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. Like Tony Stark

    Pulley system with relative motion

    Well, first I tried to understand the relation between the velocities and accelerations of both bodies and I got that the velocity of ##B## is half the velocity of ##A##. This is because a change in length of the cord "that touches ##A##" must be equal to the change in length of the two cords...
  2. Like Tony Stark

    Spaceship approaching Mars and relative motion

    As the problem asks for the spaceship's perspective, I know that I should take ##\vec v_S=0## and ##\vec v_M=24.1-19.3## because the motion is relative to the spaceship. Then, the relative velocity of Mars and ##SM## should have the same direction. If they have the same direction, that angle...
  3. Like Tony Stark

    Relative rotational motion on a disc

    The first doubt that comes to my mind is "I have to determine the acceleration with respect to what?", because the problem doesn't tell. Then, I have some problems when having to plug the data in the formula of acceleration. ##\vec a_B=0## because the origin isn't accelerated, ##\vec{\dot...
  4. E

    Can Math Model Real-World Camera Focusing Dynamics?

    This problem arose in modeling camera focusing movement, such as a control system might do. It assumes a simple (thin) lens, rays close to the optical axis, and monochromatic light. While most camera lenses are not simple, this is a first approximation. Camera lenses project an image of a...
  5. W

    Lagrange Equations of Motion for a particle in a vessel

    The final answer should have a negative b^2⋅r(dot)^2⋅r term but I have no idea how that term would become negative. Also I know for a fact that my Lagrangian is correct.
  6. W

    Lagrange Equations of Motion for a particle in a vessel

    I start out by substituting rcos(Θ) and rsin(Θ) for x and y respectively. This gives me z=(b/2)r^2. The Lagrangian of this system is (1/2)m(rdot^2+r^2⋅Θdot^2+zdot^2)-mgz. (rdot and such is the time derivative of said variable). I then find the time derivative of z, giving me zdot=br⋅rdot and...
  7. jisbon

    Circular motion with kinematics

    My working: ##s=\int v## ##v= \sqrt{\frac{a_{c}}{r}}=\sqrt{\frac{a_{c}}{\frac{4}{2t+2}}}## ##s= \int_{0}^{2} \sqrt{\frac{2}{\frac{4}{2t+2}}}## My final answer seems to be wrong. Any ideas? Cheers
  8. warhammer

    Classical Physics & Particle Motion Under Force Field

    The particle is moving under a force field with the potential energy equation described above. I find it logical that Newton's Laws can be used as in the question itself it is stated that the velocity is quite small and we could approximate its subsequent motion via the notions of Classical...
  9. dRic2

    I Periodic electron motion in a perfect conductor using a semiclassical model

    According to the semiclassical approximation, in response to a constant electric field I would get a periodic motion of the electron, like this: The sinusoidal type function is the velocity, while the function that goes to infinity is the effective mass. Thus I was wondering, since ##v## also...
  10. Like Tony Stark

    ##\vec v## and ##\vec a## expressions - motion on an off center circle

    Well, I tried decomposing velocity into its components on the radial and angular axis. But I have problems with the angles because in some parts of the trajectory the velocity is on the angular coordinate, but in other parts it isn't. I mean, I can't say ##V=V e_\theta## because it's not always...
  11. jisbon

    Simple harmonic motion -- manipulating SHM equations

    My workings: ##D(t) = Asin\omega t## ##v(t) = \frac{\text{dD}}{\text{dt}}=Acos(\omega t)\omega## ##v(t) =Acos(\omega t)\omega## When displacement half of amplitude, ## Asin\omega t## = 0.5##A## ## sin\omega t## = 0.5 ##v(t) =Acos(\omega t)\omega## ##v(t) =\omega (0.5Asin\omega t)cos \omega t ##...
  12. Santilopez10

    Relative motion between cars with different types of movements

    I think my approach is quite wrong, still I gave it a shot: First I know that ##v_A=13.3 m/s=r\omega=60\omega \rightarrow \omega=0.2 \frac{rad}{s}## Then $$\vec a_A=-r\omega^2 e_r=-2.4 e_r$$ But ##e_r=\cos{\theta}i+\sin{\theta}j## and substituing the latter in the acceleration equation I have...
  13. Ranku

    B Understanding Virtual Particles: Speed and Rest Mass

    Do virtual particles travel at the speed of light, or do they have 'virtual rest mass'?
  14. Prabs3257

    Why is the acceleration not equal to mg sin(theta) in pure rolling motion?

