Speed Definition and 1000 Threads

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. F

    I One-way speed of light

    Hi PF, as far as my modest knowledge goes, measuring the one-way speed of light is impossible / non-sensical (?). Thus I'm not here to actually propose a possible method for that purpose, but I just would like the savvy among you to point out the fallacy in my reasoning. That said, I was...
  2. L

    I General relativity question: Gravity clock

    Would a gravity powered clock increase or decrease speed? Take an hourglass. It should increase speed as gravity increases because it's powered by gravity... But it should also decrease speed because of general relativity...
  3. S

    Kinematics, is this a poorly worded question?

    Like, i don't even understand what it means by the ball accelerating at 0.5m/s2. is it accelerating upwards constantly at 0.5m/s2? what about gravity? the answer given is weird. initial speed u = 5m/s and t = 10s it says time taken to go up is = time taken to go down. (why?) so total time...
  4. S

    I Speed of information-carrying radiation

    How can an approaching observer receive the information from a source at a higher rate (as an accelerated recording), while the information-carrying radiation could not?
  5. A

    B Why doesn't a charged particle moving at a constant speed radiate?

    If I stand by a flying with constant speed charged particle, at my location the electric field will change as the particle get closer and further from me. So this change in electric field should create magnetic field and so on, producing an EM wave?
  6. S

    B Speed of light seen by a moving observer

    How can the speed of light remain the same for an observer while the duration of a light beam is shorter if you move towards the light, and longer if you move with the light? The light beam remains the same length.
  7. A

    I For a relativistic spaceship, what is the most recent news the ship gets from Earth?

    Looking to see if I am in the ballpark correct about this. Let's say a spaceship flies at 99% the speed of light (B=0.99) to a planet 12 light years away. The spaceship is going to land on the planet once it arrives and stay put ever after. Ignore acceleration of start/stop. When it lands on the...
  8. I_Try_Math

    Deriving speed equations | Elastic collision

    So far I've got: ##p_{1,i} + p_{2,i} = p_{1,f} + p_{2,f}## ##p_{1,i} + 0 = p_{1,f} + p_{2,f}## ##m_1v_{1,i} = m_1v_{1,f} + m_2v_{2,f}## According to the textbook, the final speeds should be written in terms of ##v_{1,i}, m_1, and m_2##. It looks like I need another way to relate everything...
  9. maxelcat

    B How fast are gravitational forces transmitted?

    I understand that if the sun where to 'disappear' then it would take several minutes for the us on the earth to notice as the light takes time to travel. Does this mean that it would also take the same time for us to notice that the gravitational (or electromagnetic) force would disappear too...
  10. doke

    Rpm and power for this electric motor, gearbox and pulley system

    i got confused because if the connections btw the parts
  11. isni

    I A vehicle has two wheels rolling at different speeds. What forces act on the vehicle as it follows a curved trajectory ?

    This is not homework, it's a problem I came across while trying to model differential braking for my flight simulator. I cannot really see what force pushes the vehicle horizontally and I would really appreciate a detailed explaination.
  12. E

    I Can “speed of light” distinguish all Timelapse videos from real motion?

    my question then is that can we ascertain some unknowns about the physical constants of our universe by using video altered speeds , to observe the universe through moving picture frames ? Is there a project benefit in doing more with this ?
  13. S

    I Did the Michelson-Morley Experiment Disprove the Ether Theory?

    I read on the forum that: "Michelson-Morley results strongly suggested the local speed of light being the same in all reference frames." I think that's an erroneous understanding. MME only suggested that there is no ether because speed of light is not affected in any direction and remained the...
  14. chwala

    Calculate how fast the car was travelling when brakes were applied

    My query is on highlighted. This was straightforward. I thought that coefficient of friction ought to lie between ##0## and ##1## maybe i need to check that again anyway; ##F_{lim} = 1.5 ×15000= 22, 500 N## Using ## v^2 = u^2 +2as## and ##F=ma## ##22,500 = 1500a##...
  15. E

    I Proving speed of light constant “c” is the same in all directions?

    If I’m not mistaken, the synchronization of 2 clocks to conduct a measurement of the one-way speed of light is not possible since simultaneity is not possible and 2 clocks even if they are synchronized in a midpoint and slowly moved to opposite ends, depends on c being the same in all...
  16. G

    I I want a good reference to a discussion of the meaning of "the speed of light is constant"

    I am interested in making an observation that involves the speed of light. It is widely stated that the speed of light is constant, but without saying what that means. I need to be precise, and would like a reference to an acknowledgement of the problem. When people talk about the speed of...
  17. G

    B Filming a passing road sign: speed vs bumpiness

    As part of an audio-visual I'm producing, I want to video two passing road signs while driving in a car on a country road. The videos have to fit in with commentary and music, so the timing is pretty tight, and I want to get it right the first time I visit the area – 4 hours drive from home...
  18. G

    I Standing light waves if the one-way speed of light were not constant?

