then be 30/3 leaving the answer at 10m/s?I am new to physics and this question has left me stumped
  12. wcivch

    B Ride Gravitational Waves to Increase Speed?

    This is my first post so I apologize if i am in error anywhere. I recently had a thought that I have had trouble confirming. Based on the following assumptions. 1.) As you accelerate an object near the speed of light it’s mass increases exponentially. 2.) Mass warps space time. 3.) Spacetime...
  13. kasnay

    A Wave direction and speed of propagation

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  14. O

    B Parallel light reflection for a one-way speed of light measurement setup

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  15. Svelte1

    Solving for Speed: Exploring Velocity and Magnitude

    https://ibb.co/jG6n0jZ The 15 is fine as this is clearly his overall magnitude but then v2 is equated to the horizontal velocity rather than the magnitude.
  16. mef

    I How to Measure Speed of Light & Is It Constant?

    The question constantly arises how the speed of light is measured and what does it mean that the speed is constant, including at remote points for the observer, including at points beyond the local frame of reference, as you understand it in general relativity (GR). First of all, it should be...
  17. M

    Measurement of speed using redshift or blueshift

    The solution on my textbook is 13000m/s toward Earth as the light is blue-shifted I'm able to calculate the magnitude of velocity (13000m/s), but i don't understand why thus is blue-shifted? Since in the lab, the light's wavelength observed is slightly higher than light from Ursa Majoris. So my...
  18. A

    I My textbook is deriving wave speed on a string under tension

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  19. H

    I Can we determine the one way speed of light by combined measurements?

    The idea is to have 2 clocks at position A and B. The clocks are synchronized by sending a light pulse from position S over 2 equal distances x. The receiver is at position R at a distance y rectangular to the direction AB and exactly in the middle between A and B and right below S. for proofing...
  20. Anmol Dubey

    Calculating final rotational speed from angular velocity

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  21. giulioooooh

    Calculating Vehicle Speed based on Downforce (Aerodynamics)

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  22. PCNerdz

    B Speed of Push vs Speed of Light

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  23. M

    I Light speed independent of source

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  24. J

    Find the drift speed of electrons in a wire

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  25. fluidistic

    I Extremely strange consequence of the speed of heat value

    By chance, I have read a paper that left me in shambles. I would like you to help me figure out if it makes sense or not. I tried to follow Bridgman's logic, without a complete success. The extraordinary claim is that, if you start with a system consisting of a 1cm^3 copper cube where 2 faces...
  26. C

    B Propagation speed of movements of a fluid in a pipe

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  27. T

    Average speed question about a hovercraft

    I assumed since both pass the points they must have the same speed at both instances, so the average speed would be 15 and the speed when passing the object should be 30, since the equation of average speed.
  28. bbbl67

    B Can You Become a Black Hole from Approaching Light Speed?

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  29. B

    B Can You Use a Rubber Bracelet as a Buoyancy Reference for Freediving?

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  30. dirb

    Calculating Angle & Speed to Reach Planet's Moon from Station Orbit

    A station is orbiting a planet at a distance R1, a moon is orbiting the planet at distance R2 with the period T. The planet itself has a radius rp and a mass mp. We know that when an object adds its velocity at a point in the orbit, the height of the opposite orbit will increase. Determine the...
  31. D

    B Proposed Experiment - Speed of Light

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  32. G

    B Constant Speed of Light: Low Speed Explained

    I understand the relativistic effects at high speeds, what accounts for the constant speed of light at low speeds? Example... If I travel towards a beam of light at 25 MPH, I will still measure the light as traveling towards me at the speed of light and not the speed of light plus 25 MPH.
  33. T

    I Permittivity/Permeability, the speed of light, and the meter

    Good evening all, I had a question on how the standard for the meter is defined. A simple Google search tells us that since 1983, the meter has been internationally defined as the length of the path traveled by light in vacuum during a time interval of 1 / 299, 792, 458 of a second. Many other...
  34. S

    Average ejection speed of gas

    I assume the system starts from rest so the total initial momentum is zero. Let: M = mass of trolley + cylinder m = initial mass of oxygen Δm = mass of ejected oxygen u = speed of ejected oxygen Conservation of momentum: 0 = (M + m - Δm) . vmax - u . Δm 1) Is my equation correct? 2) I...
  35. K

    B Experience Travel on a Photon: Hitch-Hiker's Guide to the Universe

    We know that as speed increases, time slows, and at the speed of light time apparently ceases. Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are...
  36. P

    I What if the Universe expands faster than the speed of light?

