Reference Frames with Blimp the Dog
Imagine you are travelling inside one of the raindrops. What would you see as Blimp the Dog scampers across the grass?
Frames of Reference
Different observers will see different versions of the same event.
Let’s say you are at one end of the park and Blimp is running towards you.
A person at the other end would see Blimp running away from them.
This means you and this other person are in different frames of reference.
A frame of reference is a coordinate system that can be used to determine positions and velocities in that frame.
Inertial Frame of Reference
There is a special name we give to the frames of reference most common in our daily lives. Going back to you standing still in the park, you are not accelerating relative to the reference frame of the Earth because you are travelling at the same speed and in the same direction. So you are seeing things from an inertial frame of reference.
Frame of Reference of Rain
We can model the rain as falling at a steady speed.
So the frame of reference of the rain, let’s call it R, is an inertial frame. This is important because your frame of reference in the park, which we can call S, is also inertial.
Now, we use a principle that is one of the central pillars of classical mechanics, Galilean invariance, which states:
The laws of physics are the same in all inertial reference frames.
It just means that you can accurately describe Blimp’s motion using either reference frame, R or S.
Blimp’s velocity in R
In S, you see Blimp running into the raindrops and the rain falling down towards Blimp.
But if you were inside a raindrop, that is your reference frame was R, you would see Blimp rising up towards you, like a blimp!
But we also need to remember that Blimp is also running horizontally towards the raindrop.
Combining the velocity components for Blimp,
Therefore, from inside a raindrop, we would get the impression that Blimp is flying through the air towards us!
