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. mattlfang

    Find the velocity and acceleration of a pulley in a mass-spring system

    This looks like a classical setup but I can't find a solution. We can calculate the energy of the system by looking at the work done by the gravity and the spring. But how do we divide the energy between the kinetic energy of the pulley and the rotation of the pulley?
  2. E

    I Relativistic Velocity, Perp. Accel., Momentum: Explained

    A stationary observer sees a particle moving north at velocity v very close to the speed of light. Then the observer accelerates eastward to velocity v. What is its new total velocity of the particle toward the north-west relative to the observer? I ask because while the particles total...
  3. Leo Liu

    Sum of oscillating frequency and angular velocity

    The "egg" initially spun around axis 1 with at ##\omega_s##. After being disturbed, it has started to possesses angular velocities along 2 and 3. The question is to find the rotational speed of ##\vec \omega=\vec\omega_1+\vec\omega_2+\vec\omega_3## to a fixed observer. It is calculated that...
  4. PhysicsTest

    To find the parameters of velocity in an electric field

    i have drawn the E field as below, hence the F will be in the upward direction for electron a. Using energy is constant, the velocity ##v_x## as it crosses A is ##0.5mv_x^2 = q*V_a## ##v_x = \sqrt{(\frac{2qV_a} m)} m/s## one doubt i have here is, the question mentions electrons, but i have...
  5. weirdoguy

    Velocity of wind based on the shape of a smoke trail

    Hi everyone. My high-school student got the following homework exercise at school which I have problem with: Smoke from the steam locomotive is carried by horizontally blowing wind. Shape of the smoke trail is shown in the attached figure. Using the drawing, determine velocity of the wind...
  6. diazdaiz

    Find the function of velocity of a person that accelerates to a constant v

    for example, I want to know velocity of a person when time is equal to t, that person start running from 0m/s (t=0s) to max velocity of 1m/s (t=1s). I am thinking that this is like rain droplet that affected by gravity and drag force, where force is directly proportional to its velocity, to make...
  7. J

    Velocity from spring pushing on 2 masses

    So i know Ek=123.48 from the potential energy that converts into kinetic energy (Ep=1/2kx^2). Now by conservation of momentum, m1v1=m2v2 So m2Sqrt(2Ek/(m1+m2))=m1v1 This is where I am making a mistake I think, but not sure how. Answer is suppose to be 4.44
  8. J

    Escape Velocity Question: Why is the final kinetic energy = 0?

    So Ekf-Eki+Epf-Epi=0. I understand that the final potential energy is 0 (distance away approaches infinity), but don't get why the final kinetic energy becomes 0. If the final kinetic energy was 0, wouldn't that mean the object no longer has any velocity and would start being effected by the...
  9. J

    Fundamental Forces: finding velocity using different fields and forces

    So I know the acceleration is 0, so the net force is 0. QE=1.6E-3*9.9E3 k hat = 15.84 k hat (thats one force) qv x B = q(v i hat + v j hat) x (0.51 i hat + 0.10 j hat) =q(0.10 v k hat- 0.51 v k hat) =q(-0.41 v k hat) =-0.000656 v k hat Now solving for velocity, 0.000656v=15.84...
  10. PolVD

    Solve 2D Asteroid Problem: Step-by-Step Guide

    Hi everyone! I don't know how to solve the next problem, and if anyone could explain to me step by step how it is solved I would thank you a lot. I know it's not hard, but I'm not seeing how to do it. Thank you in advance!
  11. JackFyre

    On Bodies Moving with a Constant Velocity

    Is the below Displacement time graph posible? if an object is in motion at a constant velocity, is it possible for it to stop instantaneously, or does there have to be a decelaration? According to Newton's first, an object will remain in its state of constant velocity unless acted upon by...
  12. Kairos

    I Velocity Addition and Doppler Effect: Explained

    The Doppler effect of light corresponds to the classical Doppler effect corrected by time dilation, but the first one is obtained with classical velocity additions (c+v at the front of the source and c-v at the back) whereas velocity addition of special relativity gives c at the front and c at...
  13. J

