Velocity Definition and 1000 Threads

The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion (e.g. 60 km/h to the north). Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.
Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, being a coherent derived unit whose quantity is measured in the SI (metric system) as metres per second (m/s or m⋅s−1). For example, "5 metres per second" is a scalar, whereas "5 metres per second east" is a vector. If there is a change in speed, direction or both, then the object has a changing velocity and is said to be undergoing an acceleration.

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  1. Ahmed1029

    I Michelson–Morley experiment and the velocity of the ether wind

    How was it justified before conducting the MICHELSON–MORLEY experiment to assume that the sun was at rest with respect to the ether? Also, was the ether assumed to have the same velocity with respect to the Earth throughout space at one instant in time, or like wind, with different velocities at...
  2. mathbrain9

    Velocity comparison -- Which thrown mass hits the ground first?

    On the test, I said they hit the ground simultaneously. However, that, in retrospect, doesn't seem to be the correct solution.
  3. mncyapntsi

    1D collision, varying masses but same initial velocity

    I know I need to look at the conversation of momentum, as well as the conservation of kinetic energy. However I get stuck with my equations. Any help would be greatly appreciated! I've already got (don't know where I am going wrong): (v)^2 + (1/2)(m)(v)^2 = (vf1)^2 + (1/2)(m)(vf2)^2 (3/2)v^2 =...
  4. M

    How to find velocity of gas pump flowing into car tank

    The general balance equation is as follow: d[m{u+gz+v2/2)sys=(u+gz+v2/2)dmin-(u+gz+v2/2)dmout +dq+dw. I understand that I would take in consideration the cross sectional area as well as the mass flow rate. However I can't figure out what else to consider as if atm pressure should be considered...
  5. Graham87

    Condensed Matter Physics - Fermi velocity, etc.

    I have made solutions a-d, but my fermi velocity seems too big.
  6. Barbequeman

    Calculating Velocity & Field Strength of Gas: Help Needed!

    For the first calculation of the velocity of the gas I use the first equation and this converted in meter would be look like this (first value as an example) v=299792458 m/s * (6.76813x10^-7-6.768x10^-7)/6.768x10^-7 =5836.03m/s or 0.0019c this was the velocity of the gas for the first spectral...
  7. A

    Is it possible to solve relative velocity problems without sine law?

    I was able to solve this question successfully by utilizing the sine and cosine law however my instructor said I was only allowed to utilize the vector component method, I am unsure how to complete this question using the vector component method as we have two unknowns(those being the angle of...
  8. A

    Relative velocity Question for a Ferryboat

    So far I have this: vbwx= 4.40c0s(28) = 3.88496409 vbwy= 4.40sin28 = 2.065674876 vpwx= 4.4ocos28 vpwy= 4.40sin28 +2.02 Find Square root of vpx^2 + vpy^2 = 5.43? Im confused as to whether we add the 2.02 to the sin28 or the cos28 though, did I do it right? Also, not sure how to find the...
  9. S

    Relative velocity and projectile motion problem solving

  10. Ahmed1029

    I What Determines the Speed and Clarity of Signals in Wave Physics?

    What exactly is a signal in wave physics? Is any wave considered a signal? Like, consider a superposition of harmonic plane waves, is the signals it carries considered the envelope(that travels at the group velocity) or the individual rippes that travel at a the phase velocity?
  11. R

    I Changes to a spinning skater's angular velocity

    Can we describe what is happening as the skater's angular velocity increases/decreases using F=ma rather than invoking L = constant?
  12. unoonu

    Relative Velocity Against the Water Current

    vcanoe_wrt_ground = 3 m/s + (-6) m/s = 3 m/s - 6 m/s = -3 m/s Thus, if I understand this correctly, the rower will never reach his goal 36 m away as his canoe's resultant velocity is negative (i.e., his canoe is effectively going downstream even though it is trying to go upstream). My only...
  13. maxelcat

    Escape velocity question - constant is wrong....

