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. V

    I How to rule out that the speed of light was different in the past?

    The constancy of the speed of light is a fundamental principle in modern physics, and it is supported by a wide range of current experimental evidence. There is no evidence to suggest that the speed of light was different in the past, and the idea that it could have been different is at odds...
  2. C

    Max speed of proton in cyclotron

    For this problem, The solution is, However, I don't understand why they say just before the proton escapes? Are they assuming that the B-field is only at a portion of region which means that only half or so of the circular path is within the B-field so only half of the path have a magnetic...
  3. J

    B Questions about the Speed of Light, Time, etc.

    1. How much is acceleration of light? 2. Why is speed of light constant if time is relative, to keep speed constant distance must be changed as well? 3.What is time by Einstien? If we state it is not absolute, it is illusion, dont exsit? 4. Can we say ratio between distance and time is...
  4. J

    B Time Travel: Can Humans Go Faster Than Ordinary Speed?

    Will humans ever be able to travel forward in time faster than the ordinary speed? According to relativity if someone were traveling at speeds close to the speed of light they would jump some years into the future. The question however is will this ever be possible? Can humans survive at such...
  5. Philip Koeck

    I Speed of Electricity in Plate Capacitor & Cable

    Consider a large, charged, plate capacitor with a distance of, let's say, 1 lightsecond between the two parallel plates and also a plain cable of the same length. Everything is in vacuum. If I first connect one end of the cable to one plate and then connect the other end to the other plate the...
  6. Drakkith

    I Speed of Two Falling Ladders (Veritasium Video)

    Just saw this Youtube Short by Veritasium. The idea is that two identical chain ladders are dropped at the same time. One hits a table shortly after release, while the other continues to free fall. Curiously, after the bottom of the first ladder hits the table, the remainder still in free fall...
  7. AlienGrey

    B Light Speed In Forward and Backward Directions

    In the illustration below, a laser beam is emitted, and the time recorded, from the rear of a frame which is 150,000 km long and is moving at velocity 0.5 c to the right. When the beam reaches a half silvered mirror at the front of the frame, the frame being shown as a rectangle and using...
  8. marcosdb

    B Speed of Light Unaffected by Moving Emitter

    Is my understanding correct that if we have a moving vehicle moving to the right at speed v, as above, with a light source in center going in both directions, that (upon emitting the light at time T), a detector at D1 & D2 would both detect light reaching it at T2? (even though in the time it...
  9. paulimerci

    Find the speed of the biker at the bottom of the hill

    I used the work-energy equation to solve this problem.Only the parallel component of force acts to accelerate the biker and increases K.E., so only the parallel component and frictional force does work on the biker. therefore, Net work done on a system = change in K.E $$(F_I{_I} + F_F) d...
  10. S

    I Arguments leading to the speed of light as a dimensionless constant

    I once read (though I don’t remember where) that in the same way that you talk about a dimensionless ratio between Y and X in ordinary space, you can conceive of c as a dimensionless ratio between T and X in spacetime. Do you know where I can find a reliable treatment of that idea? As...
  11. A

    A MOND: Two Small Masses at Great Distance - Orbit Speed

    In MOND should two small masses placed at a great distance orbit faster than in Newtonian dynamics? Would it be theoretically possible to do this experiment in the solar system, or would the presence of the sun and earth be a problem?
  12. Lluis Olle

    B What if a needle hits Jupiter at nearly speed of light?

    This is much like the "grain of sand" at near speed of light hiting Earth, but now targeting Jupiter with a needle! What if... Any way, which is the correct way to analyze this kind of scenarios? For example, I think than being the speed of the needle near c, is nonsense to think about the...
  13. Matthew_p

    B The One-Way Speed of Light: Speculation of a Tired Mind

    I perfectly understand that we cannot, as of our current methods, determine the one-way speed of light - I was, however, wondering if we could determine a relative speed using electrical transmission. the basis of which would be sending a laser light across a field into a sensor, starting a...
  14. C

    B Deep Space Speed Limit: What Prevents Exceeding Light Speed?

    This is probably a dumb question. I'm not a physicist and took basic physics a very long time ago. If an object was in deep space, a long way away from gravitational fields and was subjected to a constant 1g acceleration in a straight line what prevents it from eventually exceeding light speed...
  15. DaveC426913

    At what speed can a person jump in zero g?

