What is Motion: Definition and 1000 Discussions

In physics, motion is the phenomenon in which an object changes its position over time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and time. The motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame with change in time. The branch of physics describing the motion of objects without reference to its cause is kinematics; the branch studying forces and their effect on motion is dynamics.
If an object is not changing relatively to a given frame of reference, the object is said to be at rest, motionless, immobile, stationary, or to have a constant or time-invariant position with reference to its surroundings. As there is no absolute frame of reference, absolute motion cannot be determined. Thus, everything in the universe can be considered to be in motion.Motion applies to various physical systems: to objects, bodies, matter particles, matter fields, radiation, radiation fields, radiation particles, curvature, and space-time. One can also speak of motion of images, shapes, and boundaries. So, the term motion, in general, signifies a continuous change in the positions or configuration of a physical system in space. For example, one can talk about the motion of a wave or about the motion of a quantum particle, where the configuration consists of probabilities of occupying specific positions.
The main quantity that measures the motion of a body is momentum. An object's momentum increases with the object's mass and with its velocity. The total momentum of all objects in an isolated system (one not affected by external forces) does not change with time, as described by the law of conservation of momentum. An object's motion, and thus its momentum, cannot change unless a force acts on the body.

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  1. Uchida

    Dynamics motion problem with a disk in contact with a bar

    First, I tried to model the disk-bar as a crank connecting rod, to the OA bar, and apply this: VP = VB + ω_BP x r_P/B, where P is the contact point between the disc and OA bar. I assumed VP = VP sin 30º i + VP cos 30º j (direction parallel to r_P/B), where r_P/B = sin 30º i + cos 30 j This...
  2. H

    Is My Thinking Reasonable: A's Impact on Oscillator Motion

    I just want to check if my thinking is reasonable. Since B,C and D are all dependent or fixed by the oscillator, A is the only factor to affect the amplitude of the motion at resonance or even throughout the entire process?
  3. brotherbobby

    Projectile motion with (constant) wind velocity

    Let me start be making a small sketch of the problem, shown to the right. If the range of the projectile on a still day ##R = v_{0x}T##, then on the windy day the range becomes ##R+2H = v'_{0x}T = (v_{0x}+v_w) T##. Since the maximum height attained by the projectile ##H =...
  4. C

    I Determining Future Position of Uniform Circular Motion

    Hello, Apologies if this is in the wrong section, it's related to circles so I figured Geometry was the best place. I found a very good example online that explains how to determine a future position of an object undergoing uniform circular motion: (Note that they made a mistake by writing...
  5. L

    A Heisenberg equation of motion -- Partial derivative question

    Heisenberg equation of motion for operators are given by i\hbar\frac{d\hat{A}}{dt}=i\hbar\frac{\partial \hat{A}}{\partial t}+[\hat{A},\hat{H}]. Almost always ##\frac{\partial \hat{A}}{\partial t}=0##. When that is not the case?
  6. ZdMh

    About difference in Velocity and motion in Jet engines

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  7. Bob Enyart

    Is there any motion hidden within potential energy?

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  8. momoneedsphysicshelp

    Finding Angular Velocity in Rotational Motion Problems

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  9. J

    Harmonic motion - Find the Mass held between two Springs

    So first I find the energy using the eqn (1/2)kA^2. Since there are two springs with the same k I multiply it by two to get kA^2. Energy I get is 2.0475, Now I use E=(1/2)m(wA)^2 to find mass. Again since there are two springs I use E=m(wA)^2. m=E/(wA)^2. w=(2(pi))/T btw. I get the answer of...
  10. V

    Experimental Analysis: Forced Harmonic Motion and Resonance

  11. Mayan Fung

    I Refractive index of a medium in relativistic motion

    I once naively think that the speed of light is also a constant in a medium in all inertial frames which is not the case. I tried to derive the result yet there is a discrepancy from the results I read in some articles. For example, from [Link to unpublished paper redacted by the Mentors], the...
  12. F