    This is a multiple choice question. I assumed that this is an escape velocity question. I have been going round and round... Here's what I have done:
  14. bbsgirl10

    Velocity and acceleration graphs

  15. Ahmed1029

    I Wave number, frequency, and velocity in dispersive waves

    1) If I generate a dispersive wave, will it have well-defined constant wave number and frequency? Ones that don't change in time? 2) does the velocity of any point on the wave stay constant in time? 3) How does force interact with waves? Does a free wave act in analogy with free particles...
  16. Ahmed1029

    I Why are acceleration, jerk, etc not relative, just like velocity?

    I've been thinking about this for a while, and thought it would be nice if someone could guide me to an answer. In Newtonian mechanics, an inertial frame is coordinate system that's able to make measurements with respect to some imaginary axes attached to it. It's a well known fact that velocity...
  17. raymondd

    Calculate velocity in a "simple electric train"

    How do you calculate the terminal velocity of the train moving in a coiled copper wire with two magnets attached to the polar ends? I have attached someone else's work I've found on the internet here and need help
  18. A

    I Drag equation - relative flow velocity

    Hello! I have a question about aerodynamic drag. It sounds simple but when trying to understand why the relative velocity can be used in calculations I have some trouble. The formula is 0.5*rho*u2*cd*A where u is the relative velocity between the object and the fluid. The cd value depends on the...
  19. J

    I Cosmic Inflation Explained: Constant Velocity of Electromagnetic Radiation

    C = sqrt(E/M)...this would suppose the ratio of the amount of energy vs. the amount of mass in the universe. If not, why not. If there is no mass, just energy, or much less mass at the moment of the hypothetical Big Bang, then, there C would be significantly higher, thus explaining cosmic...
  20. More_anonymous

    A Relativistic Relative Velocity Comp.: Bini, D. et al. (1995)

    I'm trying to understand this paper (equation 2.16 specifically): Bini, D., Carini, P., & Jantzen, R. T. (1995). Relative observer kinematics in general relativity. Classical and Quantum Gravity. Am I correct in reading there is no way to express the relativistic relative velocity composition...
  21. Rikudo

    Rocket Escape Velocity from the Earth-Sun system

    I have a difficulty when making the energy-conservation-equation for the second step. When making the equation, we need to know the exact position (measured from the sun) of the rocket after it is freed from the Earth gravitation. But, where exactly does the rocket free from Earth...
  22. inigooo

    Find the magnitude and direction of the velocity

    V^2= u^2 +2gh
  23. Sagittarius A-Star

    I Only Minkowski or Galilei from Commutative Velocity Composition

    The LT can be derived from the first postulate of SR, assuming linearity an that velocity composition is commutative, and that GT can be excluded: ##t' \neq t##. Definition of the constant velocity ##v##: ##x' = 0 \Rightarrow x-vt=0\ \ \ \ \ \ ##(1) With assumed linearity follows for the...
  24. B

    Velocity of a relativistic particle in a uniform magnetic field

    d(ɣmv)/dt = qvB (dɣ/dt)mv + ɣm(dv/dt) = qvB Substituting gamma in and using the chain rule, it ends up simplifying to the following: ɣ^3*m(dv/dt) = qvB Now, I am confused on how to solve for v.
  25. S

    Initial velocity and angle when a ball is kicked over a 3m fence

    Hello there, I don't understand what I'm doing wrong I don't get the correct answer, but have done the same analysis 3x already and still get the same... Some input would be appreciated thanks in advance. Note: y-axis is upwards and x-axis is to the right. 3 unkowns i.e. 3eqs. ##x = x_0 +...
  26. hjam24

    Determine vertical velocity vector on sloped surface

    A golf is launched at a speed v,f and launch angle, β,f. The slope of the green is equal to φ. At some point the ball is located on the rim of a hole. The side view (a) and overhead view (b) looks as in the attached image.According to the author of the [paper][2] "The Physics of Putting" the...
  27. morrobay

