How to spawn an entity with the same velocity as another entity?

Question

I want to summon an arrow with the same velocity as an egg that has just been thrown. I know about the NBT tag Motion:[x,y,z], but I have no idea how to match that with the velocity of another entity.

I tried to just have the arrow be constantly be teleported to the egg, but it doesn't land where the egg lands.

Answer 1

You can transfer NBT data from one entity to another in newer versions of minecraft. To replace all eggs with arrows with the same Motion-tag, you could use these three commands, executed in a repeating command block and then two chain command blocks:

/execute at @e[type=egg] run summon minecraft:arrow ~ ~ ~
/execute as @e[type=minecraft:arrow] at @s run data modify entity @s Motion set from entity @e[type=egg,distance=0,limit=1] Motion
/kill @e[type=egg]

The first command summons an arrow at the position of every egg.

The second command sets the Motion-tag of every arrow to the same value as any egg that is in exactly the same position.

The third command kills all eggs.

Answer 2

The short answer is: you can't.

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The longer answer is: You still can't, but here is why:
You can't transfer NBT data from one entity to another. Normally you might be able to work around that by detecting the NBT data in the first entity with a few hardcoded values, and then giving that same tag to the other entity.
This however won't work for the Motion tag since that incorporates three double values, and a typical Motion tag looks something like this: Motion[0.17361829472d,1.38572958294d,0.72483746234d] (good luck finding that one with hardcoded values)