    B Experimental Confirmation Length Contraction Velocity Addition

    What if any is the experimental confirmation for length contraction and the velocity addition formula?
  14. J

    Using Orbital Energy to Calculate Velocity

    So what I did first was calculate the initial and final potential energies with Epi=-9.433*10^11 m and Epf = -1.503*10^12 m. Then I found change in potential energy, -5.597*10^11 m. Using this I determined the change in kinetic energy, 5.597*10^11. I then added this change to the initial...
  15. D

    Quickest Route Between Horizontal Points using the Initial Velocity & Friction

    re: https://www.physicsforums.com/threads/is-it-possible-to-solve-for-t.996132/post-6421205 https://www.physicsforums.com/threads/is-it-possible-to-solve-for-t.996132/post-6421230 Would it be possible to find t and r via the exact same equations as found in posts #4 and #10, and then find the...
  16. O

    How to find d2 when given d1 and d, total time, and average velocity?

    I rearranged the displacement formula to d2 = d + d1. I used cosine law to solve for d2 since the triangle is not right-angled but I am not getting the correct answer or angle for d2. The angle I used in cosine law (based on the diagram) was 32+12+90 = 134. d = v(t) = 130(3) = 390 km/h [N 32 E]...
  17. S

    I The velocity of Pilot-wave and the Double slit experiment

    Hello, Its been a while since I'm trying to understand the concept of the pilot wave theory and it's relation with the deBroglie wave. What is the fundamental difference between the two? My confusion comes from the double slit experiment. The velocity of the deBroglie wave is c^2/v so I...
  18. C

    Max velocity of a vibrating loud speaker membrane given sound intensity

    My attempt: p and T allows us to calculate ##Z=402 \frac{kg}{sm^2}## using ## Z=p*\sqrt(\frac{\gamma*M}{R*T})## . The sound intensity level at 10 meters allows us to calculate the intensity at 10 meters to be I=10``````^{-7} W/m^2 using ##50 = 10*log(I/I_0)##. Then, using the formula...
  19. R

    Solving for Time to Reach Ground: Calculating Velocity of Sound over 50m

    So I figured that to find how long did the ball take to hit the ground I would have to subtract 6 with the time it took the sound to reach back to the player. My givens are Velocity Intial = 330 m/s, speed of sound Δd = 50m Velocity Final = 0m/s because sound stops when it reaches the ear...
  20. LCSphysicist

    Special relativity — Observer measuring the velocity of a passing rocket

    I am a little confused with the text above. Actually, all is ok until 2.51, i think i am missing something. What is this? Another way to define gamma? (The doubt is not about the raising indices, neither about "n00 = -1", is just where does the equation came from. That is, the equation is easy...
  21. N

    B Earth's Hubble velocity and measuring Hubble at large distances

    Just curious. We can't figure out Earth's speed of travel through the universe due to the Hubble constant because that would be measured from the center of the universe and the center is located somewhere unknown to us except that it is beyond what we can perceive, i.e., more than about 63 Gly...
  22. M

    Help with Angular Velocity True-False Question

    1)Starting at rest, he brings the weights into his chest. His angular velocity increases. 2)A friend throws a third weight so that the student catches it in one of his outstretched hands. No matter what the direction of the throw, the student's angular velocity decreases. 3) Starting with...
  23. Chris Miller

    B Shape of Hubble sphere at relativistic velocity

    Consider the Hubble horizon as the proper distance over which Hubble expansion equals c, so that you are in the center of a Hubble sphere with a radius of about 13.5 billion light-years. As you approach light speed in any direction, does the Hubble horizon draw closer in that direction due to...
  24. D

    Brachistochrone Problem w/ Initial Velocity

    Hello, There is a physics problem called the Brachistochrone problem which I know has been solved for 0 initial velocity (assumes 0 friction and only gravity) and I know the answer is a cycloid. My question is: is there is an existing formula for finding the portion of a cycloid which is the...
  25. P