    Crest to Crest Velocity of Tsunami

    A tsunami traveling in deep ocean. So suppose a tsunami with 200km wavelength is traveling at 800km/hr. Given these are are transverse waves. How is the crest to crest velocity calculated ? For above: 200km/800km/hr would the elapsed time (period) be 15 minutes between passing crests. So...
  28. agnimusayoti

    B Understanding Average Velocity in Linear Motion with Constant Acceleration

    I read Tipler's book: Physics for Scientists and Engineer and he derived the equation of motion for linear motion with constant acceleration algebraic. He stated that for linear motion with constant acceleration, the average velocity can be determined with mean value of initial and final...
  29. P

    I Velocity addition via k-calculus

    We had a thread a while ago where a poster was particularly interested in the SR rule of velocity addition. And in that thread, I suggested a better foundation was the k-calculus approach, with a reference to Bondi's treatment in "Relativity and Common Sense". Here I would like to show how to...
  30. H

    A Calculate the group velocity in EIT (famous paper: light speed 17m/s)

    hello everyone! Recently,i'm reading a paper about slow light,that's really a famous work published in Nature.[Light speed reduction to 17 metrespersecond in an ultracold atomicgas]. But I'm trouble with some calculation about the velocity of slow light.here are below: i try to use the...
  31. Victor Correa

    I Terminal Velocity proportional to the Drag Force 𝑚𝛾𝑣² in free fall

    Can anyone help me? I know that's wrong, but i don't know where. Thanks for your attention so far.
  32. alhuebel

    I Constraints on Lorentz Velocity Transformation

    1. The 2nd line on the 3rd page of your notes, you have x=ct and x'=ct', thus ux=dx/dt and ux'= dx'/dt' =c according to Einstein's assumptiuon. 2. But near the end of the last page, you wrote dx'/dt' = (ux -v)/(1-vux/c2) . Compare with 1. This equation can be valid only for ux=c and...
  33. lindberg

    I Applying Velocity Addition in Rotating Frame: Is It Correct?

    From the top of my head, I would say that yes, the very moment our clocks are aligned, and the two bullets are launched it is perfectly ok to use the relativistic velocity addition formula to determine the speed of the bullets from my reference frame. But the more the disk keeps rotating, the...
  34. lindberg

    I Relativistic Velocity Addition: Calculating Electron Speed

    If we imagine launching an electron wave in a reference frame S with speed v, should someone viewing the electron from frame S1, which is in inertial motion referring to S, use the relativistic velocity addition to calculate the speed of the electron?
  35. A

    I Block with upward velocity tethered to a bottom block

    If I shoot a block with mass m1 with initial velocity v, and the block m1 goes to the end of the string tethered to the bottom block with m2 without lifting it up, what is the force equation involved with the block with m1? I am torn between whether it would be 1) -T -m1*g = 0, which I am...
  36. WMDhamnekar

    Angular Velocity in the Rotating systems

    Summary: Consider a body which is rotating with constant angular velocity ω about some axis passing through the origin. Assume the origin is fixed, and that we are sitting in a fixed coordinate system ##O_{xyz}## If ##\rho## is a vector of constant magnitude and constant direction in the...
  37. C

    I Velocity for uniform circular motion

    Hello everyone, I've been studying centripetal and centrifugal acceleration and derivation of their magnitude. I noticed in one of Walter Lewin's lectures that the velocity is written as both a vector and an arc length which is confusing to me. When velocity is written as a vector, it has a...
  38. Philip Koeck

    I Phase velocity and frequency of a matter wave

    The relationships for matter waves are (see e.g. https://en.wikipedia.org/wiki/Matter_wave): λ = h / p and E = h f, where E = m c2 From this the phase velocity can be derived and we get vph = c2 / v. v is the group velocity, which is also the velocity of the particle. If I consider these...
  39. lola1227