    I know the ans comes out to be mgsintheeta/3 by using f=ma and the torque eqn but my question is as stated in the question the cylinder is in pure rolling hence friction should only try to oppose mgsintheeta so that the accelration does not change hence v remains equal to rw so why is the ans...
  15. jisbon

    Projectile motion (time taken)

    The below attempt will look messy, but I'm really kind of stuck as how to proceed on here. Thanks for any advice.
  16. PhysicS FAN

    The motion of 2 objects in contact

    For the non-circular object of mass m: From Newtons second law we get that F-N-T=ma where N is the force that the cylinder acts on the object. Replacing numbers: 13-N=2a. For the cylinder: From Στ=Τa(ang) we get that T'= 1/2ma or T'=a. Where T' is the friction that acts on the cylinder and we...
  17. callekula

    Question regarging pendulum axis and its motion

    Hi, I have a question regarding a pendulum and it's motion/momentum given the axis it moves around. In the below picture, there are two version of a pendulum with a weight at the bottom. It moves in and out of the screen, around an axis resting on two nails (red). My question is, how does the...
  18. Prabs3257

    Velocity of the center of mass in constrained motion

    I tried getting the velocity of a which by using constraint came out to be 3 and by using the eqn for vcom i got the ans as 3.5 i just want to confirm is this the correct answer
  19. jisbon

    Circular motion -- Find the angular velocity at t=3

    Hi everyone. Do correct me if I am thinking wrongly. So to find angular velocity, won't I just have to integrate angular acc = 2t, which means angular velocity = t^2? Hence, won't the answer be 3^2=9? The answer seems to be 5.43 :/ Thanks
  20. J

    Sinking Object Motion Equations

    Hi guys! I am currently learning about fluid dynamics, and I am stuck on a certain equation derivation. It's about sinking motion which considers only gravity force, buoyant force, and viscous resistance. The link attached has the details...
  21. ?

    I Predicting Motion of a Swing on a Non-Horizontal Branch

    A swing is suspended from a non-horizontal tree branch. Points C and D are fixed in space. All 4 line segments in the diagram have constant distance. After some initial "kick" imparts energy to the system the only force acting externally on the system is gravity. Is it possible to predict the...
  22. Sebastian_

    In a Projectile Motion Question, How to Find Theta?

    Solving the relevant equation tan-1(30/7) = 76.865977693604 = 77 Just need confirmation or a correction since I am very new to physics in general
  23. Prabs3257

    Laws of motion and a projectile thrown from an accelerating elevator car

    At the moment of throwing the ball the ball will have a vertical vel of 4 and horizontal of 3 wrt to elevator so as seen from inside the elevator the ball will behave like a projectile where g(eff) = 8 and with time period 1 sec hence range will be the displacement which should be 3 m but ans...
  24. karush

    MHB 243 parametric equations and motion direction

    11.1 Parametric equations and a parameter interval for the motion of a particle in the xy-plane given. Identify the paritcals path by finding a Cartestian equation for it $x=2\cos t, \quad 2 \sin t, \quad \pi\le t \le 2\pi$ (a) Identify the particles path by finding a Cartesian Equation the...
  25. adosar

    B Time Dilation & Relative Motion: Who Measures Proper Time?

