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. Z

    How is Constant speed of light referenced

    The speed of light is constant but with reference to what? Distances are not absolute and neither is time. Is it that all inertial observers agree upon the ratio of the distance to the time that it takes for light to go between the two ends of some object even though they don’t necessarily...
  2. K

    Fallin with constant speed what is the force of air resistnace

    Homework Statement A 1.26 samara-the winged fruit of a maple tree-falls toward the ground with a constant speed of 1.1 What is the force of air resistance exerted on the samara? Homework Equations F=ma I thought that the force of air resistance would have to be equal to mg The...
  3. S

    Explaining the Constant Speed of Light in Special Relativity: A Beginner's Guide

    I apologize if this has been covered but I did a quick search and didn't see anything. Alright, my question concerns the speed of light and why it's a constant. I was trying to explain special relativity to my friend and after I told him that the speed of light is always c no matter what...
  4. P

    Determine the Altitude of a Satellite above Earth Given it's Constant Speed

    Homework Statement A remote-sensing satellite travels in a circular orbit at a constant speed of 8.2 x 10^3 m/s. Determing the altitude in kilometres of the satellite above Earth's surface. Homework Equations Fc = FG The Attempt at a Solution mv^2/r = GMm/r^2 v^2 = GM/r r =...
  5. O

    Varying timing of eclipses of Algol and constant speed of light?

    Take a look... http://calgary.rasc.ca/algol_minima.htm Now then, if light always passes us at the same speed then what is causing the decrease in time between eclipses when we are moving towards Algol and an increase in time between eclipses when we are moving away from Algol?
  6. J

    Exploring the Constant Speed of Light in a Moving Spacecraft

    I know there is an answer to this somewhere, so please forgive me for not being able to see it :) When people say that light will always be measured at a constant speed, no matter where you are, what do they mean? Say we are in a spaceship which is standing still. Inside the spaceship, we...
  7. T

    How many rocks/min to mantain a constant speed

    This is the problem i am trying to solve: A child sits in a wagon with a pile of 0.60-kg rocks. If she can throw each rock with a speed of 10 m/s relative to the ground, causing the wagon to move, how many rocks must she throw per minute to maintain a constant average speed against a 2.0-N...
  8. S

    A train traveling at a constant speed

    Homework Statement A train traveling at a constant speed rounds a curve of radius 200 m. A chandelier suspended from the ceiling swings out to an angle of 17.0° throughout the turn. What is the speed of the train? I am confused about the orientation of the chandelier to the turn and how...
  9. F

    How can simultaneity and the constant speed of light stay coexist.

    Watch this video and tell me shouldn't the passenger in the train see both lightning strikes at the same time considering that light speed is perceived as the same speed no matter how fast you travel or where it comes from? Watch both these vids...
  10. C

    What is the velocity of raindrops in two reference frames?

    A car travels due east with a speed of 50.0 km/h. Raindrops are falling at a constant speed vertically with respect to the Earth. The traces of the rain on the side windows of the car make an angle of 45.0° with the vertical. Find the velocity of the rain with respect to the following reference...
  11. B

    Crates with diff. mass inside elevator at constant speed

    Homework Statement Crates A and B are in an elevator, with Crate A on top of Crate B. The mass of crate A is greater than the mass of crate B. The elevator moves downward at constant speed. Compare the accelerations of Crates A and B. I'm tempted to say there are equal, and both equal to...
  12. Y

    Dynamic equilibrium - walking in constant speed

    Homework Statement A man is walking in a straight line in a constant speed V. All variables are known. Since the speed in constant, the net force is zero. But aside from the static friction which is allowing him to move forward, what other force is present (in the direction of the movement)...
  13. B

    General motion with constant speed

    Homework Statement A particle moves along any path in 3d space with constant speed. Show that its velocity and accelerations vectors must always be perpendicular to each other.[hint. Differentiate the formula v dot v = v^2 with respect to t. Homework Equations possible equations...
  14. S

    Car travelling at constant speed (embarassed to ask)

    Hello, If a car provided 10 N force and friction force is 10 N wouldn't the forces cancel out and be zero and nothing happens. If so why is that we say car travels at constant speed. If you can explain this to be I'll be very greatful. Thanks!
  15. L

    Calculating 'Push' Force to Maintain Constant Speed

    Homework Statement A conducting rail in contact with conducting wires, oriented perpendicular to both wires, is pushed with constant speed, causing an induced current of 0.69 A. B = 0.50 T and R (Resistor) = 2.0 Calculate the "push" force necessary to maintain the rail's constant speed...
  16. T

    Objects will travel at a constant speed towards the earth no matter

    When releasing two different objects at the same time, the objects will travel at a constant speed towards the Earth no matter the mass. I have a hard time believing this. If I were to release a brick and a feather at the same time, the brick will reach the ground before the feather. I know...
  17. S

    Calculating Work to Pull a Skier Up a Slope at Constant Speed

    Homework Statement A skier of mass 69.5 kg is pulled up a slope by a motor-driven cable. a) How much work is required to pull him a distance of 59.1 m up a 30.1° slope (assumed frictionless) at a constant speed of 2.07 m/s? The Attempt at a Solution I started out this problem by finding...
  18. D

    Mass pushed by horizontal force at constant speed on an incline

    3. A 52.3-kg trunk is pushed 5.95m at constant speed up a 28.0 degree incline by a constant horizontal force. The coefficient of kinetic friction between the trunk and the incline is .19 . Calculate the work done by a) the applied force and b) the force of gravity. Fy = N - mgcos28 = 0 Fx =...
  19. B

    How Many Rocks Must a Child Throw to Maintain Constant Speed in a Wagon?

