Relative motion Definition and 282 Threads

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.

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  1. S

    Doppler effect with moving medium but no relative motion

    I would like to be sure about one case of the use of Doppler effect with sound waves. If the medium (in the case of sound air) is moving, but there is no relative motion between the observer and the source there is no Doppler effect at all. (And the absence of relative motion is...
  2. A

    Why can't the acceleration of a wedge be found by simply....

    ↪ mentor note: thread moved to h/w forum so template is missing ↩ Hi, I have recently been learning relative acceleration and I have a bit of trouble with basics; In a problem with a smooth wedge on a smooth plane, a mass is gently placed on the wedge and I've been asked to find the...
  3. B

    Projectile Motion with Relative Motion (w/o Solver)

    Homework Statement Homework Equations vx = 70*cos(α) + 8.333 (30 km/h = 8.333 m/s) vy = 70*sin(α) vx*t = 350 vy*t = 60 -(1/2)*g*t^2 + vy*t + 2.5 = 60 The Attempt at a Solution This problem is conceptually very simple for me, but I can't solve it without using a solver like Wolfram Alpha...
  4. A

    Sphere rolling with slipping on a movable platform

    Homework Statement A sphere (of radius r and mass m) rotating with angular velocity ω0 is lowered onto the edge of a floating platform of length L and mass M. The platform can move freely on water. The platform is rough and the sphere rolls all the way from one edge to the other edge of the...
  5. jdawg

    Dynamics: Relative Motion Analysis

    Homework Statement [/B] If bar AB has an angular velocity of 6 rad/s, determine the velocity of the slider block C at the instant shown. 2. Homework Equations The Attempt at a Solution vB=vA+wABxrB/A vA=0 vB= -0.8485i+0.8485j So then C relative to B: vC=vB+wBCxrC/B -vCi=...
  6. Stephanus

    B Why do objects always revolve around each other in space?

    Dear PF Forum, I have a confusion about gravity. And frankly I don't know if this question belongs to this sub forum (cosmology, general physic?). Gravity attracts object - Newton Gravity curves space time - Einstein. Why we revolve around a massive object? Because that massive object curves...
  7. S

    Relative motion and conservation of momentum

    Homework Statement A 52kg man is on a ladder hanging from a balloon that has a total mass of 450kg (including the basket passenger). The balloon is initially stationary relative to the ground. If the man on the ladder begins to climb at 1.2m/s relative to the ladder, (a) in what direction does...
  8. hackhard

    How to check if frame is inertial?

    inertial frame is one in which isolated particle has constant velocity but is there actually any "isolated particle " ? how then can frame be defined as or not being inertial ? or is it that - for a system in which acceleration due to external forces is equal for all members , the frame of...
  9. S

    Relative Motion Analysis: Acceleration

    Mentor note: moved to homework forum later, therefore no template. When doing a rigid body relative motion (accel) problem using vector analysis, how do I know when it's appropriate to substitute sin/cos values vs when to use just the numbers given as the velocity?
  10. D

    Relative Velocity of Ball on Moving Board

    Homework Statement A ball rolls with a velocity of 14 mm/s [W] on a board that is being pulled [E 60o N] at 20.0 mm/s. What is the velocity of the ball relative to the floor? b = ball x = board f = floor Homework Equations V_bf = V_bx + V_xf The Attempt at a Solution I tried to work through...
  11. clydefrog

    Plane ride relative motion problem

    Homework Statement You are traveling on an airplane. The velocity of the plane with respect to the air is 160 m/s due east. The velocity of the air with respect to the ground is 41 m/s at an angle of 30° west of due north. 1) What is the speed of the plane with respect to the ground? Homework...
  12. a lone fishy

    Relative motion or Newton first law?