    Relating to Stratus' thread in this forum, I'm trying to figure out initial velocity of a jump in zero g. Lets start in 1g. In one g, a person might be able to jump 1m. From this, using : v^2 = u^2 - 2as, we can calc his initial u as 4.45m/s. Is it a safe assumption that, were an astronaut to...
  16. Kyuubi

    Speed of Current in a coaxial cable

    I've found the inductance and capacitance per unit length in a long coaxial cable. I even clearly see that if I multiply the two, I can get the speed of light. How do I begin to find the current wave and its speed though?
  17. anorlunda

    Who Limits My Internet Speed, and How Do They Do It?

    I've been bothered lately with an excessive number of freezes and dropouts when trying to conduct meetings on Zoom. As many as 10 freezes with 5 dropouts in 15 minutes. I installed a tool to monitor my connection from my PC. In 4 hours, it gathered the following stats. General stats...
  18. Tom4

    Speed of asteroid at impact

    Since the radius is "very far", I cannot find the total mechanical energy by using the gravitational potential energy formula and find the kinetic energy at impact. I can't think of any other way to find final velocity without knowing the radius.
  19. B

    Calculating the speed air comes out of a fan

    Hi, I have a task where i have to calculate the speed of which the air is flowing out of a drones propeller fan. This is all i have been given: The mass of the drone m = 0.42kg, There are four propellers the lenght of each blade d = 0.21m, the air density is 1.3 kg/m3 and g=10 m/s^2y. And the...
  20. V

    Ambiguous Problem Statement on Determining Speed of Aircraft

    When I attempted this problem, I came up with two different solutions and answers, and both seem correct to me. However, the answer obtained by the first method is correct according to the book. The methods I used are as in screenshots below. Isn't the problem statement ambiguous, or maybe I...
  21. Leo Liu

    The choice of sweep angle (low speed) and its impact on C_L, C_D, C_M

    My school's group project is to design a powered model flying wing that can fly for as long as possible. To make it stable in the longitudinal direction, we either have to use a swept wing design or an airfoil with reflex (like BR1). We are forced to use wing sweep because the airfoil with...
  22. mpross

    B Dark Energy, Dark Matter and the Speed of Light

    Quote from NASA: My understanding of dark energy is based on NASA's report: https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy; were NASA state as follows: "It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on...
  23. A

    B Speed of Point on Expanding Sphere

    Suppose we have an expanding sphere. That means that the surface ##4 \pi r^{2}## is getting bigger and bigger. For example, suppose the area expansion rate is ##b \, r##. Does this limit the speed at which a point can move on the surface?
  24. P

    I Is RMS speed dependent on pressure?

    As it is clear from the formula for r.m.s speed ,it is only dependent of temperture for a particular gas,I think the answer should be "No change happens".But I wonder whether there is more insight to this.And is my assumption true?
  25. gggnano

    Can the thrust of Ion Thruster be increased at the cost of speed?

    Based on the little I know ion thrusters seem extremely impractical on earth, even because speeds over 20,000-30,000+km/h will result in immense friction and fire, so why not cut the high speed somehow and increase thrust which is abysmall? For example: the ions passing through a medium?? Wider...
  26. H

    Calculating how many revolutions it takes to reach the final speed

    11 rpm equals 1.15 rad/ sec not sure how this helps contribute to the question but I solved for it just in case
  27. M

    Hop height of a rod and speed/ time if hit upward with force f

    I want to make a few of these if it will not cost a lot.
  28. Lotto

    B Can we use molecular speed of gas formula also for the air?

    Molecular speed of gas is generally $$v=\sqrt{\frac{3RT}{M}},$$where R is gas constant and M is molecular mass. Can we use this formula for the air as well, when the air is a mixture of different gasses?
  29. T

    How to calculate the speed of this umbrella extension arm?

    TL;DR Summary: Have a university project where we are calculating the speed of the arm at the end. I need some help on where to start on the calculations. We have tried to determine instantaneous center of rotation for the device. We want to find the linear velocioty of point B. There is a...
  30. J

    I Car low average speed vs aero drag relation

    I drive car only at country roads allways at speeds 100-120km/h, no city and no idle time-heating engine etc. Why computer allways show very low average speed , 40-48km/h?I allways have feeling that this speed is too low because I allways drive way faster then this. Indeed all my friends have...
  31. S

    How do our brains process information differently depending on the task at hand?