    I Finding the Velocity of an object due to an imparted force

    Hello folks, I was wondering how I could calculate/find the velocity of an object from an imparted force given its mass. My force is 26 MN, the object is 10 metric tons, or 10kKg, what will the objects velocity be if this force was applied to it rapidly, e.g. like a jab/punch. My goal is to...
  26. OscarF

    Physics Homework Question on terminal velocity and weight

    Below I've attached the question - I don't know why this question is so difficult, perhaps I missed a lesson or such, however I've Benn working at it for ages and got nowhere...
  27. justinpaul

    Calculating Velocity Vector of Object in 2D Space

    The question I am trying to solve is what is the velocity vector (direction and magnitude) of an object in 2 d space. We know the distance measured to the car from two different angles. We know the radial velocity of the car on both measurements. The radial velocity is the component of the...
  28. snoopies622

    I Derive Escape Velocity GR: Source for Schwarzschild Metric

    I was surprised to read that the formula for escape velocity — at least for a spherical mass like the Earth — is the same in relativity as it is in classical physics: v_e = (2GM/r)^{1/2} I'm wondering if someone can give me a good source for deriving this. (I assume one takes a radial...
  29. T

    Conversion of high velocity airflow into a pressure

    We know blower compressors i.e. compressors that use a blower to create high velocity for injecting air/fluid to higher pressure and that's available in market. Now, suppose we have such a blower type compressor that can create a flow of air/gaseous fluid of 337 m/s. Question is, what maximum...
  30. aatari

    Find the Magnitude of Average velocity using the given vectors

    Hi Guys I've attempted the question but not sure if the approach I used is correct. Would someone please have a look at my solution and let me know if it makes sense. Thanks!
  31. A

    Angular velocity and angular acceleration of a turbine

    Okay so what I've done; I've put the diammter d = 1m as r = 1m The time interval of 4s is t = 4s and the angular velocitys as; ω1 = 20 rad/s ω2 = 40 rad/s Now to get the accelaration. Angular acceleration can be split into two parts tangetial acceleration and radial acceleration What I...
  32. greg_rack

    Considerations for the FBDs of a parachutist traveling at terminal velocity

    There are a few things I'm not getting about this exercise and related diagram: -what does "force from parachutist" consists of? -if the terminal velocity is reached, then ##a=0## which means ##F_{tot}=0##, so shouldn't simply the sum of all forces pointing upwards be equal to the sum of all...
  33. B

    Fluid velocity and pipe diameter using the continuity equation

    Hi, Can anyone let me know if I had done this Q correctly? Thanks for any help!
  34. SamRoss

    I How is the weak force related to a change in velocity?

    Hi everyone, The four fundamental forces are gravity (I understand that G.R. does not look upon gravity as a force but I'm not worried about that here), the Lorentz force, the weak force, and the strong force. I'm familiar with the inverse square law for gravitation and the Lorentz force...
  35. A

    Determining the minimum velocity when given height and length

    Now I've tried looking at the problem like this. Considering that a is the length off the vehicles that he is trying to jump over I would consider that to be s. The plane from which he starts (b) should be the h. So considering that he is jumping from a horizontal plane, gravity should also...
  36. Hamiltonian

    Velocity of a cart moving in the rain as a function of time

    At time t = 0, the mass of the cart is ##M_0## and velocity is ##v_0## in a time interval ##dt## let a mass of ##dm## be added to the cart due to the pouring water and let the reduction in speed be ##dv## ##\lambda = dm/dt## applying conservation of momentum from the ground frame gives $$M_0...
  37. A

    Angular velocity in terms of Euler angles

    In Chapter 4, derivation 15 of Goldstein reads: "Show that the components of the angular velocity along the space set of axes are given in terms of the Euler angles by $$\omega_x = \dot{\theta} \cos \phi + \dot{\psi} \sin \theta \sin \phi, \omega_y = \dot{\theta} \sin \phi - \dot{\psi} \sin...
  38. Hamiltonian

