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. titasdasplus

    B Friction forces on the tires of a moving car

    When a car is moving the velocity of the connection point between the car and the ground is zero. So static friction must be act here ,mustn't it ? But my mentor said ,it is kinetic friction. Which is correct? If kinetic friction, why?
  2. I_Try_Math

    Angular Momentum Conservation | Velocity at perigee

    I'm trying to understand if I'm making a mistake because the answer in the textbook is ##10.2~km/s##. Here's my attempt at a solution: The subscript A is intended to mean apogee and P is for perigee. ##v_A = 3~km/s## ##w_A = \frac{v_A}{r_A} = 7.5 \times 10^{-5}~rad/s## ##L_i=L_f## ##I_Aw_A...
  3. M

    Two rods with fixed angle, find the velocity of the crossing point

    I apologize if this is a stupid question but how come we can't just say that for part (a) v_A = -v_1 j hat, and for (b) v_A=v2 sintheta i hat + (-v1-v2cos theta) j hat? i.e. how come we can't just do vector decomposition "normally"? I am especially confused about the given solution in (a) where...
  4. S

    Acceleration of a ball at its max height after being thrown upwards

    The answer given is -10m/s2 because of constant downwards acceleration of gravity. i would like to know why is it not 0 at its peak height. at the top, velocity is momentarily 0, since acceleration is the change in velocity, change in 0 = 0. so why issn't a = 0? thanks
  5. N

    A 2.8-kg block slides over the smooth icy hill -- Find the minimum speed to clear the pit

  6. paulb203

    Momentum: instantaneous or average?

    I get the impression that momentum, in most physics questions (at least at GCSE level), is instantaneous. Is that correct? I tried to apply the basics to a walk to the local shops that I take regularly Distance 0.9 miles Time taken 0.25 hours Avg S=D/T Avg S=0.9/0.25 Avg S=3.6mph Avg...
  7. steff-23

    Relation between Friction, Velocity, and Acceleration

    For reference I'm a high school senior in physics. My thought process: 1) If the force of friction and applied force are equal the object "should" be at rest because the forces are equal (obviously this isn't correct because my professor tried explaining it and I'm not understanding the...
  8. Rick28

    Linear momentum but quadratic kinetic energy?

    no clues
  9. fallingforfandoms

    Circular motion, coin on a rotating disk

    First I tried to convert V = 61 rev/min to linear velocity. frequency = 61 rev / 60 sec = 1.017 rev/sec time = 1/f = 0.983 s V = 2(pi)r/t = 0.52*pi/0.983= 1.662 m/s From there I tried to find the maximum radius the coin could be at by using mu(v^2/r)=g r = mu(v^2)/g r= 0.47(2.76)/9.8 r= 0.13 m...
  10. Old Man Scho

    B Tangential Velocity (maybe)

  11. T

    I Escape velocity near a black hole: Newtonian or Special Relativity?

    For a spherical body mass M and radius r, the escape velocity at the surface is ##\sqrt{\frac{2GM}{r}}##. At the surface, an object mass m has GPE ##-\frac{GMm}{r}## and 0 GPE at infinity. So ##\frac{GMm}{r}## KE must be converted to GPE which means the minimum take off speed must be...
  12. M

    B Stopping power of rifle slugs

    As a child, one of my family friends was an 80 year old neighbor, Harry R. Caldwell, who had been a famous tiger hunter in his youth. He killed tigers and Kodiak bears with a .22 or .25 caliber high power load that some felt too small for big game. [edit: the claim about the bear is false...
  13. K

    Body attached to a block by a spring, shaped like an inclined plane

    I was taught to solve this problem by first finding the velocity of the body (of mass ##m ##) relative to the block of mass ##M ##. One way of doing this is as follows: first write $$ {v _{m _{B }}}^{2 }={v _{mx _{B }}}^{2 }+{v _{my }}^{2 } (I)...
  14. J

    Trying to tie two runners of different speeds over a race

    Attempt is attached, thanks for the help :)
  15. P

    I Can Propeller Speeds and Wave Breaks Challenge the Speed of Light?