    I may be way off, but at some point in the past I understand there was super expansion where the universe expanded faster than the speed of light. If matter expanded with it it's an example of matter in separate areas moving apart faster than the speed of light. If it didn't the universe has...
  37. Dum Leme

    B Is light breaking its own speed limit when cornering?

    Being a neophyte to physics, I try to visualize a light cone as it travels about. I try to put myself in it and use my car to talk of it. When I ride in my my car, I note that when I corner, one wheel will speed up as compared to the other side. A light cone does the same, and given that the...
  38. PhysicsTest

    Control the speed of a motorcycle

    As the summary indicates I want to control the speed of a petrol bike upto 60Kmph, so that it does not go beyond that. Now my questions are can it be controlled electrically, most probably I may want to control the throttle, but how to do it? What all sensors I may require one I can think of is...
  39. Ineedhelpwithphysics

    Which object is moving to the right with a constant speed?

    I need help with this question how in the world is it A and C. In A everything is cancelling out so nothing is movingfand nothing moving to the right. In C its not moving to the right. Isnt it suppose to be D since sits going right constant to 20N.
  40. R

    I Black Hole Waterfall Analogy & Light Speed

    Recently I have seen a number of General Relativity visualisations that show spacetime flowing towards any mass, similar to water flowing into a sink hole. ScienceClic's video is an example. That model is also used in the "waterfall model" to explain the event horizon of a black hole, as the...
  41. alan123hk

    B Why is Light Speed Invariant & Finite?

    I think infinite speed is unimaginable. If something is moving at infinite speed, we can't find it at all because it has moved to infinity. Furthermore, when the maximum speed is limited, a reasonable inference should be that observers in different reference frames should find the same one speed...
  42. Mohika

    B Why is the speed of light a constant?

    The question is of what does it depend on? We take it for a constant still how do we know that its the exactly the same as in a vacuum in a center or closer to the center of our galaxy for example. Why is the number what it is?
  43. J

    Lawn/Garden Home project help please - max wind speed to tip over object

    Hi guys, thanks for helping me! I am working on a home project that includes a privacy shade for my backyard. I have a 6'x15' vertical shade attached to two portable posts that are held up right by four legs ("X" shaped footprint). the surface footprint of the legs covers an area of 46"x70". I...
  44. D

    I Light Speed in Dense Medium: Explained

    If light goes from air through glass, the reason the light bends is because I'm told that the light travels slower inside the glass. If I change my observer reference to inside the glass, the speed of light inside the medium it look like light is still traveling at the speed of light? if so, the...
  45. Hari Seldon

    Watt Rotational Speed Regulator's Lagrangian

    I understand that it is a system with two degrees of freedom. And I chose as generalized coordinates the two angles shown in the pic I posted. I am having troubles in finding the kinetic energy of this system, cause the book tells me that the kinetic energy is something different then what I...
  46. G

    B Exploring the Possibilities of a Higher Cosmic Speed Limit than c

    I think when the speed of light was measured (and predicted from Maxwell's equations) that the assumption was made that this speed was a cosmic speed limit Suppose that the cosmic speed limit was higher than c (not infinite) and that perhaps another form of radiation traveled at that speed...
  47. alan123hk

    B Measuring Light's Brightness & Power Flux Relative to Observer's Speed

    Assuming that the observer moves along the direction of the light, does the speed of the observer's measuring instrument relative to the light source (which may be close to the speed of light) affect the brightness and/or power flux density measured by the observer? I'm not sure about this...
  48. chwala

    Calculate the speed of car for maximum economy

    This is the problem, This is the textbook solution ; i think the textbook solution is not correct! My thought, ##S_{max}## will be given by $$\frac {dy}{dS}=0$$ $$\frac {dy}{dS}=\frac {-1}{400}S+\frac {1}{5}=0$$ $$S=80$$ It is maximum economy because ##y{''}<0##, i.e by use of the second...
  49. L

    B Why does a DC motor have a restricted speed?

  50. gen66

    How fast does a human sized object need to move so I won’t see it?

    Summary:: How fast does a human sized object(or a human) need move so I won’t be able to see it at all if it passes right next to me? Im trying to figure out something in regards to the Flash movies/Flash series but I’m super bad at calculations and math :( Do you think that mach 2...
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