    Tension in rope for non-uniform circular motion with air resistance

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  13. E

    Motion of a sphere in a liquid

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  14. Purpleshinyrock

    Help me solve this equation please -- Roller Coaster Motion

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  15. Pipsqueakalchemist

    Engineering Using Cartesian vs. Normal/Tangential Coordinates for Centripetal Motion

    So for this problem the solution used Cartesian coordinates but I was wondering wouldn’t it be easier to use Normal and tangential coordinate because the bar is undergoing centripetal motion? Also on the right diagram shouldn’t the acceleration be down and not up. The reason I think that is...
  16. Pipsqueakalchemist

    Engineering Questions on Forces & Friction: Analyzing Plate & Block Motion

    So I have questions on two separate questions. The first question is about the fbd of the plate attached to the pin. How can there be an Ax since there’s no other force in the horizontal direction wouldn’t the plate start moving on the x-direction due to imbalanced forces. Second question about...
  17. V

    Forces on particle in complex motion relative to ground observer

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  18. Frouel

    What is the maximum kinetic energy for harmonic motion with a reduced amplitude?

    Solutions in a file.
  19. P

    Does Raising My Arm Change the Center of Mass?

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  20. S

    Motion of box on inclined plane connected by spring to a wall

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  21. H

    Motion of a particle in a magnetic field

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  22. rg0ldberg

    Projectile Motion with a backwards Rotation/Tumble

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  23. warhammer

    Motion involving Translation & Rotation |Kleppner and Kolenkow

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  24. P

    Getting the equations of motion for this SHM problem

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  25. M

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  26. abdsaber000

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  27. A

    I Electric Field seen by an observer in motion

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  28. G

    Did I Miss Something in Disputing a Popular Book's Solution on Relative Motion?

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  29. hquang001

    Rotational motion and angular momentum

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  30. momoneedsphysicshelp

    Simple Harmonic Motion of a Mass Hanging from a Vertical Spring

    Assuming zero spring mass and zero friction, At the greatest value of x, the loss in gravitational potential energy should equal the loss in elastic potential energy. so I did (1/2)kx^2=mgx to isolate x in the formula, x=(2mg)/k then I plugged in my values so: (2*13.6*9.81)/8.8= 30.3218...
  31. RoboRaptor

    A Car on a Banked Curve Moving in Uniform Circular Motion

    First I figured out the normal force being exerted on the car using the equation above. Cos(40°)*(1050*9.8) = 7883N Next, I tried to find out the horizontal component of the normal force by doing: Cos(50) * 7883 = 5067N I figured out the angle by using certain geometrical properties. Next, I...
  32. A

    I Is Heart Rate Invariant in Different States of Motion?

    just something i think about, maybe it difficult to answer. i know, from study, that any observer moving along his world-line - in any state of motion - will not himself notice any difference to the rate which time passes for him. example: his heart-rate will feel normal (well, so long as he...
  33. John Mohr

    Constant Circular Motion Not Really Constant

    I was pondering my practice of talking about circular motion in the horizontal direction and the vertical direction. But I'd often would see in books, notes, and the internet that we assume constant velocity for the vertical case. However, when one thinks about the forces at different points in...
  34. hquang001

    Calculating Distance and Time for One-Dimensional Motion in Physics

    total time: t = 36 mins = 0.6h = t1 + t2 => t2 = t - t1 = 0.6h - 0.1h = 0.5h Vmax = a1 x t1 Vat C = Vmax + a2t2 substitute Vmax in Vat C we have : 0 = a1 x 0.1h + (-600 km/h²) x 0.5h => a1 = 3000km/h² Vmax = a1 x t1 = 3000 x 0.1 = 300km/h I check the result by: x1 = ½ a1 t1² = ½ . (3000)...
  35. Ibby321

