Science Term: inertial observer

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inertial observer

Definition/Summary

An inertial observer is an observer for whom Newton's first law is true, or can be regarded as true.

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Scientists

Sir Isaac Newton (1643-1727)

Recent forum threads on inertial observer

Breakdown

Physics
> Classical Mechanics
>> Newtonian Dynamics

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Extended explanation

Newton's first law states that a body on which the total vector sum of the forces acting on it is zero has a constant velocity.

"Constant velocity" means a constant speed (which may be zero) in a fixed direction.

Any observer on whom the total vector sum of the forces is zero and who is not rotating is an inertial observer. Note that a rotating observer always experiences a non-zero net force. For example, an observer travelling in a circular path will be subject to centripetal acceleration and hence must experience a centripetal force.

For an observer stationary on the Earth, Newton's first law is not exactly true, because the observer is rotating with the Earth, and so will regard distant stars (for example) as rotating round him.

However, Newton's first law is accurate enough for most purposes (and for most examination questions) for an observer stationary on the Earth, and so such an observer can usually be regarded as inertial.

A non-inertial observer may invent imaginary forces so that Newton's first law is true.

For example, a rotating observer may invent an imaginary centrifugal force to explain why objects appear to move round him.

Commentary

sganesh88 @ 07:36 AM Jan7-09

"Any observer ON whom.. "
Why are we talking about force ON an observer rather than the ones measured w.r.t him?
".... and who is not rotating is an inertial observer."
Rotating with respect to? This sort of leads to a circular definition.

tiny-tim @ 06:47 PM Nov18-08

This is in the "Classical Mechanics > Newtonian Dynamics" section of the Library …*in general relativity, one would probably prefer to talk about a "geodesic" rather than an "inertial observer"

Naty1 @ 05:40 PM Nov18-08

An awkward aspect to understanding inertial observers (an inertial frame of reference) is that in special relativity, (without gravity, hence flat spacetime) all are equivalent and they measure both local and distant speed of light as "c". Every inertial observser "sees" flat spacetime, both near and far. But in general relativity (with gravity, hence curved spacetime) an inertial frame is only local when in free fall (no acceleration, no forces felt). If that inertial observer trys to measure "c" other than locally (where curvature is insignificant), they will detect lightspeed as different from "c" due to the curvature of spacetime. For example, an observer a great distance from a black hole will observe light moving toward the black hole as slowing and never crossing the event horizon. A local free falling observer just outside the event horizon will see the light pass the event horizon normally and the observer will than pass it as well

Hootenanny @ 06:12 PM May10-08

Added note in rotating observers