    Hi, I have two topics: 1) Suppose a propeller is spinning very fast (say it's in a vacuum, no drag) and we arbitrarily lengthen its blades. Eventually the velocity of the tips of the propeller will approach the speed of light. That should be a speed limit, but that implies that the blades of...
  16. I_Try_Math

    Determining angular velocity of weight attached to a cord and cylinder

    ## a_t = r\alpha ## ## 9.8 \: m/s^2 = (.25 \: m)\alpha ## ## \alpha = 39.2 \: rad/s^2 ## Since the cylinder rotates through 20.01 radians after 5 m of cord has been removed: ## \omega_f^2 = \omega_0^2 + 2\alpha(\Delta\theta) ## ## \omega_f^2 = 2(39.2 \: rad/s^2)(20.01 \: rad) ## ## \omega_f...
  17. Zayan

    Ray optics: velocity of image reflected off a moving mirror

    Using equation, Vm=(Vo+Vi)/2; 2= (4+Vi)/2 =>Vi=0. I.e, image velocity is zero. I can't understand how it is possible that the image is at rest wrt ground. If the object moves with more velocity than the mirror, wouldn't it collide with the mirror at some point? And plus the object is moving 2m/s...
  18. Z

    B Speed vs. time for our Galaxy and an Observer

    Lets just say for the sake of argument that the earth, sun, solar system, galaxy were all moving due north at 1,000,000 miles an hour. If you were to travel due south at 1,000,000 mph you would basically be not moving at all. How would this affect time for you or anyone observing you?
  19. Heisenberg7

    How to Apply Derivatives in Physics Problems?

    I would like to discuss a few ways to apply derivatives in physics (I don't understand it fully). I don't need a full solution, I only need to understand how to successfully apply the derivatives First example, Thin insulating ring of mass M, uniformly charged by charge ##+q## has a small cut...
  20. Heisenberg7

    Find the change in the time required, if acceleration increases by the differential amount 'da'

    According to the problem statement: $$a = \frac{dv}{dt} = const \implies dt = \frac{dv}{a} \implies \int_{0}^{T} \,dt = \frac{1}{a} \int_{0}^{v_f} \,dv \implies T = \frac{v_f}{a}$$ Now, the distance covered is given by, $$L = \int_{0}^{T} v \,dt \implies L = \frac{1}{a} \int_{0}^{v_f} v \,dv...
  21. brotherbobby

    Man pulling a boat along the water with a rope (from above on a steep bank)

    Attempt : Let me copy and paste the question from the text alongside. Method 1 : I draw an image of the problem situation to the right. Boat B moves to the right pulled by a rope from the point O where the man is located. The angle the boat makes with the horizontal ##\alpha## changes with...
  22. helloworld2941

    Theoretical velocities at different heights sliding down an inclined plane

    At 3m: U = 3 x 9.8 x 80 = 2352J U at 6m = 6 x 9.8 x 80 = 4704J k = 1/2mv^2 (4704-2352)J = 1/2 (80) v^2 v = 7.67 m/s At 0m (bottom): U = 0 4704J = 1/2 (80) v^2 = 10.84 m/s Okay so what is bothering me here is just that my working doesn't take into account the...
  23. Heisenberg7

    A charge q approaching two stationary charges q1 and q2

    I'm only confused about one part of this problem and that is setting up the conservation of energy equation. In the solution, they just wrote this: $$\frac{mv_o^2}{2} = - k \frac{q_1 q}{r} + k \frac{q_2 q}{l + r}$$ where ##r## represents the distance at which the force created by the negative...
  24. P

    Circular motion of a Weightless rod

    I used law of conservation of energy to calculate (d theta/ dt)^2 (from:mgasin theta=1/2m(d theta/dt.a)^2+1/2mu^2(u is the velocity of the C ring at time=t)), but wasnt able to find u(velocity of C).Is there any relationship between the tangential velocity of B(d theta/dt.a) and velocity of C(u)...
  25. good jelly

