Well, you see, I don't think Alana really is the reference frame. Call me paranoid, but I think you changed the reference frame during the question, and didn't tell us.
In which reference frame is the 19 km/hr measured ? It CAN'T be Alana's reference frame. Your own reference frame moves along with you, and you can't move in it, even if your name is Alana.
If Alana is the reference frame, and she throws the tennis ball at 11 km/hr, then the speed of the ball is 11 km/hr in Alana's reference frame. Her reference frame moves with her, so it makes no difference how fast she is skateboarding in any other reference frame, who she throws the ball at, or whether or not he sees it coming and catches it.
Yes, this stuff can get confusing. And if you think it's bad now, wait till you start reading some of Prof. Einstein's stuff, where two people in the same reference frame can watch the same tennis ball, and not even agree on how fast it's moving, because THEY're both moving and their own motion makes their rulers and clocks change ! So they measure different speeds, and they're both right !
But I got distracted. I'm sorry. The point I'm trying to make, right now when you're just starting to learn reference frames, is that EVERY time you say a speed, you have to tell which reference frame the speed is in. Because, as you're starting to learn, the same object can have a different speed in every reference frame.
And, just in case you're thinking about this later today and you want to ask "Which one is the REAL speed ?" . . . THAT's the whole point of learning about reference frames ! There is NO SUCH THING as REAL speed. It ALWAYS depends on which reference frame it's measured in. They're all different, they're all real, and they're all correct.
Newton's first law of motion states that an object at rest or uniform motion in a straight line will continue in that form unless acted upon by an external force.
The applied force on the bus in the form of brakes acts as the external force on the bus.
According to Newton's first law of motion, the external force (brakes) will change the motion of the bus.
Thus, we can conclude that a force is changing the motion of the bus.
