Re: What principles of Physics are involved in the newly popular sport of kite surfing (kiteboarding)?

There are lots of physics principles to apply here especially in the 
area known as Fluid Mechanics to engineers who design the gear you use. 
Kite boarding would involve principles from fluid mechanics called lift 
and drag on both the kite and the board as well as several other that are 
interesting and complicated. These principles analyze how friction, 
density of the water and air and geometry of the kite and board affect 
your motion. An engineer or designer would look into making the board 
more aerodynamic in the water to get more speed, a word you've probably 
heard already. An engineer would also look into how you could get more 
lift from the kite so you could do more aerobatics with the board. These 
would certainly have a lot to do with this sport however if you want to 
know which of the three of Newton's laws of motion would apply and how 
then that would include all three. 

In Newton's first law he deals with balanced forces, in other words when 
things are either NOT in motion OR are at a constant (non-changing) 
velocity (speed), we call this the study of "Statics", not statistics, in 
engineering. The equation for his first law F=m*a is equal to zero, here 
the a=zero, no acceleration. In the previous equation (F) is the force, 
(m) is the mass and (a) is the amount of acceleration. The amount of 
force pulling/pushing in one direction is equal to the force in the 
opposite direction. This applies to kite boarding when you have no 
movement or a constant speed when you are on the board. 

In Newton's second law he deals with unbalanced forces, in other words 
when things are accelerating, we call this the study of "Dynamics" in 
engineering. The amount of force pulling/pushing in one direction is 
either greater or smaller than the force pulling/pushing in the opposite 
direction. The equation for the second law F=m*a is no longer zero 
because you now have some acceleration. As it applies to kite boarding 
I'm sure you can feel when your speed either increases or decreases, 
decreasing speed is called negative acceleration so the second law still 
applies here even though you may be slowing down because even though 
acceleration may be negative it's still not zero.

In Newton's third law he deals with "Reciprocity" which is just a fancy 
physics word that means we have a state of mutual actions. In this case 
you have object X exerting a force on object Y and object Y exerts an 
equal and opposity force on object X, the equation looks like this 
Fx=-Fy. This would be the case when Newton's First law applies. It would 
apply to kite boarding when the rope between you and the kite is taught, 
meaning not loose then the force you exert pulling down on the rope so as 
not to let it go is equal and opposite to the force the kite is exerting 
on you however this no longer becomes true once the kite either falls 
down towards you or you let go of the rope.