    Consider the example of flashing light in spaceship. The observer in the spaceshipe and the observer in the Earth measure different times. Both observers could argue that the clock of the other tick slower(motion is relative). But is anyone right ? I mean the one will measure proper time...
  26. J

    Equation of motion of a mass on a 2d curve

    So ##T+U=\frac{1}{2}m(\dot{x}^{2}+\dot{y}^{2})-mgy=constant##. If I derive this with respect to ##t## $$\dot{x}\ddot{x}+\dot{y}\ddot{y}-g\dot{y}=0$$ Then I use ##\dot{y}=\dot{x}\frac{dy}{dx},\ddot{y}=\ddot{x}\frac{dy}{dx}+\dot{x}^{2}\frac{d^{2}y}{dx^{2}}## to get...
  27. Kaushik

    Does the block move along the pink dotted lines

    Does the block move along the pink dotted lines as attached in the figure below? I tried to draw the FBD of the small block ##m ## at the lowermost point which is also attached below.(The direction of ## v_0 ## is actually tangential) Is the figure above correct? If not, why?
  28. L

    Motion under Gravity: Displacement of 273.6m in 5sec

    u=5m/s t=5sec the height will be the displacement s=ut - (1/2)gt^2 s=5*5-0.5*10* 25 s=-100m But the answer is 273.6m
  29. J

    Unbanked Curve Motion: Friction vs Intuition

    This is just a conceptual question. I get that when a car is turning on an unbanked curve, the friction provides the centripetal force. I don't understand why this is though. I thought friction is supposed to oppose the direction of motion. But that would imply that the direction...
  30. R

    B Does the Earth "really" revolve round the Sun?

    I think this question is important so posting it here. To cut a long story short there is a person in our country who is totally anti-science and anti-Western citilization. He says science is a big lie. < remainder of post deleted by moderator >
  31. S

    Vertical circular motion with accelerating centre

    I can do the problem if the centre is fixed. The steps are: 1) Assuming tension in the string is zero at the top most position, we calculate the velocity at top most position by mv2/R = mg 2)Now, we simply apply mechanical energy conservation when the ball is at the top and bottom positions...
  32. G

    On the motion of a particle on the inner surface of a sphere

    Could anyone please help me out with the second part of this question: I've got the first part, u = √(5ga) Here's my diagram for the second part: Distance traveled is from bottom of sphere to peg is 2πa/3 which means angle traveled is 2π/3. So the particlee is going to travel 2π/3 radians...
  33. J

    Inclined Plane, Centripetal motion: Find the Velocity that suits the Turn

    So here is what is going on in my mind: We have a turn that is 400m away from the center of the turn. The faster the car goes, the harder it is for it to maintain its radius. We have a component of the normal force that points towards the center, and static friction which does the same. I...
  34. Y

    Deriving Bernoulli's Equation Using Euler's Law of Motion

    Hi all, I have attached an image of a page out of the book I am using for context. The blue arrow in Figure 12-3 describes the motion of the particle. I figured the net force would need to be in the same direction, but apparently the net force opposes the motion. So, in Figure 12-3 the pressure...
  35. Alfredo Tifi

    B Is Mach's Principle the Key to Understanding Rotatory Motion?

    I refer to the second paragraph of 1916's book, "Die Grundlage der allgemeinen Relativitätstheorie", translated here. First issue There are two distant stellar bodies, with unchanging shapes: S₁ (spherical) and S₂ (ellipsoidal), made of the same amount and kind of matter. Their centres of mass...
  36. O

    What are the top toys that can run almost perpetually?

    Perpetual motion is fundamentally impossible. But almost perpetual motion is possible. The list of toys or devices designed to run for a very long time is short: crookes radiometer (photons, cheap), the drinking bird (heat engine, cheap), the Atmos clock (temperature, expensive), Beverly Clock...
  37. M

    Projectile motion over an inclined plane

    I have attached below the solution provided by the textbook. I understand the whole solution, except when they divide the horizontal component and the vertical component and equate it to the tangent of 30. What seems strange to me is that, while initially the equation for the vertical component...
  38. D

