# What is Relative speed: Definition and 57 Discussions

The relative velocity

v

B

A

{\displaystyle {\vec {v}}_{B\mid A}}
(also

v

B
A

{\displaystyle {\vec {v}}_{BA}}
or

v

B
rel

A

{\displaystyle {\vec {v}}_{B\operatorname {rel} A}}
) is the velocity of an object or observer B in the rest frame of another object or observer A.

View More On Wikipedia.org
1. ### B Time Dilation in Non-Stationary Reference Frame: A, B, C

Let's consider three observers A, B and C. The experiment starts at t = 0. A is 'absolutely' stationary. B immediately (please imagine it) starts moving at speed v1 with respect to A. C immediately starts moving at speed v2 with respect to A in the same direction as B. Let's say A measures a...
2. ### I Relative Speed of Light Between Objects

This is one of my thought experiments where I am drawing a big blank, If you have 2 objects approaching a 3rd object from opposite directions (just enough off to avoid collision) at 75% of the speed of light, the first assumption is that each observing the other would see the other object...
3. ### B Relative speed of two oppositely directed light beams

Can someone give a meaningful explanation that the relative speed of two oppositely directed light beams is why only one light speeds? I understand that based on the Einstein relativity theory, the relative speed of two beams is C, because nothing can be quicker than light speed. However it is...
4. ### B Can an Object Exceed the Speed of Light?

I was curious if the relative speed of an object can exceed the speed of light. Specifically, I am curious about the following thought experiment. I am not a physicists (and if I were asking the following would make me a poor one) and it has been 20 years since college physics. If a vessel is...
5. ### I Relative Velocity of Particle ##p##: S vs S

A particle ##p## is moving with a velocity ##u\hat i## with respect to S ... The velocity of ##p## with respect to S' is then ##-c \hat i##... Another part... Similarly taking the velocity of particle ##p## with respect to S' as ##-c\hat i## the velocity of ##p## with respect to S is then...
6. ### Calculating Relative Speed & Time to Meet: I Can't Find the Distance

I able to calculate this 2 things: relative speed of A and B = 3 + 1; 4m/sec Time for meeting for first time in opposite direction = 100/(3+1); 25 sec I can't able to find distance in Meeting for first time for opposite direction.
7. ### Calculate rest mass given relative speed and kinetic energy?

The answer is 3.079*10^-26 kg I tried doing E=mc^2 20*10^9*1.602*10^-19=m*(3*10^8)^2 m=3.56*10^-26 kg However when I do E=γmc^2 I get the correct answer. Is the question wrong, because isn't it 20 GeV of kinetic energy, not total energy?
8. ### Relative speed of two photons

Homework Statement What is the speed of a photon with respect to another photon if: the two photons are going in the same direction. they are going in opposite direction? 2. The attempt at a solution I think the answer to the first question should be zero and to the second one be 2xC; C⇒speed...
9. ### B Relative speed and time dilation

1. If A is static and B is moving away from A at velocity v, from Einsteinian point of view, is A also moving away from B at the same velocity? If yes, is the similarity of movements and velocities of both A and B valid in every context? If B is experiencing a time dilation from the view point...
10. ### Relative speed when someone runs on the treadmill

We are using VR technology for people with spinal cord injury ( incomplete so they can walk with assistance) for a study. They will be walking on a treadmill wearing an oculus. We have a question regarding relative speed since we are updating the software view based on that if person A runs at 5...
11. ### I Relative Speed Impact on Synchronization: Clock Comparison

If two clocks start from the same point, and one travels double the speed of another, will they be showing the same time when they get together at the destination point. Obviously, faster clock will arrive first, and will have to wait for the slower one to arrive.
12. ### B Relative Speed of 2 Beam Lights: Is it C or Zero?

here "c" is the speed of light , i have a question in my mind and i have no answer for it , if two light beams are moving in opposite directions then their relative speed is simply c using the law of velocities addition in case of special relativity but what if those two light beams are moving...
13. ### B Relative speed between Galaxies

