Constant speed Definition and 135 Threads

In everyday use and in kinematics, the speed (commonly referred to as v) of an object is the magnitude of the rate of change of its position with time or the magnitude of the change of its position per unit of time; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero.
Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph). For air and marine travel the knot is commonly used.
The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in a vacuum c = 299792458 metres per second (approximately 1079000000 km/h or 671000000 mph). Matter cannot quite reach the speed of light, as this would require an infinite amount of energy. In relativity physics, the concept of rapidity replaces the classical idea of speed.

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  1. chwala

    Find the Tension in the cable when the lift is moving at constant speed

    aaaah just realized the solution after typing here... ... at constant speed, ##a=0##, therefore ##T-mg = 0## ##T=(1050 × 10) =10, 500## Newtons or any insight...welcome. Maybe i should ask...when the lift is moving downwards and there is deceleration then would the Tension be treated in...
  2. G

    B Why is speed constant when centripetal acceleration is constant?

    It's actually getting little boring and makes me angry why all the videos/articles show centripetal acceleration formula and presume that speed is constant. I want to prove backwards, i.e we know the constant perpendicular force acts on an object from the center and why object starts to move in...
  3. I

    I Doppler Effect on Light: Constant Speed of Light Confusion

    If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed? Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy) :oldconfused:
  4. V

    I Michelson-Morley: Errors in Calculations?

    I would like to know the opinions on the paper that I saw about Michelson-Morley experiment. Michelson-Morley experiment was done in 1887 and had an impact on the future direction of physics. It is taught in schools as an experiment that proves the non-existence of the aether and proves the...
  5. G

    B Constant Speed of Light: Low Speed Explained

    I understand the relativistic effects at high speeds, what accounts for the constant speed of light at low speeds? Example... If I travel towards a beam of light at 25 MPH, I will still measure the light as traveling towards me at the speed of light and not the speed of light plus 25 MPH.
  6. Ineedhelpwithphysics

    Which object is moving to the right with a constant speed?

    I need help with this question how in the world is it A and C. In A everything is cancelling out so nothing is movingfand nothing moving to the right. In C its not moving to the right. Isnt it suppose to be D since sits going right constant to 20N.
  7. D

    Calculating Hot Air Balloon Volume & Lift

    Volume of hot air ballon V=((4/3 pi R^3)/2) + (1/3 pi h (R^2 + r^2 + Rh) = 2956.24 m3 Balloon: R=9m h=15m r=1m m = 750 kg H = 5000m T = 373 K p1 = 101300 Pa p2 = 50650 Pa M(air) = 0.029 kg/mol F = mg - 7350 N
  8. A

    MHB Finding Distance with Constant Speed: Solving for m and b

    John is walking at a constant speed in front of a motion sensor. After 1 s, she is 2.5m from the sensor, 2 s later, she is 4 m from the sensor. a) Find an equation of in the form d=mt+b b) Determine the slope and d intercept and explain what they mean c) How far will John be from the sensor 5s...
  9. J

    Tension in a rope holding a mass under a helicopter rising at constant speed

    Helicopter lift weight of 500kg ,weight is connect with load cell to meassure tension. Neglect aerodynamic drag.. If helicopter accelarate up ,tension is rope is greater than 500kg. If helicpter accelarate down ,tension is rope is less than 500kg. If helicopter fly up with constant speed...
  10. S

    Force to make 2 blocks move relative to each other at constant speed

    Let: Smaller block = m1 = 1 kg Large block = m2 = 2 kg Coefficient of friction between the two blocks = μ1 = 0.2 Coefficient of friction between larger block and floor = μ2 = 0.3 Tension connecting two blocks through two pulleys = T Angle between tension and horizontal = θ = 37o Friction between...
  11. thaiqi

    I What is the meaning of constant speed of light?