    Wasn't sure where to drop this thread, so here it is! I've searched the internet and this forum, perhaps I don't have the correct keywords, but I haven't been able to find anything that discusses this topic. I'm interested to know about your individual experience of learning. I'd like to think...
  32. Z

    I The invariance of the speed of light is not only a hypothesis?

    hello Einstein assumed the invariance of the speed of light as an hyphotesis, while I was told that : "The speed of light need not have been postulated as an invariant." in other words the invariance of the speed of light could have been proven even regardless of the special relativity is it...
  33. P

    B GTR Vacuum vs Quantum Theory Vacuum: Speed of Light

    What is the difference between the GTR vacuum and the vacuum of quantum theory? What is the speed of light in the vacuums, specifically what can be predicted and what must be measured?
  34. A

    B Speed of Light in Free Space - Einstein's Second Principle

    hello everyone According to Einstein's second principle of relativity: "The principle of the constancy of the speed of light: The speed of light in free space has the same value c in all inertial reference frames" Does free space mean vacuum? Is this principle not valid in the air and the...
  35. M

    Finding a rocket's speed at height h

    I substituted 0 for vi, as the rocket is initially stopped. I am looking for Vf. So: Vf^2=0+2asΔs Vf^2=2asΔs I then substituted a=Fthrust/m So: Vf^2=2(Fthrust/m)Δs Δs at any given moment equals h so I substituted h for Δs. Then took the square root of both sides. Vf=sqrt(2h(Fthrust/m)) It...
  36. bobacity

    Calculating Speed of a Wave | Solving Problem with Period of 0.025s

    Since the period is 0.025s, I think the frequency is 1/0.025s = 40 Hz. I don't know how can I proceed solving the problem from here. I'm assuming I will have to try to find the velocity and wavelength, but idk how.
  37. K

    A Is it possible for variable speed of light in a vacuum?

    <unacceptable reference deleted> As our universe is expanding, that is, our vacuum is becoming more and more sparse, I believe it is possible that some key characterics of our vacuum is also variable.
  38. A

    I Speed faster than light speed in a medium

    hello everyone I read this text in physics book: "Another example is the so-called Cerenkov radiation, which consists of light waves emitted by charged particles that move through a medium with a speed greater than the phase speed of light in that medium. The blue glow of the water that often...
  39. F

    I Does the speed of light imply ....

    Does the speed of light imply that in its property as a wave it encompasses the entire known electromagnetic spectrum?
  40. E

    B Problem about postulate of the invariance of the speed of light

    Hello, I have a problem with the postulate of the invariance of the speed of light. When we move away from a light source it is redshift, it is the sign that the relative velocity between us and the light source has changed. If a stationary observer observes the phenomenon, he will measure that...
  41. Bastion

    B Experimental evidence for effective mass increasing with speed

    I am new to this forum and in line with its rules I am trying not to repeat a question already asked. I find that Jeremyfiennes original question I.e Experimental evidence for effective mass increasing with speed” is the question I want to ask but I am not satisfied with the answers given so...
  42. A

    I The speed of a waves on a string in Simple harmonic motion

    The speed of a wave in simple harmonic motion on a string is $$v= \sqrt{\frac{F}{\mu}}$$ where v= the horizontal velocity of the wave on a string. Is the F the horizontal force or the resultant force (combination of Fy and Fx)?
  43. T

    B What is the recession speed today of the matter which created the CMBR?

    Hello, I am talking to a Quantum Mechanics Physicist friend who is having a hard time accepting some of the Cosmology theories and numbers and I want to be sure that my numbers were correct. 1. Firstly, what is the recession speed, today, of the matter which created the CMBR? I told him that...
  44. MrSimp

    B Does Force Travel Faster Than Light?

    Imagine a plane 1 light minute across. Now imagine 1 person on either and of that plane. Between them, is a thin indestructible bar that is 1 light minute in length. In the center of this plane, a simple device has a wire that leads to a motion sensor on the left side of the bar. If the left...
  45. F

    I Averages and average speed/instantaneous speed relation

    Hello, Given a set of numbers, we can calculate their average and there are different types of averages (arithmetic, weighted, harmonic, geometric, etc.) The choice of the average depends on the situation. The average, also called mean or expectation value, is a number that can replace all the...
  46. 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...
  47. 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...
  48. 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...
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