    Doubt related to the velocity of the center of mass

    In question 1. since there is no external force on the system of particles(and since it was initially at rest) shouldn't the ##V_{cm}## be zero? But the correct answer applies the above stated formula for ##V_{cm}## and gets ##V_{cm} = v/2## and in question 2 again as there is no external force...
  39. thequantumcat

    I Special Relativity and Velocity

    I can't seem to wrap my head around it: if an object is moving at speed v in frame S, and its observed to move at speed v' in S', what is the relationship?
  40. mcastillo356

    Average speed (or maybe average velocity) of an athlete

    Hi PF It's a multiple choice exercise, and the answer is 10 m/s. I've been wondering why, but no way. Regards
  41. N

    Analyzing Velocity and Force of a Chain in Motion

    My solution: For the horizontal portion of the chain: let at any instant the length of chain inside the tube is x, and at that instant the chain in the tube is having a velocity v. Then, at any instant: ##F = \frac{\mathrm{d} p}{\mathrm{d} t}## ##p##= mass of the chain in the tube at the...
  42. andrewasd

    What is the significance of height in a vertical spring system?

    a
  43. C

    What velocity does the particle need to get to point B?

    It was a long time ago I did these kind of problems so I’m a bit rusty. The only thing I can think of is divide it up to two parts one x and one y. In y the acceleration is sin(a)*9.82? Then put that in the equation and solve for t. In x the there is no acceleration so the formula is x=V0*t, I...
  44. peace

    Estimate the initial velocity of the cars after the collision

    What came to my mind for this question is: Consider one of the cars. The velocity and mass of this car are V and M respectively. And the velocity and mass of the piece attached to the car are m, v respectively. Before the collision, the velocity of this piece relative to this car is zero. So its...
  45. T

    Non-constant acceleration, solving for velocity

    Hello! I am trying to solve for the velocity in terms of position of a particle moving with non-constant acceleration. a=c*t (where c is a constant) I can easily solve for velocity in terms of t. dv/dt=a dv/dt=c*t I differentiate and get v=1/2*c*t^2+v0 (where v(0)=v0) However I am not sure...
  46. Sunny Singh

    Relaxation time and average electron velocity in Drude model

    If τ is the relaxation time, τ means, on average the time between two collisions for an electron moving under a constant electric field inside a metal. Now according to the assumptions of drude model, the electron acquires an additional velocity of \frac{-eEt}{m}where t is the time elapsed since...
  47. E

    Velocity of charges or bounding curve features in motional EMF?

    The motional EMF is$$\mathcal{E}_{\text{motional}} = \oint_{\partial \Sigma} (\vec{v} \times \vec{B}) \cdot d\vec{x} = \int_{\Sigma} \frac{\partial \vec{B}}{\partial t} \cdot d\vec{S} - \frac{d}{dt} \int_{\Sigma} \vec{B} \cdot d\vec{S}$$(that's because Maxwell III integrates to...
  48. L

    Maximum velocity of an electron emitted from lanthanum

    Mass of an electron = 9.1*10^-31 Well, to find the maximum kinetic energy of the electron use E=hf, E=hc/λ=6.63*10^-34*3*10^8/1.5 *10^-7 E=1.326 *10^-18 J 1/2m v^2 max=E Rearrange in terms of v: v=√2E/m v=√2*1.326 *10^-18/ *9.1*10^-31 v=1707127... ~ 171000 ms^-1 Where have I gone wrong here?
  49. M

    Determining an object's velocity in cylindrical coordinates

    I got the answer for velocity and acceleration. But I don't know how to draw the shape of the particle's motion over time. How to draw it? should we change a,b,c,e into a numbers or not? or we may not to change a,b,c,e? Please help me how to draw the shape of particle's motion over time?
  50. E

    Finding time from a velocity vector

    I've looked it up online and someone did t=40−65=0.15(h) I was just wondering why they would subtract the velocities. Could something explain this to me please? thanks.
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