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. S

    What is the Relationship Between Tension and Velocity in a Vibrating String?

    Homework Statement (Problem #1 on this page.)[/B] Homework Equations ##v=\sqrt { \frac { T }{ \mu } } =\lambda f## The Attempt at a Solution I don't think there is enough information, ##v=\sqrt { \frac { Mg }{ m/L } } ## m, the mass of the string is not given
  2. K

    Question: Shadow Speed Related Rates

    Homework Statement 2. Homework Equations Similar triangles The Attempt at a Solution $$\frac{y}{30}=\frac{50}{x}~\rightarrow~x=\frac{1500}{y}$$ $$\frac{dx}{dt}=-\frac{1500}{y^2}$$ $$s=16t^2=16\frac{1}{4}=4$$ $$\frac{dx}{dt}=-\frac{1500}{16}$$ The answer should be ##~\displaystyle...
  3. Alexanddros81

    Determine the angular speed ##\dotθ## of the arm OC

    Homework Statement 13.30 The colar B slides along a guide rod that has the shape of the spiral R = bθ. A pin on the collar slides in the slotted arm OC. If the speed of the collar is constant at v0, determine the angular speed ##\dot θ## of the arm OC in terms of v0, b, and θ. Homework...
  4. C

    Guidance on Sensor Choices for High Speed Impact Sensing

    I am a fourth year biomedical and mechanical engineering student and my final project has to do with designing a crash test dummy. The end goal is to mount the dummy on a bicycle and launch it at ~20kh/h, then it will be struck by an automobile driving at ~30km/h. Various different impact...
  5. Pushoam

    Number of molecules moving in a given direction at a given speed

    Homework Statement I didn't get it. Consider velocity space. The no. of velocity vectors for a given speed is proportional to the surface area of a sphere of radius v in the velocity space. So, a fraction of velocity vectors with speed v in an elemental area dA is##\frac{ dA}A##. A = ##\Omega...
  6. C

    How Long Does a Sprinter Accelerate Before Reaching Top Speed?

    Homework Statement The sprinter completes 200m in 19.3sec. When the starting pistol fires he accelerates constantly until he reaches his top speed of 13.4m/s. He then holds speed till the 200m point. At what time did he stop accelerating and how many meters did he cover before reaching top...
  7. S

    I Is the speed of gravitational waves non- dispersive?

    Dear all, In a recent talk, I have heard that speed of gravitational waves is non-dispersive. How is it proved "observationally" in LIGO detections that all the frequencies travel with the same speed, so one can say the speed is non-dispersive?
  8. A

    Calculating speed in circular motion

    1. Homework Statement A whirlygig is made by hanging a mass, m1 = 386.0 g, through a tube and then spinning another mass, m2 = 198.0 g around so that it forms a circle. When this happens the string makes a small angle with the horizontal as shown in the diagram. If this is done at a specific...
  9. C

    Controlling the speed of an AC motor?

    For my IB Physics Extended Essay, I have just changed my topic from momentum to this and I have really little time of doing it... (Due in Oct) :cry::cry::cry::cry::cry::cry: My research question would be The effect of angular velocity of circular motion of a moving sound source on the sound...
  10. A

    Calculate speed of block at the end of the ramp

    Homework Statement A mass of m = 1.40 kg is placed on a 2.00 m ramp. The angle of the ramp can be adjusted by changing the height of the top of the ramp. In reality there is a small amount of friction between the block and the ramp: μs = 0.261, μk = 0.119. If the ramp (with friction acting)...
  11. M

    Two object leave the same point with different speeds

    Object A starts from the origin velocity 3 m/s and object B starts from the same place with the velocity 5 m/s, 6 seconds later. When will B Catch up with A?
  12. S

    Wind speed versus the force on objects like flower pots and deck boxes

    i was wondering if anyone had a site that catalogued windspeed effect on heavy objects? I found several sites that talk about trees and shingles, I'm interested in flower pots and deck boxes. If someone could shoot me an equation would allow me to calculate various objects
  13. K

    MHB Determine the speed of the parachutist

    A parachutist is falling with speed 176 ft/s when his parachute opens. If the air resistance is Wv2/225 lb where W is the total weight of the man and the parachute and v is in ft/s, find his speed as a function of time after the parachute opens.
  14. N

    Net force from a varying speed non-uniform rotating wheel?