    Hi. According to Wikipedia: The "one-way" speed of light, from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. Mid 20th century, the most precise measurements of the speed of light were done using...
  19. M

    Finding speed of spaceship traveling to star ##a## light years away

    For this problem, My working, (a) ##d = a~ly = ac~y## ##v = \frac{d}{\Delta t} = \frac{ac}{b} \frac{m}{s}## (b) Lorentz factor is ##γ = \frac{1}{1 - \frac{a^2}{b^2}}## Thus time dilation is ##\Delta t = \frac{b}{1 - \frac{a^2}{b^2}} y##, however, I think my arugment is only valid if ##a >> b##...
  20. M

    Particle moving at speed ##bc## relative to laboratory frame

    For this problem, My working is (a) ## \Delta t = \frac{a}{\sqrt{1 - b^2}}~ns ## (b) ## \frac{abc}{(1 \times 10^9)\sqrt{1 - b^2}}~m= d_1 ## (c) By symmetry, ##d_2 = -d_1## ##-\frac{abc}{(1 \times 10^9) \sqrt{1 - b^2}}~m= d_2##, however, I'm not sure whether my explanation for (c) is enought...
  21. LarryS

    I Planck's Constant times the speed of light

    Does the product of Planck's Constant and the speed of light, hc, have any significance in physics other than an arbitrary product of two constants? I noticed that it appears in one formula for the fine structure constant. It also appears in Planck's formula for black body radiation. Thanks...
  22. R

    B Speed is distance multiplied by time -- Is this correct?

    Hello, In this study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9784821/, the distance the punch travelled from start to impact is 0.49 meters and the time taken from start of punch (that's it, they define the start of punch as the moment the elbow first start to extend) to impact is 0.1...
  23. T

    B Why 186,282?

    Is there an explanation for why the speed of light tops out at 186,282 miles per second? Of course that number depends on our definition of miles and seconds. If a mile was 3000 feet then c would be a different number. But whatever speed it is…. Why that speed? In other words… there is...
  24. felizgu

    How Long for a Beam of Light to Reach Earth?

    TL;DR Summary: This is an astronomy application. Mentor note: Moved from a technical forum section, so the homework template is missing. How long does it take a beam of light to reach Earth from the Sun when the Sun is 93,000,000 miles from Earth? Express your answer in seconds, using...
  25. felizgu

    Number of Miles In One Light Year

    TL;DR Summary: This is an astronomy application. One light-year is defined by astronomers to be the distance that a beam of light will travel in 1 year (365 days). If the speed of light is 186,000 miles per second, how many miles are in a light-year? Express your answer in scientific notation...
  26. davidortenn79

    B Why is it impossible to go faster than the speed of light?

    Ok, I took like a class in physics in college. It was a class to learn physics for majors that we really didn't care. I remember him saying, if you remember anything from this class, remember inertia. That's about all I remember from the class... lol. But, I don't understand why it's...
  27. G

    I Does the speed of light pop out of other theories than EM?

    Hi. The numerical value of the speed of light ##c## pops out as propagation speed of the wave solutions to Maxwell's equations. It seems like everywhere else we need to plug in ##c## as a parameter. Why is that? Is there a way to "derive" the numerical value of ##c## in other theories, that...
  28. S

    Refraction of light when traveling from air to glass

    I don't know how to explain it in terms of speed. I know the speed will decrease but if the ray entering the glass at certain angle, let say 10 degrees, the speed will also decrease so what is the relation of speed to the fact that the light will bend or not when entering the glass? And the...
  29. hello478

    Camera drone and speed of air below it

    my answer = F = 1.6*9.81 - 1.2*9.81 = 3.924 N -> resultant force of the drone moving (weight minus the upward force) M= 1.2kg , mass of drone a= 3.31 m/s^2 , acceleration of drone u=0 m/s , initial speed of drone t=1s , time v = 3.27 m/s, final speed of drone and its not even in the options... 😶
  30. G

    Molecular speed distribution - most probable speed V

    Can someone explain to me what I am doing wrong? Trying to calculate the Temperature T using this formula I am trying to find T using most probable speed of atoms formula. T = (V^2) m/( 2k) I am getting riduculous T like 20x10^27. Why? If number of atoms N= 2x10^24 how do i get mass m? each...
  31. H

    What is the time taken by a pulse generated at the bottom of a string to reach the top?

    I have been taught that speed of pulse is v = √(T/μ), but here tension varies at each point therefore I cannot just simply apply the formula. I think integration would be needed, I tried but ended up nowhere. Can someone help me find the time?
  32. 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...
  33. 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...
  34. petar rezek

    How Do You Calculate the Average Square Speed of Argon Molecules?

    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
  35. 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...
  36. R

    Final speed of particle far away from rings with opposite charges

    When I submitted it, this answer was incorrect.
  37. 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...
  38. 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...
  39. 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...
  40. 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?
  41. 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 :-(
  42. A

    What was the mistake in solving for the speeds of two marbles after a 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.
  43. 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
  44. P

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

    Does it violate the postulate of special relativity in any sense?
  45. 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...
  46. 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?
  47. 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.
  48. 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!
  49. 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%...
  50. cathrynliz

    I Can Electrons or Photons Exceed Light Speed?

    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?
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