Physics Help Forum question about relativity and frame of references

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 Apr 22nd 2015, 01:09 PM #1 Junior Member   Join Date: Apr 2015 Posts: 1 question about relativity and frame of references 1:why velocity is relative but acceleration is absolute . 2:what do we mean by the term "absolute" in kinematics? 3:can we find centripetal and centrifugal force simultaneously in non inertial frame of reference ?
Apr 22nd 2015, 03:32 PM   #2

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 Originally Posted by idol 1:why velocity is relative but acceleration is absolute . 2:what do we mean by the term "absolute" in kinematics? 3:can we find centripetal and centrifugal force simultaneously in non inertial frame of reference ?
1: SR takes the speed of light to be a constant, as implied by the Lorentz transformation. Under this rule we get the velocity transformation laws. Accelerating frames are not a feature of the Lorentz transformations so the law is that v < c, but you can have any value for the acceleration. There is a distinction here with General Relativity: accelerating frames are now allowed but they too have their "distortions." There is still no upper limit on the acceleration but accelerations now look different in different reference frames.

2: I'm not sure what you mean by "absolute" either. Do you have an example you can share?

3: Centripetal force is defined as the radial force an object undergoes as a result of its speed in an arc. (Typically we have objects moving in a circular orbit and constant speed which gives a constant centripetal acceleration, but if the object is not on a circular path we still define aC = mv^2/r, it's just that we no longer have a constant v and r.) The centrifugal force is similar to the centripetal force, just that it is in the opposite direction. The distinction is that centripetal force is calculated as mv^2/r and that the centrifugal force is due to an object's inertia.

-Dan
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 Apr 22nd 2015, 03:49 PM #3 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 1,035 I think more context is required to your questions, particularly as you seem to have mixed up/misconnected several terms. You have posted in pre-university physics what level to you expect an answer to be pitched at?
Apr 25th 2015, 12:41 AM   #4
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 1:why velocity is relative but acceleration is absolute . 2:what do we mean by the term "absolute" in kinematics? 3:can we find centripetal and centrifugal force simultaneously in non inertial frame of reference ?
I tend to think of it this way:

1. A body with no force on it travels with constant velocity. The velocity will be differently observed due to the different observers relative velocities. No ovserver is more correct than any other. For instance our earth is not stationary, neither is our solar system or even galaxy. There is no way to know who is standing still and who isn't.

2. Acceleration is "absolute" because it comes from F = ma (Newtons third law) and as far as I know mass is not relative and Force is not relative, they have very specific values independent of who observes them. For instance our earth has a certain mass, our sun has a bigger mass, this determines its very specific orbit around the sun.

Apr 25th 2015, 03:43 AM   #5
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 Originally Posted by kiwiheretic ...as far as I know mass is not relative and Force is not relative, they have very specific values independent of who observes them.
In Newtonian mechanics it's true that mass is invariant (same value in all frames of reference). However force is only invariant between inertial frames. If there is no force on a particle in an inertial frame of reference then the inertial force on it in a non-inertial frame of reference. For details please see: http://home.comcast.net/~peter.m.bro...tial_force.htm

When Einstein formulated general relativity he was motivated by the notion that inertial forces are "real" forces.

In special relativity neither mass nor force is generally invariant. The transformation for the force on a particle from one inertial frame of reference to another is given in:
http://home.comcast.net/~peter.m.bro...orce_trans.htm

 Tags frame, question, references, relativity

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