    Homework Statement An observer on the ground sees a person on a train just leaving the station in the east direction. explain how the observer on the ground could see the person moving to the west while the train is moving east Homework EquationsThe Attempt at a Solution At first I thought...
  13. C

    Minimum distance to avoid collision using relative motion

    Homework Statement 2 cars with velocities 10 m/s and 20 m/s are traveling in opposite directions, having uniform retardation of 2m/s^2 and 1m/s^2 respectively. Find minimum separation between them such that they don't collide. Homework Equations $$v^2=u^2 + 2as$$ $$V_{12}=v_{1}-v_{2}$$ The...
  14. kubaanglin

    How Should the Rescue Boat Adjust Its Heading if the Child Swims Upstream?

    Homework Statement Relative motion problem: A child in danger of drowning in a river is being carried downstream by a current that has a speed of 2.4 km/h. The child is .3 km from the shore and .4 km upstream of a boat landing where a rescue boat sets out. a) If the boat proceeds at a maximum...
  15. DanielleG

    Airplane Dropping a Package Relative Motion

    Homework Statement A Red Cross airplane is traveling horizontally east at velocity v_0 = 80.0 m/s and height h = 350 m above the ground. The plane carries emergency supplies for a village inaccessible by road. The plane will drop the package (without a parachute) into a pile of soft hay...
  16. K

    How can you accurately hit a trash bin while biking with an apple core?

    Homework Statement You are traveling on a bike, moving at a constant speed of 20,0 km/h. You just finished eating an apple, and you wish to pop the core into a trash bin located by a busstop.The bin is 3,00 m from the edge of the road (where you are biking). You throw it at a point 4,00 m...
  17. B

    Relative Motion Problem been trying to figure out for hours

    Homework Statement In hot pursuit, Agent Logan of the FBI must get directly across a 1200-m-wide river in minimum time. The river's current is 0.80 m/s, he can row a boat at 1.60 m/s, and he can run 3.00 m/s. Describe the path he should take(rowing plus running along the shore) for the minimum...
  18. P

    Using relative motion to find direction and time of a boat

    Homework Statement A motorboat can move with a velocity of 10.0 m/s, with respect to the water in a river. The river is flowing at 4.00 m/s due east. (a) Calculate the direction (Give the angle with respect to one of the compass directions.) the motorboat must be pointed so that it moves due...
  19. L

    Inelastic collision of plane onto a barge? (challenging)

    Homework Statement A 1000 kg plane is trying to land on the deck of a 2000 kg barge at rest on the surface of the sea. The only frictional force to consider is between the plane’s wheels and the deck, and this braking force is constant and equal to one-quarter of the plane’s weight. What must...
  20. B

    Applying Relative Motion to One Dimensional Motion Equations

    <<Moderator note: LaTeX corrected>> Problem: > Two cars A and B move with velocity ##60 kmh^{-1}## and ##70 kmh^{-1}##. After a certain time, the two cars are 2.5 km apart. At that time, car B starts decelerating at the rate 20 kmh-2. How long does Car A take to catch up with Car B? I tried to...
  21. R

    Relative motion in an elevator

    Homework Statement So I am in an elevator throwing a ball in an air, straight up and down, about 1 meter from my hand. The elevator is moving upwards at a rate of 2 m/s. From an observer on the ground, would they see the ball reach its peak at the same instant that I do? If not what is this...
  22. L

    Relative Motion - Elevator Problem

    Homework Statement A lift in which a man is standing is moving upwards with a speed 10 m/s. The man drops a coin from a height of 4.9 m and if g = 9.8 m/s2 - then after what time does the coin reach the floor. Homework Equations x = 0.5*g*t2 x = ut + 0.5*g*t2 The Attempt at...
  23. Chris T.

