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_Try_Math

    Using energy considerations to determine speed

    $$K_i + U_i = K_f + U_f$$ $$K_i = \frac 1 2 m(15)^2$$ $$U_i = 196m$$ $$U_f = 0$$ $$K_f = K_i + U_i - U_f$$ $$=\frac {15^2} 2 m + 196m$$ $$=\frac 1 2(15^2m + 98m)$$ $$=\frac 1 2m(15^2 + 98)$$ $$=\frac 1 2m(323)$$ $$=\frac 1 2m(17.97)^2$$ $$v=17.97 m/s$$ Not seeing where I'm...
  2. R

    B Help with science experiment magnets/force/acceleration/speed

    Sorry, I guess I should have remembered all of this from my school days, but right now I have forgotten so much that I need some help. I am developing some simple experiments for school children (age ca. 12). This one involving magnets. I am not asking for detailed calculations, that is way...
  3. petar rezek

    Calculate the mass of argon

    In the images you can see what I did I need someone to check answers and show me how to do C part of question
  4. chwala

    Calculate acceleration and speed of block

    Pretty straight forward, ...reason of posting is to check why i am having a negative value for ##a##. From my study, i know that ##R(||)## to plane ##F - 40 \cos \dfrac{π}{3} = 4a## ##a = -5 m/s^2## or can i as well have the equation ( friction and tension are at equilibrium) as, ##40...
  5. R

    Final speed of particle far away from rings with opposite charges

    When I submitted it, this answer was incorrect.
  6. Ineedhelpwithphysics

    Rating speeds from least to greatest from a position time graph

    My answer is R > P > Q = S Since R is more steeper than P the speed is greater, speed is a non-negative unit so i assume that speed of R will be greater than P Speed of P has a positive slope, also the magnitude is positive. Q and S are the same due to the fast that they both horizontal slopes...
  7. I

    I The speed of one photon through a transparent medium

    We know from double-slit experiments that singular photons behave like waves, so I expect that one photon would undergo refraction when entering, with an angle different than 90 degree, into water, glass or other transparent material. Is that true? If the refraction occurs, than the speed of...
  8. V

    Speed of ball on pendulum with Mechanical Energy

    Started by analyzing the change in energy from the initial position to the final position which gives us mgh=1/2mv^2 Since we are trying to find speed, we rearrange the equation to solve for v, which gives us √2gL. My question is, do we need to take a component of L for √2gL because it is at...
  9. U

    B What limits a rocket's max speed if in space drag is zero?

    What limit rocket max speed if in space drag is zero? If drag is zero and thrust is constant that mean rocket accelerate all the time?
  10. T

    Speed of a mass falling into a star given the mass and radius of the star

    I tried the square root of ((2)(6.67*10^-11)(3.90E+30))/(5.70E+7) I got 1.55*10^-5 and that is wrong. Maybe I am using the wrong equation but this is the one of professor gave me and I don't know what I am doing wrong :-(
  11. A

    Collision problem, two marbles, one starts moving and one starts at rest, find the speeds of both after the collision

    Solved equation 1 for v1f and then substituted into equation 2 and solved for v2f. Got 2.22 as the answer, but it said the answer is incorrect.
  12. ahira

    Frames of Reference: Find the speed and heading of the airplane

    so far what i have gotten to is that 300/0.75 = 400km/h but I dont know how to draw the diagram for this
  13. P

    B Is the Speed of Light Consistent Across Rotating Reference Frames?

    Does it violate the postulate of special relativity in any sense?
  14. P

    Help with question on motion: Avoiding a rear-end collision

    The question : A car and a truck are both traveling with a constant speed of 20 m/s. The car is 10 m behind the truck. The truck driver suddenly applies his brakes, causing the truck to slow to a stop at the constant rate of 2 m/s2. Two seconds later, the driver of the car applies their brakes...
  15. B

    B Speed of separation of two masses

    Hello. Let's say we have two masses, each moving in 90 percentage of light speed in opposite direction. Then what will be the speed of the one mass according to an observer in the other mass?
  16. S

    Kinematic car overtake problem: Don’t know what to do...

    This is what I've done so far: 54 km/h = m/s 72 km/h = m/s a = 2,0 m/s^2 A = d_1 = v * t = 15 * 10 = 150 m B = d_2 = v * t = 20 * 10 = 200 m d_3 = d_1 - d_2 = 200 - 150 = 50 m Don‘t know how to continue to solve the problem.
  17. sjordannc

    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!
  18. Username34

    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%...
  19. cathrynliz

    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?
  20. S

    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...
  21. DaveC426913

    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...
  22. CHAIM123

    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?
  23. H

    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...
  24. Slimy0233

    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...
  25. T

    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 ?
  26. Gwen49

    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 ...
  27. Aromalsp

    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?
  28. Aerodyn

    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...
  29. Grizzly_1

    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...
  30. Kyuubi

    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 just...
  31. K

    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...
  32. Mustafa Bayram

    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.
  33. Justforthisquestion1

    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...
  34. ergospherical

    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?
  35. HauTo

    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.
  36. K

    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...
  37. M

    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?
  38. C

    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!
  39. S

    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...
  40. 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...
  41. L

    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
  42. K

    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...
  43. C

    Why does the war-wolf reach its maximum speed at the vertical position?

    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...
  44. J

    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...
  45. F

    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...
  46. I

    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:
  47. A

    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...
  48. 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...
  49. 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...
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