My question is following: Is there another Galaxy in the Universe that has a relative speed to the Milky Way of 0.999c (near of light speed)? The implication of that is that our life 'observed' by extraterrestrial beings in that Galaxy might be millions of years, an eternity! This is because of...
14. ### Relative speed : Two trains crossing each other

Two trains having lengths of 120 m and 100 m are running in the opposite directions with velocities 40 km/h and 50 km/h . In what time they will completely cross each other? In the solution it's given that Time taken by the two trains to cross each other = sum of length of trains/relative speed...
15. ### B Is the mass of a photon always negligible compared to that of a proton?

Dear PF Forum, Yesterday (on Sunday) my friend texted me, "the mass of a photon is 10-18 Ev/c2## If my number is correct. And I also I check it in Wiki, and it's true. Now what I want to ask is this first. I Understand, my wording is confusing. Now, I'll rearrange my question for a new question...
16. ### Conservation of momentum (relative speed)

Homework Statement Consider a gun of mass M (when unloaded) that fires a shell of mass m with muzzle speed v. (That is, the shell's speed relative to the gun is v.) Assuming that the gun is completely free to recoil (no external forces on gun or shell), use conservation of momentum to show that...
17. ### Relative speed and coin dropping

Hello Forum, We are on planet Earth that is rotating at an angular speed of omega = 7.27 × 10-5 rad/s. The Earth atmosphere is also rotating with Earth at the same speed (that is why it is at rest relative to the Earth surface). Let's say I have a coin in my hand that is 1 meter above the...
18. ### Measuring Relative Speed of Frames Using Spacetime Interval

Hi all, Say two events happen in the same place according to one observer (1). They are separated in time by 3 years. According to another observer (2) that is moving relative to the first, the events are separated by 5 years. We can calculate, using the invariance of the spacetime interval...
19. ### Relative speed of orbiting rockets in Schwarzschild metric

Homework Statement Two rockets are orbiting a Schwarzschild black hole of mass M, in a circular path at some location R in the equatorial plane θ=π/2. The first (rocket A) is orbiting with an angular velocity Ω=dΦ/dt and the second (rocket B) with -Ω (they orbit in opposite directions). Find...
20. ### Relative Speed of Two Inertial Frames | Proving It Must Be Same

I'm wondering .. if there is an argument could prove that the relative speed of two inertial frames must be the same .
21. ### How many observers are needed to calculate the relativistic speed equation?

I am studying the relative speed equations but I confused in one point. We need (at least) how many observers to calculate relativistic speed equation. Three or two.
22. ### Relative Speed of Two Masses Connected by a Rod

Homework Statement The system shown in consists of two balls A and B connected by a thin rod of negligible mass. Ball A has three times the inertia of ball B and the distance between the two balls is ℓ. The system has a translational velocity of v in the x direction and is spinning...
23. ### Vectors and finding relative speed

Homework Statement Find Particle A's speed with respect to Particle B. I am given: Particle A: (15i - 10j) or 18.03 m/s angle A: -33.69 degrees Particle B: (5i + 15j) or 15.81 m/s angle B: 71.57 degrees (Both angles with respect to x axis) Homework Equations Had there not been any angles, I...
24. ### Relative Speed Problem: What is Matt's speed from Mary's point of view?

