Speed Definition and 180 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. 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...
  2. Harli

    Mechanical Waves Recorded after an Earthquake

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  3. rudransh verma

    B A Question about a Quote from the Feynman lectures

    “incidentally, to a good approximation we have another law, which says that the change in distance of a moving point is the velocity times the time interval, Deltas=vdeltat This statement is true only iF the Velocity is not changing during that time interval, and this condition is true only in...
  4. rudransh verma

    Calculation of Speed

    "We know only where she was at intervals of one minute from the table; we can get a rough idea that she was going 5000 ft/min during the 7th minute, but we do not know, at exactly the moment 7 minutes, whether she had been speeding up and the speed was 4900 ft/min at the beginning of the 6th...
  5. Huzaifa

    B Trying to understand this paragraph on speed, acceleration and velocity from my textbook

    I am not able to understand the following paragraph from my Physics textbook;
  6. J

    A Derek Muller and UCLA Prof. Alexander Kusenko -- Downwind Physics Wager

    Derek Muller is the Veritasium YouTube channel host and he had a wager against UCLA professor Alexander Kusenko on whether a unpowered car going downwind can go faster than the wind. Derek Muller said it can go faster than the wind, while Alexander Kusenko said it only seems that way because...
  7. OscarF

    Calculate speed from elastic and inelastic collisions? (momentum)

    So to cut to the chase, I missed my class' lesson on momentum - have tried to catch up, quite successfully but am baffled about this question. I know the conservation of momentum etc. but after trying for ages it's just not happening this question so any help would be much appreciated, Oscar.
  8. L

    Which way is correct and why?

    I get two different answers even the area of the graph. I think there is something wrong with the equation I constructed for average speed part. I know the method 1 is correct. But I'd like to know why the average speed cannot be used. And why I get two different areas for the graph Thank you
  9. TammyTsang

    Engineering Scaling of units for equations of motions

    In the picture, there is a problem where the t is in units of square root(l/g), and V in square root(gl) I am wondering 1. What it means when time is in units other than time? Does it mean that when solving I have to take time/squareroot(l/g) 2. How did they get square root(l/g). Thank you...
  10. EEristavi

    Water Pressure and 2 dimensional motion

    $$ H = \frac { V^2 - V_0^2 sin \Theta} {-2g} $$ $$ H = \frac {V_0^2 sin \Theta} {2g} $$ So, I need to calculate ## V_0 ## I'm thinking about pressure. $$ P = \rho g \Delta h $$ $$ \Delta h = h - L sin \Theta $$ $$ F_A = P S_A $$ $$ F_A = P S_B $$ Dead End here...
  11. naji0044

    Inelastic collision problem: Bullet striking a wood block

    Hey everyone I'm struggling on the last part of this assignment. I need to find the total work done by the block and the bullet, when the collision happens. The informations is: mblock=0.3 kg mbullet=0.01 kg vg=700 m/s Height=0.72m The final speed after the collision is vf=22.6 m/s and the...
  12. Uricucu

    Relationship between speed and flow

    Hi! I'm currently doing a project to simulate a water slope, and want to find the maximum speed of a person going downhill in function of the water flow. At first I thought using a combination of the Manning equation and the Newton Law for viscosity, through the sheet of water, but I'm in a dead...
  13. matai

    Integral for the linear speed of the Earth

    I need to make an integral to fine the speed of the earth. Say the radius is 6378137 meters. How would I account for things closer to the axis that have a radius of 0.0001 meters? I don't think I can just take the speed at the radius. So I found that the earth rotates at 6.963448857E-4 revs/min...
  14. D

    Calculate the speed of a point based on a graph

    Homework Statement A point moves rectilinearly in one direction. The figure shows the distance s traveled by the point as a function of time. Using the graph, find: The average speed of the point during the movement time. Maximum speed The moment t0 in which the instantaneous speed is equal to...
  15. J

    Speed at the top of an elliptical roller coaster loop

    Homework Statement This isn't really a homework but a project I'm doing that's due soon. In our multivariable calculus class, we're creating a 3D roller coaster, and I need to explain the physics behind the roller coasters. For a roller coaster loop, if it were perfectly circular, we would...
  16. J