    Vertical motion through an electric field

    I know that the answer is 0 J (no NET work was done) because there is symmetry to the problem and this symmetry comes from the fact that the direction of force changes, BUT I don’t know why the force changes (I have an idea; TBD below in #4). When I did this problem I thought I could find the...
  36. R

    Projectile Motion -- Help please understanding these basic problems

    1)Find the range of the gun when the muzzle speed is 400m/s and the elevation 24.5° - I tried to find " Time " via t= [v-u]/a by substituting it through cos--> t= ( 0- [400cos24.5] )/-10 and got [ 36.39 ]. Which I then substituted in S=ut---> S= ( 400cos24.5 ) x 36.39=13245.3. [ the answer...
  37. L

    Circular Motion Questions (energies, forces, angular velocities, etc.)

    Question 1: I believe that the ratio would be b. 8:1 because by combining the formula for kinetic energy and momentum the expression Ek=p^2/2m can be obtained. Thus, for a body of mass 2kg with twice the momentum: Ek=2^2/2*2=1 For a body of mass 4kg with half the momentum: Ek=1^2/2*4=1/8...
  38. T

    Change in the direction of motion of the particle

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  39. J

    Equation of motion of a chain with moving support

    In the figure assume the "ceiling" moves with motion ##Y(t)##, i.e. it is a point support. Applying Newton's law in the vertical direction ##T(y).\hat{y}=\rho y[g+\frac{d^{2}Y}{dt^{2}}]## If ##\theta## is the angle between ##T## and ##\hat{y}## that means ##|T|\cos\theta=\rho...
  40. V

    Circular motion of a particle around a track -- what provides the centripital acceleration?

    Suppose a particle is moving around a circular track of radius R at speed v. To bend around a circle some agency has to exert an acceleration towards the center of the circle. I analyze the forces acting on the particle, its weight and the normal force and there is no acceleration in the...
  41. E

    Kinematics: Analyzing circular motion of a particle

    Help please
  42. Matejxx1

    Describe the motion of yoyos suspended from the ceiling

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  43. E

    Classical The motion of cats and stuff in free fall

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  44. sergiokapone

    I What does motion mean in quantum mechanics?

    Consider the Schrödinger equation for a free particle: \begin{equation} -\frac{\hbar^2}{2m} \partial_i^2\psi = i\hbar\partial_t \psi. \end{equation} Let us be interested in the motion of a free particle in quantum mechanics. We say ok, we have a solution to the Schrödinger equation for a...
  45. J

    Wave problem - when they meet given their equations of motion

    So what I did was made the two equations equal each other. A lot of stuff cancels out and I end up with x=-vt. My issue is that t isn't given and I am not entirely sure how to get it. I don't think taking the partitial derivative of time will be any help nor the partial derivative of...
  46. guest948

    Force and motion -- Person falling from 20m onto a cushion (EASY)

    APPROACH 1 (correct): Height above ground = 3 - (v+u)/2 * t = 3 - 18.263/2 * 0.25 = 0.717 m ----------------------------------------------------- APPROACH 2 (incorrect): Let d be the height from the ground when he is stopped by the cushion. PE loss = Work done against motion by cushion mgh =...
  47. H

    Oscillation of a particle on a parabolic surface [equation of motion]

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  48. S

    Projectile Motion: Using an arrow to shoot a coconut dropped by a monkey

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  49. FEAnalyst

    Modeling impact forces in rotary motion

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  50. J

    Harmonic Motion Problem - Finding oscillation of charges in a circuit

    So since V(cap) + V(ind)=0 then Q/C + L dI/dt=0 Now since I=dQ/dt, I can replace dI/dt with d^2Q/dt^2 resulting in Q/C + L d^2Q/dt^2 =0 Now L d^2Q/dt^2 looks like a harmonic motion thing I can solve, where w^2=L. This means I can find w. I get 0.0005385. Now my issue is using this w gives the...
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