Homework Statement Mary sits on the perimeter of a merry-go-round undergoing uniform circular motion. Matt stands on the ground 12 m from the centre of the merry-go-round. Matt observes Mary coming towards him at a speed of 1 m/s. What is Matt's speed from the point of view of Mary? (Ans...
25. ### Find relative speed of fast moving partical with dilated time

Homework Statement The muon is an unstable elementary particle that decays via a weak-force interaction process into an electron and two neutrinos. The life time of muons in their rest frame is 2:197 s  2:197106 s. Nuclear reactions in the upper atmosphere, precipitated by the impact of...
26. ### Find the upper bound on the relative speed of the Earth and the ether

Homework Statement "The Michelson-Morley experiment was conducted using an interferometer with L1 = L2 = 40m, lambda = 632nm, and maximum fringe separation d = 0.0022 fringes. Find the upper bound on the relative speed of the Earth and the ether, and clearly state the significance of the...
27. ### Relative speed with respect to mirror image in two dimensional motion

Homework Statement A particle P is moving in a straight line with velocity u=10 m/s along the ground. Its line of motion makes angle θ=60° with the X axis drawn on the ground. A flat mirror oriented perpendicular to the ground as well as the X axis is moving with constant velocity of v=20...
28. ### Can Two Different Coordinate Systems Explain the Zero Relative Speed of Light?

Suppose a beam of light is approaching you from a distant source. As it comes closer to you, it doesn’t hit you; instead it misses you by a 10 meter distance. How can the rate of acceleration of that beam relative to you, be explained(in non-jargon language)? What would the speed of that be...
29. ### Relative Speed of Two Cars

Homework Statement Your car is going 78 mph on the freeway. Another car is going 58 mph in the opposite direction. How fast is the person in the other car going, relative to you? Homework Equations Your Car's Speed: 78 mph --> Another Car's Speed: <-- 58 mph The Attempt at a...
30. ### How Can the Relative Speed of Photons Exceed the Cosmic Speed Limit?

https://www.facebook.com/groups/258790870808892/#!/photo.php?fbid=2797441950335&set=o.258790870808892&type=1&theater[/EMAIL] Can anyone explain how is here relative speed exceeding the cosmic speed limit?
31. ### Relative speed, relative angles

Homework Statement A kayaker needs to paddle north across a 100-m-wide harbor. The tide is going out, creating a tidal current that flows to the east at 2.0 m/s. The kayaker can paddle with a speed of 3.0 m/s. Homework Equations v= v + V Let the Earth frame be S and a frame attached...
32. ### Relative speed, center of mass, kinetic energy

Homework Statement Let m1, m2 and m3, the mass of 3 particles and v12, v23, v13 their respective relative velocities. a) Show that the total kinetic energy of the system in relation to the center of mass is: ( m1m2v12^2 + m2m3v23^2 + m1m3v13^2 ) / (m1+m2+m3) b) generalize this result...
33. ### Trying to calculate relative speed between two moving points

There are two points on the 2D rectangular coordinate system, namely P and M. Their positions are function of time and are: Position \, of \, P: \, (p_x(t), \, p_y(t)) Position \, of \, M: \, (m_x(t), \, m_y(t)) Distance between them is: R(t) \, = \, \sqrt{(p_x - m_x)^2 \, + \, (p_y -...
34. ### Electron-Proton Relative Speed: Solving for 0.9c and -0.0994c

Homework Statement A laboratory experiment shoots an electron to the left at 0.9c. What is the electrons speed relative to a proton that is moving to the right at 0.9c? I have no idea how to attempt this problem i know the answer is -.0994c but I am not sure how to get this answer?
35. ### Another relative speed and size question

if the universe was the size of my laptop, and i moved my hand over my laptop from one end to the the other in let's say ... 1 second . relative too the universe my hand would have moved millions of times faster than the speed of light. Why is this? is speed relative to size?
36. ### Can Spaceships Travel Faster Than the Speed of Light?