    Relative motion of two cars (velocity and acceleration)

    Homework Statement Cars A and B are traveling on adjacent roads. Car A travels at a speed of 70 mi/h in the positive y direction, and its acceleration is zero. Car B travels along a circular path of radius 0.7 mi, and has a velocity of 50 mi/h at an angle of 30 degrees right of vertical. Car...
  24. _N3WTON_

    Relative Motion Analysis: Acceleration of rotating disk

    Homework Statement A slotted link AC is is driven by the peg P connected to a rotating disk. Point A is fixed. Determine the link's angular velocity, \omega_{ac} and acceleration, \alpha_{ac} when the angular velocity and acceleration are \omega and \alpha , respectively. \omega = 6...
  25. H

    Fisherman's boat Relative motion

    Homework Statement A fisherman wishes to cross a river to the shore directly south of his position. The river is 400 m across, and the fisherman's boat is capable of a speed of 2.5m/s. If the river's current has a velocity of 1.5m/s [W], determine a) the time it takes the fisherman to cross...
  26. T

    Relative angular velocity in a rotating rod

    Consider a rod AB rotating about one of it's end A with angular velocity w. Now angular velocity of each point of the rod is same i.e. w. But if we have to find the relative angular velocity of the end B w.r.t. mid point of rod, what it will be? Will it be zero because, w-w=0 or first we...
  27. H

    Relative Motion & Rotation of Axes of Reference

    I'm still very early on in my reading, so forgive me if this question isn't coherent. In the "historical introduction" section of the 1920 University of Calcutta translation of the original papers of Einstein and Minkowski available via the MIT online archive, mention is made of the fact that...
  28. R

    Physics homework help? Grand Canyon problem

    Homework Statement If you were to throw a large log over the edge of the Grand Canyon and it took 5.65 seconds to hit the ground, calculate the velocity of the log at impact in m/s and calculate the distance the log fell in feet. Homework Equations vf=vi + at, s=d/tThe Attempt at a Solution...
  29. R

    Relative Motion Homework Problem?

    Homework Statement "To illustrate this principle, consider a plane flying amidst a tailwind. A tailwind is merely a wind that approaches the plane from behind, thus increasing its resulting velocity. If the plane is traveling at a velocity of 100 km/hr with respect to the air, and if the wind...
  30. R

    Relative Motion problem involving a moving person and a moving bus

    1. Problem Statement: What is the velocity of John relative the bus in each of the following situations: A) John is sitting on the street and the bus is moving at 30m/s toward him B)John is sitting in his car traveling North at 25m/s toward a stationary bus? C)John is sitting in his car...
  31. R

    How Does Relative Motion Affect Perceived Speeds in a Police Chase?

    A police car is chasing a stolen car ahead of it. If the police car is traveling at 150 km/h (N) and the stolen car is traveling at 120 km/h (N), then determine: a) velocity of police car relative to ground b) vel. to the pol. car relative to the stol. car c) vel of stol car relative to the pol...
  32. R

    Relative Velocity of Ball Tossed on Moving Train

    While traveling on a train, two boys play catch in the aisle. The train is moving north at 30.0 m/s. The ball is tossed front to back at 5.0 m/s relative to the boys. A bystander on the highway observes the ball being tossed toward the back. To the bystander, what is the relative speed of the...
  33. 2

    Deriving the doppler shift equation for sound

    I am looking at derivations of the Doppler effect for sound and light, and I am very confused about what stays the same when it comes to sound. In this video, at 5:10, it is said that the wavelength will be the same. I'm not sure I agree with this because I'm pretty sure both the observed...
  34. ajlen

    Mybe stupid but i would like to know (vehicles in relative motion)

    So we have moving truck at 100mph, on truck is truck moving 100mph, then on this truck is truck moving 100mph, and on this truck is car moving 100mph. Then the last car is moving 400mph in space but only 300 from first truck?
  35. RAHIL008

    Perception of far object that is in motion

    Why do objects that are distant and moving appear to be moving slowly than the objects that are close?
  36. AlanC

    Is it Possible to Have No Motion in Space? Exploring the Possibilities

    In theory, Is it possible to have no motion at all in relative to all of space? A point in-between galaxies perhaps? Or, given enough energy, to suspend motion in absolute? If this were possible would everything rush away? We are moving through space at approx. 830 km/s.
  37. S

    Calculating Required Heading for North-East Sailing with Current at 5km/h

    Homework Statement A ships captain wishes to sail his ship north -east. A current is moving his ship with a velocity of 5.0km/h . If the ship has a maximum speed of 30 km/h what is the ships required heading? Homework Equations Cos angle=A/H The Attempt at a Solution Heading=cosine (5/30)...
  38. G

    Does the speed of sound depend on the observer's relative motion?