    Which one does it mean: "phase velocity" or "group velocity" or "speed of the wave front"? In the postulate of constant speed of light .
  12. R

    B Constant speed between 2 objects close to the speed of light

    If i am moving away from an object at a certain constant speed close to the speed of light, is that object also moving away from me at the same constant speed?
  13. R

    Elliptical motion: An object is moving at a constant speed?

    I am confused why the acceleration doesn't point to the center of the ellipse or one of the focus, since it moves in circular motion. Shouldn't the acceleration be just in the radial direction
  14. N

    How to show that P moves with a constant speed u

    For particle Q: The resultant force on particle Q would be zero since it is at rest. Thus$$ T - Mg = 0$$ which gives $$T = Mg$$ For particle P: This is where I am struggling. I can't seem to write out the polar equations of motion. I have to show that $$u^2 = (Mgb)/m$$ I know that $$\vec a =...
  15. G

    B Accounting for the constant speed of light

    I keep hearing that the relative speed of light remains constant because time and lengths change with speed, what I believe is called the Lorentz factor. At slow speeds the Lorentz factor is extremely small, so what do people believe accounts for the rest of the change? If I travel...
  16. N

    How is static torque calculated for constant speed?

    The way I have understood it is that dynamic torque involves angular acceleration, while static torque has zero angular acceleration: α =Δω/Δt = 0 But static torque still has centripetal acceleration with constant speed: ac = v2/r where v is linear velocity, and r is the radius Then F = mac...
  17. I

    A particle P2 chases particle P1 with constant speed.

    Homework Statement [/B] A particle P1 moves with a constant velocity v along x-axis, starting from origin. Another particle P2 chases particle P1 with constant speed v, starting from the point (0, d). Both motion begin simultaneously. Find - 1. Initial acceleration of P2. 2. Ultimate...
  18. HappyFlower

    A plane, diving with constant speed at an angle of 53.0 degrees....

    Homework Statement A plane, diving with constant speed at an angle of 53 degrees with the vertical, releases a projectile an altitude of 730m. The projectile hits the ground 5.00s after release. A plane, diving with constant speed at an angle of 53 degrees with the vertical, releases a...
  19. M

    Is there work for object moving at a constant speed?

    Homework Statement An object of mass 100kg is acted on by a constant force as it moves through a displacement at a constant speed of 7.2m/s. What is the net work done on the object? Homework Equations Wnet = ΔK Fnet = 0 The Attempt at a Solution I know that the answer is zero because the...
  20. N

    B Would All Observers See a Warp Drive Spaceship at Constant Speed?

    If you were in a warp drive spaceship traveling at say 2c. Would all observers see your ship traveling at 2c?
  21. G

    A Assumptions behind Maxwell's equations for constant speed

    I need some help in defining what are the assumptions needed to derive a constant speed of light from Maxwell equations. Is it correct to say that this result applies to a sinusoidal wave as an assumption? In my understanding that is (more or less) equivalent to planar waves in vacuum: is it...
  22. annanause

    Muscle tension required to lift a child at a constant speed

    Homework Statement When a mother lifts he child with her foot (as shown in figure) what force should the upper muscle exert to lift her child at a constant speed? Homework Equations W = F x D f = mg I think these might be the right equations to use, correct me if I'm wrong. The Attempt...
  23. F

    I Does Noether theorem explain the constant speed of light?

    I learned in Analytical Mechanics: "Emmy Noether's theorem shows that every conserved quantity is due to a symmetry". The examples I learned where conservation of energy as symmetry in time and conservation of momentum as symmetries in space. Now I wonder, do universal constants are also due to...
  24. B

    A person walks on a rotating platform at constant speed

    Homework Statement A large flat horizontal platform rotates at a constant angular speed ω. A person on the platform walks in a circular path of radius R0 centered on the axis of the platform with a constant linear speed v relative to the platform’s surface. The coefficient of friction between...
  25. J

    Showing particle travels at constant speed (geometry)

