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Why is velocity relative but acceleration is not.. why is so hard in physics.. so many things are relative and some are absolute, its easy to figure out which is which.. but WHY. I've been far down the rabbit hole and it never ends..
That's begging the question a little bit, as acceleration is absolute only because we restrict ourselves to inertial (i.e. lacking acceleration) reference frames. If you allow non-inertial reference frames, acceleration can be relative - Coriolis acceleration, for instance, may be nonzero relative to a rotating reference frame but zero in inertial coordinates.
@khird @sda @mandlebro @mngrif
So if we look at position and all its derivatives we see:
Position - relative
Velocity (1st derivative) - relativederrivative
Acceleration (2nd derivative) - non-relative
Jerk (3rd derivative) - non-relative
Every additional derivative is also non-relative...
Sort of. Acceleration isnt force though. It is however the result of a force applied to a mass.
Otherway around. If you sitting in a ship and want to know the force applied to the ship as well as the acceleration it is undergoing you would directly measure acceleration and then can find the force applied to the ship if you know or determine the ships mass
If you had a spring and a standard mass (known to be 1kg or something) inside the ship with you and you used that to measure you would not be measuring the force applied to the ship but rather the force applied to teh weight attached to the spring, which would be an entierly different quantity of force than the force applied to the ship.
So its a little convoluted in practice we would use a fixed weight and spring to determine acceleration from the force applied to the fixed weight, and then from that acceleration use it to determine the force applied to the ship (if you know the ships mass).
But it might help give you some insight on the relationship between force and acceleration if you consider how we might measure it if we didnt know anything about mass. If you stick some unknown mass in a tube and obsere how fast it accelerates from one end to the other then we know the acceleration on the ship directly. In this setup the mass in the tube doesnt need to be known as all masses would behave the same.
@freemo @sda @mandlebro @mngrif
What you've written is true in the case where all reference frames are inertial. It is *not* true if the reference frames themselves are allowed to accelerate relative to one another.
Let's take a 75kg man standing in a windowless cube as an observer. He can stand up, and let's say there's a 735N normal force between his feet and the floor. Even with accurate measurements of his mass and this normal force, he still cannot determine his acceleration. Perhaps he is in an otherwise empty universe, and the box is being driven "upward", accelerating at 9.8m/s. Perhaps he is in a uniform gravitational field of 9.8N/kg pulling him "downward", and the box is stationary. Since he can't distinguish between these cases to determine his own acceleration, there is no way he can guarantee that he and an outside observer would agree on its value - it's relative to the observer.
@khird @sda @mandlebro @mngrif
Your response only indicates that a person can not determine the SOURCE of their acceleration, moreover that apparent motion also is not an indicator of acceleration.
You also seem to be mistaken about what a non-inertial and inertial frame is. In the example I gave in my last response I explained both inertial frames and non-inertial frames
Second a person appearing to stand still ont e surface of a planet but undergoing gravity is, in fact, experiencing acceleration, it just doesnt look like it to us because we can not see space time accurately with our eyes. But one of the biggest discoveries that is central to general relativity is that a spaceship accelerating in space is equivalent to a person standing still on the surface of a planet.
@khird @sda @mandlebro @mngrif
Sounds like you guys are misunderstanding what the term relative means in a physics standpoint.
Relative just means that there is no process whereby all frames of reference could agree on some value without first agreeing on a reference point with which to use as a privileged frame.
Another way to think of it is this. If there are two observers in two different reference frame and neither can see or communicate with the other, and these observers were in an empty universe except for one other object. Is there some process they could use to determine a value of a property. If so then the property is non relative.
So lets take that back to velocity for a second. There is no way to determine an objects velocity in any way unless all frames of reference have some agreed upon privilaged frame, this often tends to be the earth.
Acceleration is non-relative because I can measure the acceleration in my own reference frame by measuring (just as you feel a car accelerating by being pushed back in your chair). You can then measure the acceleration of an outside object by using your own reference frames measured acceleration as the reference point and ultimately calculate the acceleration of the observed object. If all observers use this same process to measure acceleration then they will all agree, thus non-relative.