# Relative Velocity

Relative Velocity Vs Actual Velocity,

We measure the velocity of many things on a regular basis. Sometimes we say things are at rest, I say nothing is actually at rest. If it sits on the planet and the planet is spinning on its axis, and also rotating around the sun, and within the solar system ....

and I can hear you say yes but with respect to us the object is at rest !!

Think can you play table tennis on a boat, or on a train, http://faraday.physics.utoronto.ca/PVB/Harrison/Flash/ClassMechanics/Relativity/Relativity.html

Imagine a wall, most walls do not move, but put yourself in a car with a lunatic driver, who decides to drive towards the wall. It seems like the wall is getting closer ? but does the wall move ?

Imagine a dog in the back window of the car you are driving, when you and the car behind are travelling at the same speed the dog notices no actual movement between the two cars. If you are going slow, the dog notices cars pass with a velocity, is this velocity the same as the velocity that the speedometer in the car reads? If you are going fast and overtaking cars then the dog notices that other cars have a negative velocity.

This relative velocity is important, if you can run with an actual velocity of 8 m/s, and you are on a train travelling at 20 m/s, what velocity does a rabbit sitting on a platform that the train is passing see you running at ?

Ever on a train in a station, with another train along side ? Which train is moving

We say that the Velocity of A relative to B (where B is the dog or rabbit, the observer) is the Va - Vb

This is read as the velocity of a relative to b

this has to be considered in the i axis and the j axis

Keep in mind in the exam, if they ask for the relative velocity you might also state it in terms of magnitude and direction

so work out the relative velocities for the 2 cars above,

If 2 ships leave the same port one has a speed and direction 5km/hr angle N36 52E

and the other going 13km/hr S 22 37 E

work these speeds into i & j vectors

now calculate VAB and VBA

what do you notice about VAB and VBA ??

Two Gents, Michelson & Morley, succeeded in proving there was no ether.

This movie simulates the set-up used in the Michelson-Morley experiment, including the non-existent aether wind they were trying to detect! The basic idea is to detect the time difference between light going "upstream then downstream" and light going "across shore and back."

http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/mmexpt6.htm

http://en.wikipedia.org/wiki/Speed_of_light#Astronomical_measurements

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