Two spaceships are traveling relative to Earth (stationary). One spaceship is going to the right at 0.8c the other is going to the left at 0.8c. Could you say their relative speed to one another is .8c+.8c? I'm thinking not since you can't possible go faster then the speed of light, doesn't...
37. ### Relative Speed and Size of Two Spaceships Traveling in Opposite Directions

Homework Statement Two spaceships are traveling relative to Earth (stationary). One spaceship is going to the right at 0.8c the other is going to the left at 0.8c. a. What is their relative speed to one another? b. what is their relative size? Homework Equations The Attempt at a Solution
38. ### Relative speed of two particles

Homework Statement One particle is shot in the x direction at speed u and a second is shot in the y direction at speed u as well. Show that the relative speed of one to the other is: u(2-(u/c)^2)^1/2. Homework Equations velocity addition: u = (u' +/- v)/(1 +/- u'*v) Lorentz Trans...
39. ### Does Relative Speed Affect Gravitational Attraction?

If i am moving at a constant speed and i see someone moving towards me , According to relativity i can't tell if I'm moving or they are moving. But if the heavier person is moving towards me they would have more kinetic energy and a stronger gravitational field, then if i was moving and they...
40. ### Finding relative speed of space shuttle

Homework Statement The space shuttle, with an initial mass M = 2.81E+6 kg, is launched from the surface of the Earth with an initial net acceleration a = 17.5 m/s2. The rate of fuel consumption is R = 8.20E+3 kg/s. The mass of the shuttle when it reaches orbit is Mo = 6.74E+5 kg. What is the...
41. ### Average relative speed

Homework Statement For a gas in which all molecules travel with the same speed v, show that average relative speed = 4/3v (rather than sqrt(2) v which is the result obtained when we consider the actual distribution of molecular speeds.) Homework Equations The Attempt at a...
42. ### Find Relative SPEED of Image to Object: My Solution

This problem came in an exam . An object is approaching a mirror with a SPEED V . Find the SPEED of the image relative to the object . My solution - The object approaches the mirror with a SPEED V relative to the ground . Thus the image approaches the object with a SPEED V relative to the...
43. ### What's the relative speed of electricity?

I've searched for this question on these forums and google and everyone seems to have the same question as me, but hasn't stated it clear enough for people to understand exactly what they want to know. I'm wondering how long it takes for electricity to turn something on. I don't want to know...
44. ### Relative speed of light paradox

I'm no physicist, but I read that the speed of light is constant from all observers (regardless of relative speed) and in doing a thought experiment I encountered a problem. Here is the situation: John shoots a laser at you. You turn and run away at near the speed of light, but you look back to...
45. ### Time and the relative speed of an Expanding Universe

I would appreciate assistance in answering the following. Does the existence of time depend on the universe expanding at an accelerated rate?
46. ### Relative speed and Time dilation

Homework Statement What is the relative speed if two observers record a time difference of 1% Homework Equations t=(gamma)t' The Attempt at a Solution I use the above equation and solved for v, and got v=sqrt(-c2((t'/t)2-1) however I am gettin 2.999x108m/s, and I am pretty...
47. ### Need help with the 2nd part of a frequency/ relative speed quesiton

Homework Statement A galaxy known to be traveling away from the Earth at 0.650 percent of the speed of light is observed on Earth to have a strong peak in its spectrum at 649.1 nm. However, one small portion of the galaxy (a "dwarf") has a similar observed peak with a wavelength that is 4.50...
48. ### Relative speed between two objects

Homework Statement Two jets of material from the center of a radio galaxy are ejected in opposite directions. Both jets move at .750c relative to the galaxy. Determine the speed of one jet relative to the other. Homework Equations u'x = ux-v/1-ux(v)/c^2 The Attempt at a Solution...
49. ### Solving Relative Speed and Flashes in S' Frame

Homework Statement A red light flashes at position xR = 3:00m and time tR = 1e-9s, and a blue light flashes at xB = 5:00m and tB = 9e-9 s, all measured in the S reference frame. Reference frame S` has its origin at the same point as S at t = t0 = 0; frame S' moves uniformly to the right...
50. ### Is the Relative Speed of Light Constant?

I read somewhere that relative speed of light always remains constant. That is there is no difference between relative speed and absolute speed of light. If we move with 99% velocity of light in the direction of light , the speed of light is NOT 1% of actual speed of light , It still remains...