    If we have a circle rotating then a force v^2/r is produced but no net directional force exists as it cancels. But if the wheel was not uniform in mass ie not rotating around its centre of mass AND the speed changes over time as a function of the "wheel"'s rotation angle could a net force be...
  15. rkatcosmos

    What is the rotational velocity of hard disk actuator arm?

    0 down vote favorite There is a lot of material discussing the rotational speed of the magnetic disk in HDD but not about the rotational speed of actuator arm. What are the typical actuation speeds of the actuator in HDD? This information will be used in the design of a fast shutter system...
  16. T

    Net Neutrality, Bandwidth, Speed, Physics, Greed?

    Net Neutrality is being overturned as ISP telecoms claim they need to throttle bandwidth and speed as too many customers want to stream HD videos and play HD online games. EM waves are fast and unlimited, so is Tiered Premium Billing absurd? ISP telecom expenses include transmitters...
  17. M

    Effect of electrical permittivity on the speed of light

    Why does an increased electrical permittivity reduce the phase velocity of light in a medium? Furthermore, what interactions do we see on an atomic level? I am aware of the equation that defines the speed of light in terms of the electrical permittivity and magnetic permeability, but I do not...
  18. M

    Effect of electrical permittivity on the speed of light?

    Why does an increased electrical permittivity reduce the phase velocity of light in a medium? Furthermore, what interactions do we see on an atomic level? I am aware of the equation that defines the speed of light in terms of the electrical permittivity and magnetic permeability, but I do not...
  19. S

    I Traveling at a speed faster than light

    Imagine yourself to be bat. You can't see anything. You don't have eyes. All you can do is echolocate, using ultrasound. Now imagine something is moving away from you faster than the speed of sound. Can you locate it ? Can you perceive its existence ? If the answer is yes, how ? If no , then...
  20. A

    Calculating Speed at Time of Collision

    First, let me apologize if I don't give you enough info. If I'm missing something, please let me know and I'll try to get the info. Here is the set up: Car A is traveling West at 5 mph. Car B is traveling South at X mph. Car B strikes A on the rear quarter panel (centered on the rear tire)...
  21. Pushoam

    Final speed of ball : collision with vertical plank at top

    Homework Statement Homework EquationsThe Attempt at a Solution I have solved the first part already. For the 2nd part, I am writing the relevant eqns. Can I take motion of the plank as a pure rotation about the pivot ? Quantities corresponding to pivot is subscripted by p. Conservation of...
  22. Pushoam

    Finding the speed of rolling ball

    Homework Statement Homework EquationsThe Attempt at a Solution In rolling motion, friction doesn't work. So, applying work energy theorem, ## \frac 1 2 m v_0 ^2 =\frac 1 2 mv^2 + \frac 1 2 I_{center} \omega ^2 ## ## taking~ I_{ center} = \frac 2 5 m R^2 ~and ~ v =\omega R , v = \sqrt {(...
  23. Alexander350

    Finding angular speed by conservation of angular momentum

    Homework Statement Homework Equations Angular momentum = moment of inertia * angular velocity Change in angular momentum = impulsive moment v^2=u^2+2as The Attempt at a Solution First I used v^2=u^2+2as to find the velocity of the particle the moment the string goes taut. I got v=\sqrt{8ag}...
  24. Pushoam

    Final speed when the mass hits the post

    Homework Statement Homework EquationsThe Attempt at a Solution A) Assuming no gravity The only external force acting on the mass m is tension and the torque due to it about the axis of post is 0. Hence, applying conservation of angular momentum about the axis of post gives, ## m v_0 r = m v...
  25. K

    I Does the wave function shorten when approaching light speed?