    Relationship between linear velocity and angular velocity

    I find the velocity vector that is perpendicular to the radius which is v*cos(theta). Once I obtained the velocity that is perpendicular to the radius I used the equation : v = wr. But my answer seems to be wrong, the actual answer to this question is v = wr*cos(theta). Why?
  26. M

    B Direction of drift velocity vs direction of current

    I've done internet searches on this. There doesn't seem to be any agreement. Is the direction of the drift velocity in a wire the same as the (conventional) current? [AI Chatbot reference deleted by the Mentors]
  27. S

    I What if one is in 1905 and debating foundational fundamentals?

    Einstein's theories are accepted since 100+ years. Modern science built on it somehow provides a proof of truth of his theories but what if one is living when Einstein proposed his theories and debating its merit? Einstein did two things: 1. He declared velocity of light as invariant c for all...
  28. Anisur Rahman

    I What Happens to Rotational Velocity When AC Part is Dropped?

    In figure AO = OB = 4m. C is the midpoint of AO. The rod rotates with a velocity of 5 rad/s about the axis PQ. What will be the rotational velocity of the remaining rod if AC part is suddenly dropped from the rod? Assume that the mass of this uniform rod is M.
  29. chwala

    What distance will be covered before coming to stop?

    Phew; back n forth on this anyway, my lines ... ##v_{1} = \dfrac{1000}{20} = 50## m/s in the first ##10## seconds. ##v_{2} = \dfrac{1000}{30} = \dfrac{100}{3} ##m/s in the first ##35## seconds. where ##v_1## and ##v_2## are the respective velocities in ##10s## and ##35s## respectively...
  30. U

    AC gaussmeter in static magnetic field

    When you shake back and forth AC gaussmeter ,he significantly increase reading(magnetic field), because device is moved in Earth static magnetic field. Does his acceleration or velocity cause increase in reading and why he even increase reading if magnetic field from Earth is static and...
  31. robphy

    Insights Addition of Velocities (Velocity Composition) in Special Relativity

    Continue reading...
  32. 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...
  33. A

    I Question Regarding Force Being Equal, Even Moving Upwards (if V=0)?

    I understand based on the equation F = ma that if there is no acceleration, the forces on the object all balance out to 0 in all directions. What I don't get is for example, slowly lowering a heavy stone slab at a constant velocity v, and raising it way above my head as high as I can at a...
  34. highschoolstudent454

    Resistance force changing the velocity of an object

    Mentor note: Moved from a technical section, so is missing the homework template. A Body of mass 1kg moves with a uniform velocity of magnitude 12m/s, a resistance of force of magnitude 6x^2 (newton) where x is the displacement (meter) which the body travels under the action of the resistance...
  35. H

    I Freqeuncy of Matter Waves Approaches Infinity as Velocity Approaches c

    I have heard that the phase velocity of matter waves can be represented as c^2/v. But if the wavelength of these matter waves goes to zero as momentum approaches infinity and v approaches c, then does this mean that the frequency of the matter waves approaches infinity, to give the matter wave a...
  36. rosedog09

    Drag Forces and Terminal Velocity

    Teacher says Im wrong on both these questions. I have consulted with other teachers and they say im correct. What do you guys think? 3. As mass increases, so does terminal velocity. 5. Fnet = FDrag - Fg ma = FDrag - mg FDrag = 79(8) + 79(10) FDrag = 632 + 790 = 1422 N Teacher Comments 5...
  37. Manish_529

    Average velocity in different coordinate systems

    <v>=?
  38. K

    B Time and right angle traveling twins paradox

    So in scenario in which two ships(A and B) are moving relative to each other, and towards each other; each ship will measure the other to have a velocity. Within special relativity no observer himself moves. Do you agree? If you agree, assume each of the above two ships sends out shuttle along...
  39. hello478

