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. J

    Finding power without given speed or time

    Homework Statement The lawn tractor in figure 18 goes up a hill at a constant velocity. Calculate the power that is developed by the tractor. Mass: 120.0 kg Angle of incline: 21 degrees Distance: 12.0 m Velocity: no velocity given other than it is constant Time: idk Homework Equations None...
  2. A

    How to calculate the speed of an object attached to a string

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  3. J

    Maximum speed of car on downhill

    Homework Statement The exercise is this: The mass of the car is m=500 kg. The maximum speed of the car on flat road is v1=35 m/s and on 10 % up hill (tan(alpha)=0.1, alpha is with respect to flat) the maximum speed is v2=20 m/s. The friction force is double on up hill because it has been made...
  4. M

    What Is the Escape Speed of an Electron from a Charged Glass Sphere?

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  5. M

    Determining speed of electron in a parallel plate capacitor

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  6. VSayantan

    Speed of a Magnet Falling through a Conducting Wire

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  7. physicsnewbie101

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  8. karush

    MHB 243.13.01.19 For which path is the particle's speed constant

    $\tiny{243.13.01.19}$ $\textsf{The following equations each describe the motion of a particle.}$ $\textsf{ For which path is the particle's speed constant?}$ \begin{align*} \displaystyle R_1(t)&= t^7\textbf{i}+t^4\textbf{j}\\ R_2(t)&= \cos(3t)\textbf{i}+\sin(8t)\textbf{j}\\ R_3(t)&=...
  9. C

    Relative speed when someone runs on the treadmill

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  10. M

    Finding the speed of the block for one and two springs

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  11. M

    Speed after gliding with friction force present using work/E

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  12. A

    Computing angular speed wrt CM I get a contradiction

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  13. N

    B Answer Questions on Relativity & Speed of Light | Beginner Physics

    Can anyone help me with a very basic question please. Is it really not possible for anything to travel faster than the speed of light, as dictated by relativity theory ? I am a complete beginner to Physics and only have very basic mathematics knowledge and I'm trying to understand the basics of...
  14. Jay Addy

    Why do objects fall at the same speed in free fall?

    This is something I haven't been able to wrap my head around yet. In physics, I've always been told that gravity is a force that ALWAYS works between "objects" with mass. Now, it seems clear to me that if a feather and a hammer were to be dropped at the same time on Earth (without air...
  15. A

    Distortions in time when going near light speed

    So I was wondering that why does time move slower the faster you get ? And does it happen at all or is this is just fake ?
  16. NTL2009

    How to speed copper oxidation?

    I'm working on some decorative items made of copper. I'd like to have them achieve the brown color of an old copper penny (or the old copper pipes in my home). I've read you can do this with some chemicals, but due to size/shape I'd like to do this some other way. I'm also concerned that any...
  17. A

    Finding the final speed of a Positive and Negative Charge

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  18. S

    Question about speed and vehicles

    Say a super fast car is going car is going at twice the speed of sound, an there is an aircraft going at the same speed. Normally, maneuvering and controlling at high speeds (slowing down, speeding up, stopping and changing direction) is very hard. Now, I just want to make sure by asking, what...
  19. C

    MHB Speed of Perigee: Find Satellite Velocity at 750 km

    Dear Everybody, How to get started for this problem? An Earth satellite has its apogee at 2,400 km above the surface of Earth and perigee at 750 km above the surface of Earth. At apogee its speed is 6,800 m/s. What is its speed at perigee (in m/s)? Earth's radius is 6,370 km (see the figure...
  20. MarkFL

    MHB Firefox Revamped: Update Brings Speed & Efficiency Boost!

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  21. Joe Fatuch

    I How does the speed of light affect expansion acceleration?

    I may have a fundamental misunderstanding of the concept, but I was wondering, how does the accelerating expansion of the universe calculate for the time dilation in light travel? From my understanding, we know that the universe expansion is accelerating because the farthest galaxies that we...
  22. C

    I Relative Speed Impact on Synchronization: Clock Comparison

    If two clocks start from the same point, and one travels double the speed of another, will they be showing the same time when they get together at the destination point. Obviously, faster clock will arrive first, and will have to wait for the slower one to arrive.
  23. D

    What is the relationship between force (in lbs) and 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...
  24. L

    Finding net force from acceleration, speed and distance

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  25. C

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  26. S

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  27. A

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  28. A

    Finding the speed to orbit Deimos

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  29. S

    Momentum: Freight cars problem

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  30. marmstrong941

    B Is anyone looking for this: reaching speed of light

    is anyone looking for what might be the effect of something going into or coming out of the speed of light?
  31. C

    B Speed of light and modern physics

    What will happen if something traveling faster than light is found?What effect will it have on modern physics?
  32. A

    I Theorem Proved: Universal Speed C in SR?