    Relative to the observer, objects shorten when approaching the speed of light exponentially. Does this rule also apply to the wave function? Does this rule also apply to massless particles like Photons? Or am I just simply forgetting something?
  26. T

    How Does the Interface Determine Sensor Speed?

    What determines the speed of a sensor (keyboard and cameras) transmitting data from the external world (pictures, sound) into the processor (CPU)?
  27. PainterGuy

    The shell strikes the water at the same speed?

    Hi Please have a look on the attachment. Do you also agree that the shell strikes the water at the same speed? It seems false to me. The vertical and horizontal motion is independent of each other. When the shell is fired horizontally, it starts descending toward the ground from less height...
  28. E

    How do you find speed given the beat frequency?

    Homework Statement Using a radar gun, you emit radar waves at a frequency of 2.6 GHz that bounce off of a moving tennis ball and recombine with the original waves. This produces a beat frequency of 730 Hz. How fast was the tennis ball moving? Homework Equations fbeat = fa - fb v=λf f=1/TThe...
  29. J

    Final speed and direction after a collision (elastic+inelastic)

    Homework Statement A billiard ball moves at a speed of 4.00 m/s and collides ELASTICALLY with an identical stationary ball. As a result, the stationary ball flies away at a speed of 1.69 m/s. Determine a. the final speed and direction of the incoming ball after the collision b. the direction...
  30. J

    Find speed and direction of a particle after collision

    SOLVED THANK YOU
  31. genphis

    I Galaxies moving away at the speed of light?

    I have as advised done some catching up on basic cosmology, looking at various discussion and lectures on the nature of the universe. Something that has always played on my mind, is that i remember reading and watching a documentary about the potential to achieve the velocity of light. It was...
  32. R

    Finding the Time of Max and 0 Angular Speed

    Homework Statement Mod note: Fixed thread title. OOPS NOT Acceleration, Speed! (Thread title is incorrect) Homework Equations w=d∅/dt v=rw x={+,-√(b2-4ac)}/2aThe Attempt at a Solution I solved this with help from Chegg study, however, I'm still not entirely sure what I am doing. Obviously, for...
  33. T

    Do all humans have the same visual processing speed or not?

    Do all humans have the same visual processing speed or does it differ from person to person? By ‘visual processing speed’, I mean the speed at which one takes to process visual information. For example, if a light has a frequency of 50 hertz where it switches on / off 50 times a second, do all...
  34. J

    Speed of Light Thought Experiment

    A thought experiment about the speed of light. Say I build a 600,000 km long tube around the circumference of the Earth at the Equator. The tube's inner diameter is constant at 54.4505 mm. A snooker ball is perfectly manufactured to its lowest tolerance by a special new machine. Each one is...
  35. K

    Speed of sound in steel dependance on temperature

    Hello, Nice easy question for you all today: I would like to know how the speed of sound in steel varies with temperature. A google search didnt reveal a great deal, so if a physicist could confirm the relationship, that would be much appreciated. I know that: v = √shear modulus/density...
  36. A

    A Speed of Gravitational Waves confirmed by Experiment?

    It seems that with the 2016 LIGO and VIRGO confirmation of the gravitational chirp that we have also experimental confirmation that the speed is indeed the speed of light. True? https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves#Direct_observation
  37. R

    Estimate average speed of electrons in gas?

    Homework Statement Suppose that a heated gas comprised of electrons glows bright orange when it is in use. Estimate the average speed of the electrons in the gas. Model this gas as an ideal gas. Homework Equations None. I have no clue. This is a "review" question on a chapter titled "Current...
  38. S

    Is the speed of light always constant in nature?