    Rank the four objects from fastest to slowest

    Homework Statement Rank the four objects (1kg solid sphere, 1kg hollow sphere, 2kg solid sphere and 1kg hoop) from fastest down the ramp to slowest. (Please see the attached screenshot for more details.) Homework Equations KE_rot = 1/2Iw^2 (where omega = w) The Attempt at a Solution Since we...
  17. S

    Bernoulli lift

    Homework Statement We have a plane which lift area is A=500 m2 and weight of plane is m= 540 000 kg. That the plane can stay in air the pressure difference between top and bottom of the lifts have to be: B>G --> Δp= (m*g)/A = 10 594.8 Pa. Homework Equations Bernoulli equation The Attempt at...
  18. X

    How can I calculate the maximum speed of a go kart?

    Hello, I'm going to build a solar-powered go-kart. To determine the sprocket that I want to use I need a formula to calculate the top speed which also considers vehicle weight and other resistance forces. All the formulas I found just used sprocket tooth and rpm to calculate top speed but of...
  19. I

    Setting two expressions equal to each other

    Homework Statement Two cars are facing each other on a long straight airport runway. They are initially separated by a distance of 1 km. Car A begins to accelerate towards the other car at a uniform 0.5 ms^-2. Ten seconds later car B begins to move towards the other car with a uniform...
  20. H

    In how many seconds will the n-th wagon pass next to me?

    Homework Statement Train starts to travel in a straight line at increasing speeds. The first wagon passes next to me in 4 seconds. In how many seconds will the n-th wagon pass next to me? Homework Equations s= vo.t + 1/2at^2 v= vo + at The Attempt at a Solution First of all we didn't learn...
  21. vincent

    I Does a photon carry a velocity component of it's source too?

    Earth is moving through space. If I throw a ball straight up, it falls down to me and is not left behind. This is obviously because it already carries the momentum of earth's movement. But if I fire a photon straight up, does it also carry a velocity component in direction of earth's movement...
  22. R

    B Speed of light if observer is VERY Big

    How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this...
  23. SamRoss

    I Where is the prediction of the speed of gravity in the EFE?

    Newton's law of gravitation cannot be compatible with relativity because the gravity from a massive object applies a force to all other masses infinitely fast. General relativity is supposed to correct this flaw by setting a speed limit on how fast the effect of gravity can reach a distant...
  24. H

    Speed/acceleration calculations for an Electric motorcycle

    Good day all, This is my first post here, hope you all are able to help me out here. I am thinking about converting an old Suzuki GS500E from an internal combustion engine to an electric motor. At the moment I am having some difficulties with the physics side of things, I would like to...
  25. Suyash Singh

    Speed of airflow to lift a man

    mass of air X velocity = mass of man x gravity (10000)x(v)=60x10 velocity=600/10000 how come velocity is so less? I am still in school so forgive me if my calculations look stupid.
  26. Natus Homonymus

    What is the fastest speed around a curve?

    I understand that the centripetal force on an object of mass 'm' is (mv2)/r However, isn't this for an object going around in a circle? Suppose I have a curve (0.0033x2+−1.0038x+98.2331). What would be the fastest speed around this curve on the bounds x -> Please note that we would...
  27. T

    Opening a car door at a high speed

    What would happen if a car is driving down a highway at 80Mph and one of the doors is fully opened. Will it make a big change and make it slow down or start turning?
  28. M

    Sources of Error in a Speed of Sound Experiment

    Homework Statement Determine sources of error which could have affected the results. Some background information: The lab consisted of playing a tone generated by a tone generator above a standing open-closed tube which had water in it. The first harmonic was measured through a computer system...
  29. Skrphys

    B Table cloth trick

    I have my students doing a lab where they have to do and explain the tablecloth trick using Newton’s Laws. The problem I am having is that the one question asks why the quick removal of the tablecloth matters. I know friction is not affected by speed and yet this is the only force acting on the...
  30. T

    What does power have to do with the speed calculations of a Train?

