kindly keep the explanation rather simple plz
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Things can only *really* move when compared to the position of some other thing.
So imagine a particle out in the vastness of space. Is it going anywhere? How could you tell? You can’t until you do something like add Earth to your map, note where the particle is relative to Earth, and observe how that relationship changes over time.
Inertial frame of reference often comes up when thinking about the motion of planets and stars. We know that Earth is rotating on its axis and rotating around the sun, but for everyday purposes we ignore that motion. The intertial frame of reference is the nearby parts of Earth. We don’t say a car was travelling 1,060 miles per hour just because it was moving along with the Earth’s rotation.
As you zoom out more, a clear inertial frame of reference becomes even more important. Our model of the solar system puts the sun as a fixed point in the center that everything moves around. That is, the inertial frame of reference is anchored by the sun. But, if you instead compare it to other stars, the sun is *also* moving. All the stars in the Milky Way are themselves rotating around the center of the galaxy. But wouldn’t you know it, the whole Milky Way is *also* moving with respect to other galaxies.
So depending on your frame of reference, you can go all the way between “sitting still” to “hurtling through space at millions of miles per hour”. It doesn’t actually change what you’re doing, just how you think about it.
An inertial frame of reference is where F=ma applies. So if there are no outside forces there is no change in momentum, so a=0.
A non-inertial reference frame is an accelerated reference frame. If I place a ball on an accelerated trolley, it’ll appear to be rolling towards the back, from the reference frame fixed to the trolley. So F=ma isn’t true. We can modify it account for the acceleration by introducing a fictitious force. If the trolley is accelerating with a0 if F=0 the motion is a0 for the ball in the opposite direction so F-a0m=ma. If F=0 we have a=-a0. So the acceleration of the ball is the same magnitude and in the opposite direction of the acceleration of the trolley.
A rotating frame of reference is also like that but we will have different fictitious forces: centrifugal, Coriolis and Euler.
Newton’s law of inertia states that in the absence of outside forces, a body at rest will remain at rest and a body in motion will remain in motion. You need external forces to make something start moving from rest, or to slow down or change the direction of something that’s moving.
An inertial frame of reference is a frame of reference where this is true. In other words, it is a frame of reference that is not experiencing any acceleration.
Imagine you’re in a car that is moving at a constant speed in a straight line and you have one of those tree-shaped air fresheners hanging from your mirror. From your frame of reference inside the car, the air freshener is at rest relative to you and the rest of the car, so it should not and does not move by itself. This is an inertial frame of reference, because the law of inertia is true here. The air freshener is at rest and remains at rest.
Now imagine the car slows down. The air freshener swings forward. From your frame of reference inside the car, this violates inertia. The air freshener, which was at rest relative to you and the rest of the car, suddenly moved by itself. Therefore, this is not an inertial frame of reference.
The difference is acceleration. Because the car experienced a change in its velocity in the second case, it cannot be an inertial frame of reference.
For an ELI5, its when an observer can not feel a force.
Floating in space you are in an inertial frame.
Moving at constant speed in space you are in an inertial frame. Indeed these first two cases are indistinguishable if there is nothing for the observer to see.
Standing on earth we are NoT in an inertial frame. You can feel the force of gravity on your feet.
Falling in a gravitational field you are in an inertial frame (ie being in orbit around a planet).
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