    Remain at constant speed? A child sits in a wagon with a pile of 0.47 kg rocks. If she can throw each rock with a speed of 9 m/s relative to the ground, causing the wagon to move, how many rocks must she throw per minute to maintain a constant average speed against a 3.5 N force of friction...
  20. D

    Force, constant speed, friction, acceleration

    I would really appreciate any help on this. I'm confused with this. Homework Statement An empty crate is sliding witha speed of 7m/s over a frictionless surface when it hits a rough patch where .25 is the coefficient of kinetic friction. Find the distance it takes to stop the box. The...
  21. D

    How hard must you push a box to move it with a constant speed?

    Homework Statement How hard do you need to push a 2kg box which has .2 coefficient of kinetic friction so that it moves with a constant speed of 5m/s? [The Attempt at a Solution (I'm calling g 10m/s2 not 9.8) F(kinetic friction)=(.2)Fn=(.2)mg=(.2)(20N)=4N so...since the acceleration is...
  22. D

    What Is the Coefficient of Kinetic Friction for a Box Pushed at Constant Speed?

    Okay, here's the question: A box full of physics books (31 kg) is pushed across the floor at a constant speed by a force of F = 85 N at an angle theta = -40 degrees. Determine the coefficient of kinetic friction between the box and floor. My question is (for later calculation) if the box...
  23. DocZaius

    Concept of the constant speed of light

    I am having trouble wrapping my head around the concept of the constant speed of light. Imagine I am moving in a ship at a certain speed (let's say 1/4 the speed of light). I send two probes out at the same speed, one directly ahead of me, one directly behind me. They are set to activate a...
  24. michael879

    Unravelling the Mystery of Light's Constant Speed: Challenges and Proofs

    I get the theory of special relativity, it is the logical conclusion drawn from the two facts that: a) the laws of physics are the same in all reference frames b) the speed of light is constant in all reference frames what I don't get is why einstein thought the speed of light was constant...
  25. M

    Understanding Potential Energy Change in Constant Speed Motion

    Hi, Now, we know that the work done on a system equals the change of energy of that system, amd if there is no work, then the energy change must be zero. Now, suppose if I hold a stone by a string and let it drop while still holidng it so that it falls at a constant speed to the ground. My...
  26. V

    Constant speed of light in vaccum?

    Just find something very interesting.. If the speed of light is constant, at some distance from the black hole where light can neither escape the attraction of the black hole, nor succumb to its gravitation, what state is the light in? does it have a speed there? ...confused..
  27. T

    Coefficient of Friction at constant speed

    Hello, please help me out on this: Two equall masses lie on each side of a rough wedge, connected by a string, the wedge makes anlges 53, and 47, to the horizontal, find the coefficient of friction for the masses to move at a constant speed. MY THINKING: since masses are equal: mgsin(53) =...
  28. P

    Exploring the Constant Speed of Light

    As there are no stupid questions ... Do we have an idea why speed of light is constant ? Is it because a photon has no mass because it is impossible to apply acceleration to something that has no mass. Isn't time in itself absolute but observation of time relative because we have to take...
  29. G

    Constant Speed for a Sliding Wood Box

    A 2.0 kg wood box slides down a vertical wood wall while you push on it at a 45 degree angle. What magnitude of force should you apply to cause the box to slide down at a constant speed? (Problem 51, Chapter 5, Knight) I can't figure out this problem and do not have the text to refer to...
  30. A

    The Constant Speed of Light: Investigating its Properties

    The speed of light is not always the same but the speed of light does not alter regardless of the speed of the source of the illumination?
  31. P

    Questioning the Logic of Constant Speed Around a Roundabout

    I am sure i am missing somthing here so if somone could point out the mistake in my reasoning it would be appreciated. If say you are traveling in a car around a roundabout you must contine to put more power into the engine to remain at a constant speed e.g. 20 mph. When you reach the maximum...
  32. D

    Acceleration and constant speed question

    Can someone tell me which of the these is true or false and possibly explain it to me? a)The acceleration of a car is zero when it is turning right at a constant speed b)The acceleration of a car is zero when it is driving up a long straight incline at constant speed c)The acceleration of a...
  33. G

    Sliding Box: Constant Speed or Increasing?

    Let say we have a sliding box. The box is sliding because of an constant force perpendicular to the box. There is no friction and no air restistance. Does the speed of the box increase or is it constant. ?
  34. B

    Doesn't light's constant speed dictate an 'ether'?

    firstly, I'm an ignoramus secondly, doesn't the fact that lights speed cannot be affected by the speed of a body that emits it suggest the existence of an ether? sound waves wouldn't exist without the 'ether' of air and so you can say that the air is the only thing that slows down sound...
  35. P

    Towing a 25.0 kg suitcase at constant speed with handle at an angle

    A woman at an airport is towing her 25.0 kg suitcase at constant speed by pulling on a strap at an angle of above the horizontal (Fig. P5.6). She pulls on the strap with a 30.0 N force, and the friction force on the suitcase is 20.0 N what is the angle she pulls the suitcase with
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