Can someone simply explain the difference between inertial and non-inertial? thank you?

Answer 1

Inertial: Newton's laws hold.

Non-inertial: they fail.

So for a frame to be inertial, it has to be moving at a constant velocity through completely empty deep space. Or it can be free-falling in a uniform gravitational field (like the space shuttle).

Non inertial frames:
--are accelerated by a force other than gravity
--are spinning
--are held fixed in a gravitational field

Non-inertial frames cause pseudo-forces on objects.
--Like when your sports car accelerates, you get pushed back in your seat
--When your reference frame is turning, you experience coriolis and centrifugal forces
--When you are in a gravitational field, you get pulled down by a gravitational force

Answer 2

I assume you mean inertial and non-inertial frames of reference.

An inertial frame of reference is one in which Newton's first and second laws of motion are valid. That is to say, an object accelerates only when acted on by a force, and F = ma.

A non-inertial frame of reference is one in which one or both of those laws is invalid, due to the acceleration of the frame itself. For example, a moving car that starts, stops, and changes speeds is a non-inertial frame of reference. If a small weight were tied to a string and suspended from the roof of the car, inside the vehicle, it would rock back and forth and at times stand at an incline to the apparent vertical orientation, all without being acted on by a force. Or, in other words, any reference frame in which fictitious or pseudo forces exist must be non-inertial.

Answer 3

We usually assume that our laboratory walls represent an inertial frame of reference. In fact, those walls are accelerating toward the center of the Earth, and the Earth is accelerating toward the sun. You can get closer to inertial by placing the lab on a westbound train, moving equal and opposite the Earth's rotation speed, on a perfectly circular track around the world at constant latitude. To find a truly inertial frame of reference, you must place your laboratory walls in the middle of a cosmic void where gravitational potential is constant; with no external force applied, the lab will be truly inertial. When you get to cosmological scales, a truly inertial reference frame will be affected by the expansion of space. The expansion will be equivalent to a gravity hill centered on the origin, regardless of where you place the origin. The shape of that "hill" will be parabolic until relativity comes into play. Then it takes on some kind of hyperbolic trig function shape. I'm not a mathematician, so I don't know how to calculate it. Cosmologists, prefer to use comoving coordinates instead of inertial coordinates. In comoving coordinates, relative motion due to expansion is considered to be "apparent motion" and relativity does not apply to it.

Answer 4

An inertial reference frame is a place (say a lab) that is moving in a straight line at a constant speed. A non inertial system is accelerating or rotating.