ELI5: Explain Like I'm 5

Inertial frame

An inertial frame is like when you are sitting in a car and the car is not moving, everything inside the car is still because the car is not changing its speed or direction. This is an example of an inertial frame.

But when the car starts to move, you feel a push back in your seat because now the car is changing its speed and direction, causing you to feel the effect.

In a similar way, when we talk about an inertial frame, we mean a reference point or a set of coordinates in space that is not moving or changing its direction. This is a very important point to understand because when we observe motion or any action in the universe, we need to have a fixed point of reference.

For example, imagine you are standing on a train station platform and a train is moving towards you. If the train is moving at a constant speed, it might be difficult for you to tell how fast it's moving or whether it's moving at all because you are stationary and the train is moving with you. But if you are on another train, moving alongside the first train and at the same constant speed, then you are in an inertial frame because you have a fixed reference point.

This fixed reference point helps us to make sense of what we observe in the universe. It helps us to understand the laws of physics, such as the laws of motion, which describe how objects move and behave under different conditions.

So in summary, an inertial frame is like a stationary platform that helps us observe and understand motion and other physical phenomena.