    Question about a Pendulum's motion

    It was always my understanding that a Pendulum has equal time at different swing heights and teachers teach that but in fact, it is not true. I downloaded the LabinApp Pendulum Amplitude Demo App and it shows a slightly different time as you drop higher and higher. My question is why don't they...
  39. J

    Motion dropping a stone: Time question

    what I know d=35.75 t=? what am I meant to do with this problem? I'll add more to this area once I know some more. thanks. Is the leafs velocity considered inital? if so I know Vi
  40. J

    Motion Q: Dropping/Throwing Stone in Water-Vi, Vf, a, t, d

    What I know for Number 1. t=2.7s d=? Vi=0m/s^-1 a=9.8m/s^-2 Vf=? Equation to use? Vf=Vi+at= 0+9.8m/s^-2x2.3= 26.46m/s So for number one the final velocity is 26.46m/s d = 0.5 * g * t2 = 0.5x9.8x2.7^2=35.721 Number 2 I know t=2.3s d=35.721 vi? Vf? a=9.8? what formula do...
  41. Physics lover

    Motion of a particle in a uniform magnetic field

    All i could think is that the z component of velocity will remain unchnged as there is no force in that direction.And for the x and y component can we imagine the helical motion as a superposition of a circle and a straight line.So for x and y component can we solve for a particle moving in a...
  42. EEristavi

    Rotational Motion / Static Equilibrium - Mechanics

    I have a solution, However Cant understand 1 point.Now, This is the solution: ##N_2 l cos\theta + \frac 1 2 F_g l cos\theta - f_2 l sin\theta = 0## ## N_2(1 - \mu tan\theta) + \frac 1 2 F_g = 0## This is the the point that I don't like - yes it is less that 0, but it's even less that...
  43. A

    Motion Along a Straight Line: Calculate velocity of a stream

    Not sure how to go about it
  44. humancentered666

    What Exactly Does Equation (2) Mean? (Equations of Motion from PE function)

    What exactly is this equation telling me? How can I use it to work out the Equations of Motion given an equation of potential energy? Won't I have to solve a PDE? I'm extremely sorry if this question comes off ignorant.
  45. Jason Louison

    Straight-Line Vehicle Motion problem

    Summary: Impossible System? Hello, I am trying to simulate the dynamics of a vehicle accelerating from a standstill to top speed. The vehicle acceleration equation is: Av = (Ft+Ff)/m Where Ft = Traction Force, Ff = Friction Force (From Drag and Rolling Resistance), and m = Vehicle Mass My...
  46. M

    B Can Canceling Orbital Motion Improve Rocket Efficiency?

    Would accelerating a rocket in a vector that "cancels" Earth's orbital motion relative to the center of the milky way in some cases improve the energy efficiency of a rocket for a given amount of change in velocity relative to the earth?
  47. S

    Velocity, acceleration, jerk, snap, crackle, pop, stop, drop, roll....

    Edit: I see this was discussed in the related thread sorry for a repost. If acceleration causes a change in velocity, and jerk causes a change in acceleration, snap causes a change in jerk, crackle causes a change in snap, pop causes a change in crackle, stop causes a change in pop, drop causes...
  48. EEristavi

    Energy Conservation in Angular motion / Moment of Inertia

    I write Conservation of Energy: Potential Energy loss(change): U = m g ##\Delta##h = m g (R+r) (1-cos##\alpha##) kinetic Energy gain(change): K = (##\frac {m v^2} 2## + ##\frac {I \omega^2} 2##) + (##\frac {M v_2^2} 2## + ##\frac {I_2 \omega_2^2} 2##) U = K m g (R+r) (1-cos##\alpha##) =...
  49. poseidon721

    Motion of a rod hinged at one end

  50. M

    Calculate the tension in a rope attached to a ball in circular motion

    The solution to the problem simply states: "Use of mv^2/r = 2000. T = (2000 + 7500) = 9500N". I don't understand this solution. Nothing more is provided. I don't know how you are supposed to find the radius (in order to use the centripetal force formula) merely from the information provided...
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