# 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. ### I What was expansion speed at the time of the CMB?

To be clear, I'm looking for the speed at which points in space were moving apart from one another in the universe as it existed 370000 years after the big bang, not the Hubble parameter (expansion rate) derived from our current distance within our current time. Thanks in advance!
2. ### How much impact mass does a boxing strike need to compensate for slower speed?

I'm trying to wrap my head around the mass vs speed equation of mechanical power. If you double mass, you double power. If you double speed, you quadruple power... Fine, but what does that tell us about the relative importance when things aren't doubled? Let's say boxing Agent 1: has 3%...
3. ### I Where Do You Stop? (Do electrons or photons move faster than the speed of light?)

The title is from a great book by Eric Kraft, who plays around with one's physical-being in elemental terms in an excellent novel. He is very funny. To get down to my question: Do electrons or photons on anything move faster than the speed of light?
4. ### B Do changes in speed always affect orbit size, and vice versa?

For example, does even a tiny boost to an orbiting object widen its orbit a tiny amount? Let's use a hypothetical teleporting asteroid in orbit around the sun. If we teleport the asteroid a millimeter outward, then a kilometer inward, then an AU outward, does its orbit slow a tiny amount, then...
5. ### B Highway: Deriving speed from Doppler shift

I passed a motorcycle on the highway going the opposite direction. I know I was doing 125/km/h. I estimated that the frequency of his motor dropped by an entire octave, so that's a doubling of the wavelength. My intuition is telling me that's extremely unlikely. I can't actually calculate how...
6. ### B Measuring the speed of light on moving systems

A spaceship moves at half the speed of light, what will be the results of measuring the speed of light for it: in relation to the light source it is approaching and in relation to the light source from which it is moving away?
7. ### Why is Desmos so insanely fast?

Why is Desmos (graphing calculator app for Android, ios, windows, etc) far faster than everything else in calculating? I've done the same calculations in Mathematica, Maple, hp prime pro app on windows (ios and Android), various calculator apps on ios and Android, etc This is one of the things...
8. ### Velocity vs Speed (What's more relevant here?)

This is a famous book in India. I was wondering if one could say if the answer should include velocity or speed. I mean, I don't think there are any details which hint at velocity. We are gives speed in the question and we are asked to find out the distance traveled, this hints we are asked to...
9. ### I Why is there nothing faster than light?

Hi guys! Is there a simple explanation to describe why light owns the faster speed at the universe ?
10. ### I Generate a trackmap with speed and g forces

Hello everyone, I hope you are all great. I am writing down to all of you concerning a problem of mine. I am struggling to generate a trackmap based on Speed (m/s) and G-Forces on excel. I followed a topic created 12 years ago but unfortunately it is closed ...
11. ### I Constancy of Speed of Light: Postulate or Assumption?

The theory of relativity is based on two key principles: the principle of relativity and the constancy of the speed of light. The constancy of the speed of light is one of the fundamental postulates of the theory of relativity. Which statement is wrong?
12. ### B Can Relativistic Mass Create a Black Hole?

Hello dear colleagues from the physics forum, Following question came to my mind, can you tell me which statement (s) is/are wrong?. Since relativistic mass is weighed with the Lorentz factor for an external observer, it is expected an increase with speed. Such mass (m1) would tend to...
13. ### The advisory speed for a car on a sloped bend

Hello all, I am sadly stuck on the last part of a circular motion question sheet I was given for homework. I have a mark-scheme with me, but it has actually given me more questions than answers. I have attached my working, and how I arrived at my answer, and the differences it has with the...
14. ### Solving Orbital Speed with Energy & Angular Momentum Conservation

I've already solved the orbital speed by equating the kinetic and potential energy in the circle orbit case. $$\frac{1}{2}mv^2 = \frac{1}{2}ka^2.$$ And so $$v^2 = \frac{k}{m}a^2$$ Now when the impulse is added, the particle will obviously change course. If we set our reference point in time...
15. ### Measuring the Speed of Sound in Water, Using Faraday's Law

Hi! This project involves both mechanical and electrical elements, so I'm discussing it in this forum since I'm not sure which one it would fit better into. I'm working on an experiment in which I'm trying to measure the speed of sound through water. The approach is simple: I have a long...
16. ### What happens when a plane slows down from hypersonic speed?

Is it results in a new shockwave or does the shockwave continuously created by hypersonic flight suddenly disappear? I guess slowing down from hypersonic speed will cause a lot of vibration on the plane.
17. ### Solving for Speed & Revolution: F * delta t = p

Honestly i have very little idea. F * delta t = p F * delta t /m = v So i know the speed of the rod And i know that however high the rod is supposed to go, when its back down it should have done excactly one revolution. I have the feeling that I should So probably i have to use something like...
18. ### Speed of function vs lambda calls in python

An observation I made earlier- something like def f(...): ... return ... def g: ... = f(...) was quite a bit slower than doing def g: f = lambda ... : ... ... = f(...) any reasons why?
19. ### Help to calculate rotation speed of rotor

Hello everyone, i have a question, how to find wind turbine rotor rotation speed based on freewheel rotation speed of rotor (RPM)(torque = 0)? Thanks for your attention.
20. ### I One-Way Speed of Light: Is it Possible?