    Momentum Collision Homework Problem -- help please

    Parallel: M1V1+M2v2=M1V1’+M2V2’ (0.5)(3)+0=(0.5)(cos60)(3)+V2’Cos(x)(0.5) V2’cos(x)= Perpendicular: M1V1+M2v2=M1V1’+M2V2’ 0=(0.5)(0.3)(sin60)+V2’sin(x)(0.5) V2’sin(x)= And the divide 2 by 1 Which is tan(x)=2/1 And then plug then back into solve, but I don’t think we do it like this because...
  40. HelloCthulhu

    How does an electric field create velocity in a water bridge?

    I've been researching water bridges and electrowetting to learn the effects of electric fields on water molecules but something continues to confuse me: if polar molecules can only rotate in an electric field, how is the water moving? Anyone familiar with this phenomenon? Any help is greatly...
  41. Shreya

    A new take on a relative velocity problem

    I can solve this question using relative Velocity or using 2 kinematics equations. But a peer of mine, tried to do it in a different way. He substituted the above said values in the 4 equations and solved for u in terms of t and differentiated them and equated to to 0 to find a maxima. He found...
  42. M

    AC Drift Velocity: Understand Reactive Power & Movement of Electrons

    Hi All I hope this topic makes sense for this forum and somebody can help me to understand. This is what I understand (please allow numerical approximations): 1. Electrons move in a conductor at a speed anywhere near the speed of light. 2. Electrons gain actual velocity (drift) only when...
  43. P

    Escape velocity of solar system

    I'm pretty confused by this but I have a few thoughts. Since the sun takes up most of the mass of the solar system, I was thinking maybe I'm really looking for the escape velocity of the sun? So I would use the mass of the sun for M and the radius of the sun for r. My other thought was to add up...
  44. Svelte1

    Solving for Speed: Exploring Velocity and Magnitude

    https://ibb.co/jG6n0jZ The 15 is fine as this is clearly his overall magnitude but then v2 is equated to the horizontal velocity rather than the magnitude.
  45. mopit_011

    Classical Understanding Derivative of Position Function: Is Velocity Wrong?

    Hello! So, I was beginning to skim Kleppner and Kolenkow for an upcoming course I’m taking over the summer. I saw this on pg. 17 and was wondering if I’m making a silly mistake in understanding what the book is saying. When they take the derivative of the position function, isn’t the velocity...
  46. R

    Understanding Velocity Addition Laws for People on Train & Ground

    I kind of just made up the questions. I realize this is a basic question but my knowledge of physics is very limited.q1 answer v_left_ball = v_left_ball - v_train v_right_ball = v_right_ball + v_train q2 answer To get the speed from Bob's frame I would use v_Bob = v_Bob + v_Alice To get the...
  47. F

    I Comparing Clock Rates in Accelerated vs Constant Velocity Frames

    @PeroK sent me here based on a discussion starting at https://www.physicsforums.com/threads/some-ideas-on-interstellar-space-travel.1006562/post-6550265. Summary: Someone claimed that an accelerated observer's clock (observer at 1g from Earth to halfway to Alpha Centauri, then -1g the rest of...
  48. A

    I Constant Velocity Motion in Relativity: Radiation Effects?

    Does relativity imply that everything that goes at a constant velocity must not emit radiation of any kind?
  49. P

    Calculate Earth's escape velocity using different methods

  50. LCSphysicist

    Rocket ejecting mass in a direction perpendicular to its velocity

    That was my approach: $$P_f - P_i = [(m-dm)(v + dv) + dm(u+v+dv)] - [m(v)]$$ $$= mdv - dmv + dmu + dmv = mdv + dmu = 0$$ Since the variation of the rocket's velocity is perpendicular to itself, $$ dv = v d \theta => m v d \theta + dm u = 0$$ So we have $$\frac{dm}{m} = \frac{-v d \theta}{u}$$...
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