What is Velocity: Definition and 1000 Discussions

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. 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...
  2. 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...
  3. 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...
  4. 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...
  5. 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...
  6. 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...
  7. 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...
  8. 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...
  9. 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...
  10. 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.
  11. 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...
  12. 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...
  13. 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...
  14. 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?
  15. P

    Calculate Earth's escape velocity using different methods

  16. 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 $$\frac{dm}{m} = \frac{-v d \theta}{u}$$...
  17. C

    Velocity Formulas for Projectile Motion

    Hi, so I solved this using energy. I also tried solving this using kinematic equations. The issue I am running into s for calculating the Vy of the problem. I am able to find the time it takes the rock to fall using d=(1/2)gt^2. Then, I considered two different ways of going about finding the...
  18. Anmol Dubey

    Calculating final rotational speed from angular velocity

    I have no idea how to go about this. Any help would be appreciated thanks :) Edit: I converted the 1.5 rev/s to rad/s = 9.4 rad/s
  19. Eobardrush

    Ball rolling down a slope problem: Find an expression for time taken

    Question: Galileo released a metal ball from rest so that it could roll down a smooth inclined plane. The time t taken to roll a distance s was measured. He repeated the experiment, each time recording the time taken to travel a different fraction of the distance s. Write an expression for the...
  20. Anmol Dubey

    Finding time taken to stop a skater given mass, velocity and force

    I derived the formula for t but don't know how it works Δt = p/F I got p = mv = 60*8 = 480kgms-1 Δt = 480/120N = 4s Is that correct?
  21. T

    Low pressure, high velocity flow vs static pressure

    In this scenario, we have a compressible flow at 0.99 barA pressure and flowing at 300 m/s velocity and is released into 1 barA pressure. Point is, whether the flow can be released at 1 barA or not at the release pressure is higher than the pressure of the compressible fluid inside the tube. And...
  22. D

    I Solving for Angular Velocity After a Disc Collision

    Hello to everyone, first of all shame on me I has to ask and can not figure out it by myself... The problem is I am trying to code game where two homogenous discs with same mass and same diameter, no fricition due to gravitational forces, can collide. I can figure out the speed and direction...
  23. H

    I Group velocity for an electromagnetic wave inside glass

    Hi, I saw that the group velocity for an electromagnetic wave can be calculate with the following formula ##v_g = v_p + k \frac{d v_p}{dk}## Thus, since ##v_p = \frac{c}{n} = \frac{\omega}{k}## Is it correct to say that ##v_g = \frac{c}{n} + k(- \frac{\omega}{k^2})## where ##k =...
  24. A

    I If we throw a ball with initial velocity v0, is there a force?

    If we throw a ball with initial velocity v0, and the ball progresses in a projectile motion, ignoring air resistance, will there be a force in the x direction? If so, what is that force in the x direction's value mathematically? I know there is a force in the y direction due to gravity.
  25. Zeeshan Ahmad

    Understanding Velocity Transformation

    I have used velocity transformation ibut a little confused on it so do solve the problem
  26. L

    I Interference - two waves of different frequencies - beat velocity

    Hello, I'm considering the "beats" phenomena. I have two plane waves in some medium with a refractive index n(ω), one propagates in a z direction and second in a direction making an angle θ with z axis. Waves have frequencies ω1, ω2 (not necessarily equal) and k-vectors k1, k2 (not necessarily...
  27. J

    Velocity of two masses due to electric potential energy

    We can find the potential energy by finding the potential difference between the two masses. the minimum distance between the two masses is 10 cm. The maximum is 30 cm because they can be 3 string lengths apart as they repulse each other once the string is cut. So, to get potential difference...
  28. A

    How Is Delta_h Calculated in Physics Problems?

    b) My question is here! 0.9v_t = v0 + a*t = a*t t = 0.9v_t/a delta_h = v0*t + 0.5*a*t^2 = 0.5*a*t^2 = 0.5*((0.9*v_t)^2)/a = where a = (-m*g + b*v)/m delta_h = (0.5*((0.9*v_t)^2)*m)/(-m*g + b*v) plugging in everything delta_h = 1.65 m which is not what the answer is. Please help
  29. Anmol Dubey

    Making a Velocity vs Time graph for a bullet

    Here is my working in case it helps :)Ignore the v = 0.6/0.0001 = 6000m/s
  30. P

    Engineering Velocity Decay Curve for a Rocket

    I am trying to solve the problem below. I have previously calculated from 0 to 4 seconds how far the rocket will travel in each second. I am stuck now as to how to start this problem. I have searched but unable to find the answer. Do i need to rearrange this? A is currently 14 which does not get...
  31. J

    Find net velocity of charged particle in electric field (symbols only)

    We know the net force on the charged particle in the uniform electric field pointing up is mg - qE. To get acceleration, divide the net force by mass to get g - qE/m Plug into kinematic equation and get velocity by itself and substitute$$\sqrt{h(2g - \frac{q \sigma}{\epsilon_o m})}$$
  32. Y

    Horizontal velocity of a basketball as it goes through the hoop

    hey i don't understand why my answers are incorrect, here is my solution i would like if someone can help me understand what I am doing worng and how i should solve this problem
  33. Keith Koenig

    B Length, Time, and Velocity -- Which are fundamental quantities?