From what I've read, it is not possible to measure the one way speed of light. We must reflect it off a mirror and then divide its travel time by 2, giving us its round trip average speed. Time dilation makes synchronizing two separated clocks impossible. We just assume light goes at C in all...
21. ### How to calculate Earth speed of the Moon induced orbit?

It didn't work and I don't know how to do it.
22. ### Initial speed of an object launched from Mars

U1 = -GMm/r K1 = (1/2)mvi^2 U2 = as r approaches infinity, U2 approaches zero K2 = (1/2)mvf^2 (1/2)mvi^2 - GMm/r = (1/2)mvf^2 + 0 vi = √(vf^2 + (2GMm)/r) = √(250,000 + 2(6.7 E-11)(6 E23)/3400) = 153776.815 But that is not the correct answer, can anybody see my mistake/misunderstanding?
23. ### Speed of sound from resonant length of tube vs tuning fork wavelengths

The graph is, I do not understand why how it is possible to find the speed of sound from the gradient for this graph. Can someone please help? Many thanks!
24. ### Engineering Measuring rotational speed for Tachometer vs Oscilloscope

Question: Why does the oscilloscope double almost the exact value of rotational speed measured by Tachometer? Rotational speed from Tachometer = 1930 [RPM] Frequency of 1 period = 64.3 [Hz] which means 3857.91 [RPM] The output waveform of hall-effect sensor is attached. Can you have any...
25. O

### I Has anyone measured the speed of light in one direction?

I don't know if there are some here familiar with "Veritasium", a YouTube video channel dedicated to science and engineering. It was created and is hosted by Dr. Derek Muller. It has over 8 million subscribers and many of its programs have been watched millions of times. Today I was watching an...
26. ### Find speed of block using energy and spring equations? Help please

So here's what I did but it isn't right: W = (Kf + Uf) - (Ki + Ui) (2.6)(9.81)(0.45)(-0.01)=(1/2mvf^2 + 1/2kxf^2) - (1/2mvi^2 + 1/2kxi^2) -0.1 = (1/2(2.6)(vf^2) + 1/2(855)(0.02^2)) - (1/2(855)(0.03^2)) 1.3Vf^2 = 0.114 Vf^2 = 0.09 Vf = 0.3 m/s
27. ### B Simple yet effective way to reliably control the DC Motor speed?

As a part of our physics high-school self-study, we are making a stroboscope. We have a small 5-V DC motor that powers the strobe disk. It works as expected, but, clearly, the motor makes the disk spin at a constant speed. Is there an easy, but effective and reliable way to control its speed...
28. ### Max speed of mass on war-wolf

For this problem, For part(a) the solution is, However, how did they know that the max speed is reached in the vertical position? For part (d) the solution is, However, I thought the solution would be because there is net gravitational torque in clockwise direction (torque due to small mass...
29. ### B Speed of light and this Accelerating Rocket scenario

Hello, I have a question linked to gravity and speed of light : According to the Special Theory of Relativity, the speed of light cannot be exceeded because it would need an infinitive energy to accelerate the relativistic mass of a moving objet (a space craft for example) : indeed if the...
30. ### Flooding and Stream area and speed

Hello, I am thinking about a real-life problem: the flooding of a stream in may area of town. A stream discharge, ##Q=A v##, represents the volume of water passing through the cross-sectional area ##A## in one second as the water moves with speed ##v##. Let's assume that the stream has 2...
31. ### I Doppler Effect on Light: Constant Speed of Light Confusion

If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed? Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy) :oldconfused:
32. ### Finding the final speed of a space probe using work and kinetic energy

First I found work: W=(3.85x10^5)(2.45x10^8) W= 9.43x10^13 Then used that for difference of kinetic energy: 9.43x10^13 = (1/2) (4.55x10^4)v2^2 - (1/2)(4.55x10^4)(1.22x10^4)^2 9.43x10^13 = (22750)v2^2 - 3.386x10^12 9.43x10^13 + 3.386x10^12 = (22750)v2^2 9.77x10^13 = 22750v2^2 9.77x10^13/22750...
33. ### 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...
34. ### 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...
35. ### 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...
36. ### 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...
37. ### 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...
38. ### 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...
39. ### 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...
40. ### 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...
41. ### 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...
42. ### 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...
43. ### 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?
44. ### 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...
45. ### 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...
46. ### 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...
47. ### 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...
48. ### 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?
49. ### 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...
50. ### 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.