    Graph of viscous force and velocity

    my answer was A but i dont understand because when the viscous force equals the weight speed becomes constant but why does the viscous force needs to be equal of weight? is it weight = viscous force? or speed = viscous force?
  40. chwala

    Find the initial velocity ##U##

    Now in determining the initial velocity; in my understanding, if ##s=1.8## then we consider the stone's motion from the top to the ground. Why not consider ##s=3.6##, the total distance traveled by stone from start point ##t=0##? Is it possible to model equations from this point? The stone...
  41. chwala

    Find the value of ##T## and distance of particle in the first ##4## seconds

    solution is here; I just need to understand this part ##14.7 = -14.7 =9.8T##... why initial velocity upwards is a negative value? or i am interpreting it wrongly. ........... In my reasoning, ##v=u+at## ##0=14.7 + (-9.8)t## ## t_1=1.5## in reverse direction, from top to start point ##T##...
  42. billtodd

    A small disk and a bigger half disk

    I need to find ##v_1## and I know what are the initial conditions: ##\theta(0)=\pi## and ##\dot{\theta}(0)=0##. Then what is ##v_1## and how to find it? Thanks!
  43. hello478

    Change in the kinetic energy of a car driving up a hill

    part i) i did 1/2 * 1700 * v^2 i dont know what v is... so how do i solve it? part ii) i calculated it correctly by 440*25 please explain in detail why i used 440? and part d) i did 1.7*10^4 = 48000/t my t= 2.82 s but correct answer is 3.5s
  44. J

    Found PhysicForums while working on ballistics problem

    Several different sites, including PhysicsForums, mention/discuss a formula for computing a bullet's Ballistic Coefficient based on measuring two downrange velocities where the measurements are made a distance L between them. This can be done with traditional skyscreen (optical) chronographs or...
  45. chevywaldo

    I Why Does Differential Pressure Vary As The Square Of The Flow?

    Can someone explain to me Bernoulli's principle as to why the pressure across a restriction varies as the square of the velocity? I'm looking for an understanding (conceptual) as to why this is without a gazillion math examples please. Thanks.
  46. G

    A The effect of perpendicular wind at channel inlet

    Hello I'm thinking of how to attack this problem. Outside a channel-inlet, wind is passing perpendicular to the opening with a wind speed of 9 m/s and a fan sucks in a certain amount of air into this channel so that the velocity in the channel is 3 m/s. When the outside air is 0 m/s I have no...
  47. M

    Is this projectile motion situation possible?

    Hi everyone, I have created a question which I thought would have a single simple solution, but have noticed there are two possible answers. This makes me think that the question's scenario is impossible with the numbers I made up. I think we all can agree that the horizontal component to...
  48. I_Try_Math

    B More convenient mathematical notation for a simple use case

    So in my textbook there's a basic problem where you solve for the final velocities of two hockey pucks, which happen to have different colors which are red and blue, using conservation of momentum. The notation that the textbook uses to express the final velocities of the pucks is ##v_{1,f}##...
  49. F

    Find the distance a particle travels

    My answer is d = (e+3)/e x(t) = ∫01 3t2 dt (0 ≤ t ≤1) = t3 |01 = 13 - 03 = 1m x(t) = ∫1∞ 3e-t dt (t > 1) = -3e-t |1∞ = lim(t->∞)[-3e-t] - [-3e-1] = 0 + 3e-1 = 3/e m Therefore total distance = 1m + 3/e m = (e+3)/e m However, the textbook answer gives...
  50. A

    A massless disk with an embedded particle rolls down an inclined plane

    This is the problem: I am a little confused at the solution below. Since ##\vec r_{P/C} = (R/2)*(5 + 4cos(\theta))^{0.5}\ \vec e_r##, I am wondering why there is no vector e_r component for the velocity. I know that d(R)/dt = 0, but you can still get a derivative in the ##\vec e_r## direction...
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