    Homework Statement the trajectory γ: ℝ→ℝ3 of a charged particle moving in a uniform magnetic field satisfies the differential equation γ''= B x γ'(t) . where B = (B1, B2, B3) is a constant 3-vector describing the magnetic field, and × denotes the vector product. (a) Show that the particle...
  26. Zaephou

    Stopping acceleration and starting to move at constant speed

    I have an example: Let us say you are on a large object with a mass of 5x10^10kg accelerating upwards at a constant velocity of 5m/s^-2, and there is a ball 0.1kg on top of that object. This ball should be accelerating at the same pace. The ball is then held up 5m from the surface of that...
  27. Lars Krogh-Stea

    Acellerating car vs constant speed car

    Hi, I've been dsicussing this with som friends, and would be thankful if someone took the time to explain it to me :) The scenario: Car A travels at constant velocity (80km/h) along the road and hits a brick wall. Car B acellerates upward towards 100km/h, and at excactly the moment it passes...
  28. Quandry

    B Can I get clarification on the constant speed of light

    I do not have a problem with the concept of the constant speed of light as it has no mass and therefore no inertia and therefore no relationship to any IFR. However it seems to be expressed as constant in all IFR's which I do not understand. This seems to say that if I am traveling at 1/2c and I...
  29. A

    I Stanford: objects in spacetime all move at constant speed c?

    In this http://web.stanford.edu/~oas/SI/SRGR/notes/SRGRLect6_2007.pdf, it is stated: Likewise, objects in spacetime all move at constant speed c in spacetime but if you change its direction, say by moving at speed v in the x direction, then spatial speed will change and so will the speed along...
  30. Isabella Wang

    Car Driving At A Constant Speed

    Hey guys, This might sound like a simple question, but I'm just starting to look in depth into Einstein's theories. I'm confused on the question about a car driving at a constant speed o 65 mph northwards. If it changes it direction to drive northeast, it then loses some speed. How is this...
  31. Maka42

    Velocity change due to load dropped vertically onto trolley

    A trolley is moving at a constant speed down a track, with no net force acting upon it. A heavy load is dropped vertically on top of the moving trolley. What happens to the trolley's speed? a) It stays the same, b) It decreases, c) It becomes zero, d) It is impossible to say, e) It increases...
  32. Guidestone

    Understanding Resultant Forces: Exploring Constant Speed and Zero Acceleration

    So force equals mass times acceleration. What happens then if we pushed a box horizontally without fiction at constant speed? The box keeps moving but acceleration equals zero? Is there still force? Thank you!
  33. Gaz

    Exploring the Constant Speed of Light in a Thought Experiment

    Hi I want to create a thought experiment involving waves of any kind. I am going to create a thought experiment I am on a boat that is at rest in the ocean. I am standing on the boat measuring the size of the waves and the frequency. So I measure the wave along the side of my boat to have a...
  34. J

    Power needed to keep conveyor belt moving at constant speed

    1. The problem statement, all variables and given/known Homework Equations P=Fv K.E = 1/2 mv2 The Attempt at a Solution This is the examiner report: Why infinite acceleration? Is it because the mass is too small?
  35. S

    A powered toy car moving at a constant speed

    Okay, so please help settle a dispute. A toy car is moving at a constant speed of 1 m/s. The car is powered by a battery. Now, when the car is going at a constant speed, there is obviously no net force on the car. The dispute comes when trying to draw a FBD. Is there friction (traction)...
  36. B

    What is the acceleration of a car at constant speed?

    If a car is traveling at a constant speed of say, 60MPH, is the acceleration of the car 0, or is it equal to the drag and rolling resistance it's facing?
  37. S

    Reconciling Constant Speed of Light & Relativity

    If the speed of light is constant regardless of the state of motion of the source, then doesn't this imply that it is possible to calculate the velocity of a reference frame by measuring the time it takes for light to traverse some known distance in that frame. For instance if our frame is...
  38. J

    Book sandwiched between your hand and table

    Hi, I have been working on this problem for a while: You place a book flat on a table and press down on it with your right hand. The hand-to-book and table-to-book coefficients of kinetic friction are 0.48 and 0.43 respectively. The book's mass is 1.14 kg and your downward push on it is 9.70 N...
  39. S