    When I read the speed of light in vacuum is c, does it imply that light doesn't actually travel at this speed in nature? My guess is no, light always travels at c, it's just that in the definition, we're trying to ignore the affects on light from other stuff like the comological constant and the...
  39. A

    What is the relationship between displacement, speed, and time in a random walk?

    http://imgur.com/a/hFubT http://imgur.com/a/fquuF In the first image, first line in the last paragraph. The first sentence define that carriers move with an average velocity component Vd in the x direction(in fact, carriers also move with an average velocity component in the y...
  40. G

    KE+PE when a rocket's speed is less than escape velocity?

    Hey people, I just want to ask that what will happen to the total mechanical force of the rocket if its speed is less than escape velocity? a. KE+PE=0 b. KE+PE>0 c. KE+PE<0 d. Depends upon initial speed of the rocket Pick one. And Why??
  41. N

    Why Isn't Average Speed Simply the Arithmetic Mean of Two Speeds?

    Homework Statement If you walk to school at 4 mph and jog home at 6mph, what was your average speed? Homework Equations S = D/T The Attempt at a Solution Is the average speed not just (4 + 6)/2 = 5mph ? If not, why not?
  42. FallenApple

    Why was the speed of light even considered to be infinite?

    Wouldn't the first thought be that we would be burned to a crisp if we where hit with all the light(in our path) across the span of the entire universe simultaneously? It seems there could be paradoxes even before the advent of relativity. Or maybe it was shown mathematically that the energy...
  43. C

    Can Negative Mass at near Speed of Light go Back in Time?

    Sorry if the title was a little vague, character limits. The question at hand is this; If you were to accelerate an object to near speed of light, time around it would speed up, thus forward time travel. We all know this and I am no expert in this area but am incredibly curious. (Assuming we can...
  44. M

    Explain speed limits on curved ramp with Newton's Laws

    Homework Statement Discuss how Newton’s Laws can be used to explain the introduction of transportation safety features such as: Speed Limits on curved ramps Homework Equations Newton's 2nd Law: F=ma The Attempt at a Solution I believe that according to Newton's first law objects that are in...
  45. A

    Finding Velocity Components given Speed and Heading

    Homework Statement Given the Speed of vehicle at any instant (say 30 meters/s) and Heading of a vehicle (say 270 degrees ) , how can i find the velocity components which is Vx and Vy . Lemme tell you about the Heading ...This is the convention.. I don;t know if this pic is viewable or...
  46. Amaterasu21

    Kinetic theory of gases: rebound speed and force questions

    Hi everyone, I remember years ago at school memorising the derivation of the formula for pressure in the kinetic theory of gases, as explained in this Youtube video: Thinking a little more deeply about this derivation there are two things I don't get: 1) At 0:53, the video says the molecule...
  47. Strange design

    I Observing Orbital Speed of stars within Galaxies

    Can someone point me to a resource that describes, or describe, how the orbital speed of stars within galaxies are observed and measured? I'm struggling with how these values can be attained with any confidence in their accuracy. Thanks :)
  48. A

    Calculate Acceleration using relative distances

    Homework Statement There are two cars traveling in a same direction. The Second car has a camera mounted on it and has ability to give relative X and Y (in meters) distance of first car with respect to it. Given the Speed of Second car , Calculate the Speed and Acceleration of first car. The...
  49. Kaneki123

    A Question about the Speed of Waves....

    Okay...I read that speed of a wave for a particular medium is constant, i.e it is independent of its frequency and wavelength...Hence we can conclude for a particular medium, all waves travel with same speed...Is' nt it contradictory with our usual observed phoenomenon..Like, if we take air for...
  50. EEristavi

    How does a 5th wheel contribute to the accuracy of car speedometers?

    I'm wondering how does speedometer work. To be specific let's talk about car speedometer. I googled it and found out that there are different kind of speedometers, but not quite understood it's working principle. My question is not about how it shows us speed (Mechanically with arrow or...
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