    Hi, I wanted to ask if the speed of a sound wave, given a fixed medium, depends on the speed of the observer. That is to say, does the speed of sound obey to the laws of relative motion, implying addition of relative speeds? In case it doesn't depend on relative motion, can you explain why? I...
  39. K

    Velma and Mort relative motion question

    Homework Statement 1. Velma passes Mort at a speed of 0.75c. At this speed, the "time dilation factor" is 1.5. In other words, Mort observes a 1-second tick of Velma's clock to take 1.5 seconds. Suppose that they are born at the same time, and that Mort observes himself to live 30 years. As...
  40. K

    Special relativity and relative motion question

    Homework Statement Hello all, I have a couple of questions that I am a little unsure of. Could you please let me know if my answers are correct? But more importantly could you let me know if my reasoning is correct? Thanks in advance! 1. Mort stands on Earth. Velma moves away from him in a...
  41. O

    Relative motion of a cannonball/submarine

    Homework Statement 1.) A projectile is fired from a submarine traveling horizontally at 20 m/s with respect to the water as shown in the figure below. According to an observer on the submarine, the projectile is fired at 45° with an initial velocity of 60 m/s. After firing the projectile, the...
  42. PhysicsKid0123

    Need help understanding relative motion and frame of references

    Okay guys, I still feel like I don't quite understand these concepts well enough. Do you guys have any websites or links to help me understand these concepts better? I feel like I only partially understand these concepts. I also feel like I partially understand the application of the mathematics...
  43. K

    Frictional force opposes relative motion?

    Frictional force opposes relative motion and not just motion. Can you please give an example to further explain this statement? :confused:
  44. W

    Relative motion of two reference frames

    Homework Statement Two clocks located at the origins of the K and K' systems (which have a relative speed v) are synchronized when the origins coincide. After a time t, an observer at the origin of the K system observes the K' clock by means of a telescope. What time does the K' clock...
  45. I

    Explain how light moves in relative motion due to special relativity?

    I've been told on many occasions that light will always be measured at the same speed regardless of measuring equipment speed as long as there is no acceleration and you are moving the same speed relative to that object you are measuring from if that is incorrect please ignore what i say...
  46. A

    Doppler Effect Relative Motion

    To calculate the detected frequency from the source frequency, we use this formula: { f }_{ D }=\frac { v\pm { v }_{ D } }{ v\pm { v }_{ s } } { f }_{ s } where {v}_{s}, {v}_{D} are velocities of the source and the detector respectively with reference to the medium. My question is why do...
  47. L

    How Does a Rower's Speed and River Current Affect Crossing Time?

    Homework Statement A rower's maximum speed in still water is 6.0 ft/s. He decides to cross a 60 ft. wide river (at his maximum speed), with an apparent heading of 30° north of east. The river's current (relative to the ground) is flowing south at a speed of 1.0 ft/s. Find the rower's speed and...
  48. S

    What Is the Wind Velocity Given Airplane and Ground Speeds?

    Homework Statement An airplane flies with a velocity of 55.0 m/s [ 35o N of W] with respect to the air (this is known as air speed). If the velocity of the airplane according to an observer on the ground is 40.0 m/s [53o N of W], what was the wind velocity? Homework Equations...
  49. T

    Understanding Relativity: The Effects of Relative Motion and Mass

    Well, relativity says whatever moves, it's volume shrinks and mass increases (as observed). So if i am traveling at 99% c with respect to a massive neutron star, do that star become a black hole for me?
  50. O

    How Far From the First Station Do Two Oppositely Accelerating Trains Meet?

    Homework Statement Two trains start from rest from two stations 5.0 km apart and travel, on two parallel railroad tracks, in opposite directions. One train accelerates uniformly at 2.5 m/s^2 and the other has a constant acceleration of 4.0 m/s^2. How far from the first station will they...
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