    Spaceship constant speed problem

    Homework Statement The proper length of a spaceship A is 60 m and the proper length of spaceship B is 120 m. The proper mass of spaceship A is 15000 kg. An observer on Earth watches the two spaceships fly past at a constant speed and determines that they have the same length. If the speed on...
  40. M

    Clarification on force on frictional surface constant speed

    for the following 2 examples from: http://www.physicsclassroom.com/class/energy/Lesson-1/Calculating-the-Amount-of-Work-Done-by-Forces A 10-N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right. Would it be more accurate to...
  41. A

    Box sliding down wall at constant speed

    Homework Statement The 2.0 kg wood box in the figure slides down a vertical wood wall while you push on it at a 45 ∘angle. https://session.masteringphysics.com/problemAsset/1070374/4/06.P51.jpg Homework Equations ΣFY = FPY - mg The Attempt at a Solution I know that since there is a constant...
  42. R

    An object moves with constant speed

    Homework Statement What relationship exists between the velocity and acceleration vectors? Prove your claim Homework Equations Since speed = |v| = constant, acceleration_tangential = d|v|/dt = 0. My question is on the "prove your claim part" Can I see simply just say let v(t) = a*i +...
  43. M

    Centripetal Force and Constant speed

    I'm confused about how it is possible for speed to be constant when a centripetal force is being applied to an object. If something is orbiting a planet, when it is at what one person considers the top of the planet, all of the objects velocity is in the horizontal direction, and all of the...
  44. R

    Ball Thrown Up: Constant Speed Despite Height

    When a ball is thrown up straight overhead. It just follows the curved space with the speed it is being released and seems as if the ball gradually slows down to zero at the top most and return to earth. If I'm right, according to Newtonian physics, the ball speed is zero at the top. And...
  45. J

    Calculating Torque for Rotating Disk: Specs, Friction, and Weight Considerations

    Hi all, I am trying to calculate the torque required by the motor to rotate a disk at a constant speed. Specs are as follows: 1. Disk radius =80mm 2. Disk Thickness =2 mm 3. Disk Weight =50gm 4. Constant speed= 3000rpm(CONSTANT) 5. Motor friction torque= 1mNm 6. Disk position parallel...
  46. E

    What is the magnitude of acceleration for a block released up an inclined plane?

    Homework Statement 1st example: A 10 kg block is pushed up a ramp at a constant speed of 5.7 m/s. The ramp makes an angle of 30 degrees to the horizontal and the ramp is frictionless. The block is released and allowed to slide up the ramp until it stops. What is the magnitude of the...
  47. L

    Calculating Car Mass with Constant Speed on a Bridge

    If a car is traveling towards a bridge at a constant speed of 20ms-1. The bridge takes 30s to cross entirely. The car creates a force of 3000N. Calculate the mass of the car when it is on the bridge. a=(v2-v1)/t F=ma I figured that a = 0 due to the fact that the car is traveling at...
  48. C

    Implications of constant speed of light in all inertial frames

    Hello, Lately I have been wondering about the implications of the speed of light being constant for all observers, to me this seems to imply that there are different versions of reality for different observers. I will use a scenario to illustrate my reasoning. After you read this I would like...
  49. U

    Can I go up with a constant speed?

    I'm trying to think about ascending in the atmosphere with constant speed, that means I will have no acceleration and that means that the net force on me will be zero, BUT, there is always gravity pulling me upward so i must have some force that's pushing me upwards, thus I must be going up with...
  50. T

    What force is needed to produce a constant speed?

    Homework Statement An empty wagon with mass 10 kg is pulled by a force of 75 N at a constant speed. Rain falls collecting in the wagon at a rate of 375mm^3/s. The density of water is 1g/mm^3. A) Make a table showing how much force must be exerted on the wagon to keep it moving at a constant...
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