    We think of length and time as the first fundamental quantities and velocity as the first derived quantity but any two determine the third so we would be completely justified in defining velocity as a fundamental quantity and one of length or time as the other, with the remaining being the first...
  34. rudransh verma

    B Is the final velocity of a Tossed tomato the same as its initial velocity

    When a tomato is thrown up with a velocity ##v_0## it’s kinetic energy is 1/2mv_0^2. It will stop at the top and then again comes back to the launch point where it’s kinetic energy will be same as before, 1/2mv_0^2. How is this possible? Also when we throw the tomato up how can you be so sure...
  35. Y

    The velocity of the particle as a function of time

    this is how i try to solve it: can someone please help me with that because i don't know what I am doing worng here.
  36. V

    Direction of of the velocity vector for particles in a sound wave

    Using the equations mentioned under this question, I came up with following analysis and directions of velocities on either side of ##x_1##. Also, I'm not sure if there is an easier qualitative way to know the velocity directions rather than do a detailed Calculus based analysis?
  37. dirb

    The time for a proton to reach a certain velocity

    A nuclear reactor is built to fuse two hydrogen atoms that are already ionized to protons. However, the electric field of the protons are becoming a significant obstacle. If the reaction was to be defined as H2--> 2H++2e-, if the mass of a proton is mp, the radius of a proton r the charge of an...
  38. dirb

    Calculating Angle & Speed to Reach Planet's Moon from Station Orbit

    A station is orbiting a planet at a distance R1, a moon is orbiting the planet at distance R2 with the period T. The planet itself has a radius rp and a mass mp. We know that when an object adds its velocity at a point in the orbit, the height of the opposite orbit will increase. Determine the...
  39. Physics_learner

    Kinematics - can't find the initial velocity according to the image

    I tried to write the data I understood from the image: y0=160m yf=0 x0=0 x1=192m I tried to express the total change in time using the position over time equation on the Y direction: y(t)=y0+v0y*(t2-t1)-0.5a(t2-t1)^2 but then I stuck with 2 variables and didn't know what to do any help?
  40. crism7

    Calculating Time of Flight & Velocity of a Ball Thrown Upwards

    Vertical components: dy = 0m ay = 9.8m/s^2 [down] t = 1.34s V1y = required V2y = 0 i first tried to find V1y dy =vi t + 1/2 a t^2 and got V1y = -6.566 then i solved for time of flight dy =vi t + 1/2 a t^2 0 = -6.566t + 4/9t^2 and for 1.34 seconds does this mean the time of flight is the...
  41. Lay1

    Finding the acceleration of the ball in contact with the ground during bouncing

    the v before hitting the ground immediately=4.85m/s the v after hitting the ground immediately= 3.96m/s I considered the down positive, then v= u+at 3.96= 4.85+ (a*0.16) so a= -5.56m/s*s The answer is 55m/s*s The parts that I don't get are why it must be -3.96 and why that velocity becomes...
  42. A

    Finding kinetic energy and initial velocity of a cart over time

    Here's my list of variables and things to account for: m=100kg Wnc=5000J Wfriction=-500J -Kinetic energy will be doubled (though I don't know how that plays into it exactly) -I don't think there's any PE because it's on level ground My idea of what the equation might be: Wnc +1/2mv^2initial =...
  43. Stewkatt

    Motion of an Object: Displacement & Average Velocity

    An object starts from rest and accelerates at 3.0 m/s2 for 4.0 s. Its velocity remains constant for 7.0 s, and it finally comes to rest with uniform deceleration after another 5.0 s. Find the following: a. the displacement for each stage of the motion b. the average velocity over the whole time...
  44. B

    The Traditional Bernoulli Eqn Derivation of Rocket Exhaust Velocity

    If you've seen it, they chose one point in the combustion chamber and the other in the exhaust nozzle. I think they're assuming that we have a gas both places. They say that the pressure in the nozzle is atmospheric pressure, or it you're in outer space, zero. That makes perfect sense...
  45. frost_zero

    I Hypothetical Velocity Faster Than C: Exploring Relativity

    I know it isn't possible for any mass/massless body to have velocity more than the speed of light in vacuum but what if it was done hypothetically? As far as I know length and time of an object undergo a transformation so that the laws of physics remain same between observers at rest and...
  46. P

    B What gives satellites their tangential velocity?

    What gives satellite a tangential velocity?
  47. J

    Velocity, traveling distance and friction force on a sliding object

  48. Remixex

    Estimating the velocity of seismic waves through an idealized Earth

    Hello, this is a repost from a much less-clear question I posted before (link to question: https://www.physicsforums.com/threads/triangles-inside-a-circle-to-represent-raypaths-inside-an-ideal-earth.1011998/#post-6596165). It's kind of a loaded question, however it can be expressed as triangles...
  49. J

    What is the relationship between angle and static friction?

  50. Istiak

    Velocity along the cylinder axis

    Since the question says that "velocity along the cylinder axis" and "magnetic field perpendicular to the cylinder axis". So cross product of velocity and magnetic field becomes their magnitude. ##\vec v\times \vec B=||v|| \\ ||B||## So ##\vec F=qvB## ##mg=qv\frac{\mu_0 nI}{4\pi r}## At first...
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