    I recall reading on this forum that a theorem had been proved to the effect that is there was a effective speed limit c in SR then there would only be one such limit for all relevant phenomena. Is this true and if so can you point me at it as I don't seem to be able to formulate the question...
  33. E

    I Speed of gravity vs. speed of light

    GW170817 showed that gamma rays came 2 seconds after gravitational waves. (after 130M years) Are these two seconds uncertainty at determination of difference of these speeds? Or is it possible to explain, why 2 seconds of difference, or is it possible to explain, at least, a part of this...
  34. H

    B Does a gravitational wave change the speed of light?

    Suppose at the instant a gravitational wave passes through an interferometer, one of the interferometer's arm get stretched by 1%. Would the wavelength of the photon traveling in the arm also get stretched by 1%? If so, then would the frequency of the photon remain the same and hence increasing...
  35. A

    Finding speed on a banked turn

    Homework Statement The question wants us to find the velocity that the car can take without assistance from friction. The friction, in this case, is the normal friction, weight friction, and the static friction (pointing towards the right). I understand how it got ncos(15) - w = 0 for the y...
  36. yrjosmiel

    Speed of objects as they hit the ground

    Let's say there are two objects. One is thrown directly downward at v0 and the other is thrown horizontally (90 degrees) at v0, both from height h. Which object will have a higher speed by the time they hit the ground? Would they have the same speed? For the object thrown downward... h = (vf2 -...
  37. 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...
  38. L

    What is the speed of the boat relative to the water?

    Homework Statement A 0.14-km wide river flows with a uniform speed of 4.0 m/s toward the east. It takes 20 s for a boat to cross the river to a point directly north of its departure point on the south bank. What is the speed of the boat relative to the water? I'm mostly concerned about the...
  39. M

    Initial Speed of Hanging Mass (HW Check)

    Homework Statement A mass hangs from a light string of length 45cm attached at its other end to the ceiling. If the mass is given an initial speed of 5m/s, what is its speed when it reaches point A in its circular path (see Figure 1)? https://imgur.com/a/ERdlD Homework Equations ##E_i = E_f##...
  40. N

    Centripetal Acceleration Problem: Finding Angular Speed

    Hi all, I've been lurking around the forums for a while to get help with homework but I figured I'd finally make an account to get direct feedback. I'm having problems with this centripetal acceleration problem, Homework Statement "In an old-fashioned amusement park ride, passengers stand...
  41. A

    Finding the Formula for Speed and Distance on an Icy Hill

    Homework Statement Maxine (mass m) slides down an icy slope then along a flat surface before coming to a stop. The slope has angle θ and height h. a. Derive a formula for speed at the bottom of the hill in terms of the angle of the hill θ, the height from which she starts h, and the...
  42. L

    What is the velocity of a decelerating boat after 4.9 seconds?

    Homework Statement A boat, moving at the velocity of 9,9m/s turns off its engine and starts decelerating at the rate of a=-kv^2. What is the velocity of the boat after 4,9 seconds if the constant k is k=5,5m Homework EquationsThe Attempt at a Solution I tried to integrate deceleration to get...
  43. E

    Speed regulation: Isochronous vs Droop Control of Generator

    Hi! I am confused with the speed regulation for Isochronous control. In Droop control, we can surely solve for a value of speed regulation because droop allows the generator to have a varying frequency. However, in Isochronous control, the frequency and speed from no-load to full-load is the...
  44. G

    B Concept of the stability of the speed of light

    Help with the question of the concept of the stability of the speed of light. I gather that current atomic clocks are based upon the speed of the electron around their nucleus, which is a function of the speed of light. Measurements of the speed of light that depend upon atomic clocks therefore...
  45. L

    Finding the correct formula for the speed of waves in shallow water

    Question: The speed of waves in shallow water depends only on the acceleration due to gravity (g) and the water depth (h). Which of the following formulas describes the speed of the wave? a) (1/2)gh^2 b) gh c) h/g d)(gh)^(1/2) Attempt: Change in V = at t= ΔV/a Δx=vt+(1/2)at2 Δx =...
  46. G

    Train traveling at speed of E field question

    This is a thought experiment so the mentioned values are only for example purpose. I was wondering about an electrical train. So here is the setup, we have rails and an electric train with an overhead wire. In one case the train gets its power from DC and in the other case the train is powered...
  47. 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...
  48. C

    MHB Find The Distance And Speed By Using The Work And Energy Theorem

    Dear Everybody, I need some help to help the distance for part a. b) If the block comes back down after it was push, what is the speed back down. A 3.25 kg block starts with a speed of 15 m/s at the bottom of a plane inclined at 35° to the horizontal. The coefficient of sliding friction...
  49. J

    I Speed of light on a moving platform

    1. We have a moving platform traveling 700 miles per hour to the Right-Direction. 2. In the middle of the platform, we have a gun which fires a laser both at the front and back of the platform at the same time. 3. We have at both the front and back of the platform, a mechanical device. These...
  50. P

    What is the impact speed of the proton in a 4850V capacitor?

    Homework Statement A parallel-plate capacitor is charged to 4850 V. A proton is fired into the center of the capacitor at a speed of v0=2.81 * 10^5 m/s. The proton is deflected while inside the capacitor, and the plates are long enough that the proton will hit one of them before emerging from...
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