    Hi there again, I have been wondering why train simulators(and trains themself) have a kW specification, AND a maximum force indication. If I would calculate the speed of a train using some formula, then where is the power value used for? Does it have something to do? Or is only the maximum...
  31. T

    Where do I find coefficient values for these 2 formulas? (drag coefficients)

    Hi there, So I asked a question here how I would calculate the air drag and the wheel drag of a train and I got an answer that made me very happy. Unfortunately i don't know where i could find the coefficient values required for the formulas. So i am wondering if someone knows where i can find...
  32. T

    How can I calculate speed based on previous speed?

    Hi, I am developing a Train Simulator kind of program where I calculate speed each frame. I currently do that by: Speed = (f/m)*elapsed frames*time between frames This works, but goes wrong when there are framerate drops or if the force changes, thats why i want to calculate the speed with the...
  33. T

    How to calculate Train speed with friction? (developing a little experiment game)

    Hi there, I am new here so please don't be too hard on me yet and move this post if i placed it in the wrong forum. I am developing a little experiment game and i want the train to drive using real physics, but I am really bad at physics. So my question is is how do I calculate the friction of...
  34. pat65

    Need some advice on an appropriate computer language to use

    Hello All, OK, I have been programming since the early 1980's. Programmed in many different languages along the way such as Pascal, C++, C, Visual Basic, and some C# (among others). I pretty much program so that I can model physics. I find it GREAT way to actually understand the physics...
  35. doublev231

    Find speed given mass, starting speed and time

    A cart which weighs 500g is moving on a horizontal table with the starting speed of 7 m/s. At the end of the cart there's a non-elastic wire attached, which is thrown over a pulley, and on the other end of the wire there's a 200g load. I need to find the speed of the cart and the distance it...
  36. M

    Determining speed of electron in a parallel plate capacitor

    Homework Statement A proton is released from rest at the positive plate of a parallel plate capacitor. It crosses the capacitor and reaches the negative plate with a speed of 50,000 m/s. What will be the final speed of an electron released from rest at the negative plate? Homework Equations...
  37. physicsnewbie101

    Relation between centrifugal force and speed

    Does centrifugal force generate in increase in overall speed? For example, if I am driving down a highway on-ramp which goes in a circular fashion, with my speed slowly increasing, does my overall speed increase because of the centrifugal force generated from the circular on-ramp? Is more force...
  38. D

    Force (in lbs) to speed?

    Hello, I have been looking at physics calculators, but cannot seem to find precisely what I need. I don't need a very precise answer, merely a realistic ballpark one. I need to know how we find the rough speed of an object's movement when a certain amount of force is applied to it. For...
  39. A

    Speed of an object sliding down

    Homework Statement An object of mass m=2 kg slides down on a track from a height of h = 10 m. The track is frictionless except the horizontal range between the points A and B. The friction force is constant between A and B. What is the speed of the object at point B if the magnitude of the...
  40. S

    Momentum: Freight cars problem

    Homework Statement Three freight cars of equal mass are coupled together. The freight cars are travelling at 6.7m/s down a straight track when they collide with two more stationary identical cars. If all five cars are coupled together after the collision, then what is their speed if they are on...
  41. S

    Two cars driving toward each other (non-uniform speed)

    Homework Statement Two cars are 1.4 km apart and driving towards each other. One car starts from rest and accelerates uniformly at 1.2 m/s^2. The other car is driving at a constant speed of 18 m/s. When will the cars meet? Homework Equations d=vt d=ut + 1/2at^2 d=vt-1/2at^2 v^2 = u^2 + 2ad...
  42. M

    Finding speed when given deceleration and distance?

    Hi! I have this homework problem that has me completely stumped. Any help is greatly appreciated! In coming to a stop, a car leaves skid marks 80m long on the highway. Assuming a deceleration of 7.00m/s2, estimate the speed of the car just before braking. I just can't figure out what formula...
  43. S

    Velocity and Speed

    Plane A flies from Paris to New York, while plane B makes the same trip via London. Assume that the total time taken for both planes is the same. Considering the two trips, both planes have the same average velocities but different average speeds. Why? How is this possible?
  44. S

    What is the mass of the book?

    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
  45. 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?
  46. 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?
  47. 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...
  48. 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...
  49. 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
  50. 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...
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