If a person were standing on the sidewalk, how fast would she say the apples are moving?
Since she will see them exactly the same way as me (we're both in the same reference frame), she will say 15m/s.
Now the truck starts to move forwards at 20m/s. I am still throwing apples forwards, exactly the same as I was throwing them before, at 15m/s.
If I am really not paying attention to what’s going on around me (like the fact that I am standing in the back of a moving truck), how fast would I say the apples are moving?
Still 15m/s! Relative to me, I can only make an apple move away from me at 15m/s, so that’s how fast I measure the apple moving away from me.
How fast does my friend on the sidewalk say the apple is moving?
Well, even before I throw it, she’ll say that the apple is moving at 20m/s (the speed of everything on the truck).
When I have thrown the apple forward, adding more velocity to it, she will say it is going at (20m/s + 15m/s) 35m/s!
Now I turn around and start throwing the apples from the rear of the truck, backwards!
I will still say that my apples are moving at 15m/s, because from my way of looking at it, that’s how fast the apple is moving. The only thing I might say that is different is that it is -15m/s, since even I should be able to notice they are going in the opposite direction now.
My friend on the sidewalk will say that the apple is moving at (20m/s + -15m/s) 5m/s!
