While skateboarding at 19 km/h throwning a tennis ball at 11 km/h what is the speed of the ball

According to whom ?

So YOU're on your skateboard, and there's somebody else, sitting on HIS porch, watching you skate by on your board.

-- The man on the porch says you're skating by him at 19 km/hr .

-- You throw a tennis ball.  

. . . . . Do you throw it in the same direction that you're skateboarding, or do you throw it away behind you, toward the place you just came from ?

. . . . . Does it fly away from YOU at 11 km/hr ?  Or does it fly past the man on the porch at 11 km/hr ?

There are 4 possible combinations.  One of them is not possible.  Each of the other three combinations leads to two different answers to the question.  And ALL six answers are correct !

1).  You throw the ball forward, in the same direction you're skating.  It flies away from your hand at 11 km/hr.

To you, the speed of the ball is 11 km/hr, in the direction you're skating.  To the man on the porch, it's 30 km/hr, in the direction you're skating.

2). You throw the ball forward, in the same direction you're skating.  It flies past the porch at 11 km/hr.

This isn't possible.

3). You throw the ball backward, toward where you just came from.  It flies away from YOU at 11 km/hr.

To you, the speed of the ball is 11 km/hr, in the direction backward from you.  To the man on the porch, the speed of the ball is 10 km/hr in the direction you're skating.

4).  You throw the ball backward, toward where you just came from.  It flies past the porch at 11 km/hr.

To you, the speed of the ball is 8 km/hr, in the direction backward from you.  To the man on the porch, it's 11 km/hr in the direction you're skating.

NOW you're going to ask me "But what's the REAL speed of the ball ?"

The answer to THAT one is:  There's no such thing !  It all depends on WHO's measuring it ... where that observer is and how HE's moving.

The displacement, speed, velocity, acceleration, and energy of the ball, ALL depend on who's watching it and measuring it.

I'll be interested to see whether you mark this answer 'Brainliest', or report it because it's weird, confusing, and ridiculous.

i'm thinking more like energy

scientific laws are the result of testing theories and proving them to be facts.