Two major forces dominate the dynamics of surfing; gravity and those which arise from changes in fluid momentum. Both are critical, and both are always at play in some way or another.
Most readers are likely to be familiar with gravity, how it operates and its relative strength, the same is not likely to be true for notions of momentum, or changes in momentum.
Formally, momentum is equal to a thing’s mass times its velocity. All things that have a mass have momentum. This is true even if they appear to be stationary, for if a thing is stationary (it has zero relative velocity) it will, by definition have zero momentum (at least from the standpoint of the observer who is claiming that the thing is stationary.) To change a thing’s momentum requires a force, or alternatively a force must be applied if you want to change the speed or direction of motion of some thing.
(Formally, force is equal to a thing’s mass times its acceleration. Acceleration, of course is the rate of change of velocity. Hence, the rate of change of momentum is force.)
A surfer uses his weight (gravity), actually he uses both his weight and that of his surfboard and the surfaces of his surfboard to change the momentum of the fluid rushing up the face of a wave. Exactly how the momentum gets changed is determined by how the wetted surfaces of his board (those surfaces in contact with the flow) are presented to the flow (which are to a some degree under the control of the surfer.) An example may help clarify all this.
Say you’re a protester and your out for a weekend’s protest. The riot police are also out and have decided to use fire hoses to control this week’s malcontents. You find yourself facing a police officer who happens to be holding a fire hose. The police officer turns on the water and the water jet hits you square in the center of your chest. You, of course are thrown directly backwards and wind up square on you ass. Undeterred, you stand up, but this time as you see the police swing the hose around your way again, you angle your body a bit so the jet just glances your chest. To your delight you are merely thrown off to the side.
So, stand directly in front on the flow (jet) and you’re thrown directly backwards (in the same direction as the flow), angle your body a little and you’re thrown off to the side.
Reviewing your experience, you also recall that when you angled your body to the oncoming jet, the water leaving your body seemed to head off at an angle after hitting you - you and the water jet sort of went off in different directions as a result of the impact. When hit directly square on the water seemed to spray right back towards the police. “Mmmm?”, you wonder if some basic principle might be operating here.
How’s this relevant to surfing? Consider, take ‘you’ to be the surfer/surfboard and the jet to be the water rushing up the face of the wave. Mind you, in this analogy gravity is working in the wrong direction, but surely you can appreciated the ‘force’ of the jet of water hitting your chest, and how changing its impact angle changes the resultant force on you. But lets complete the illustration by introducing a force which will play a role similar to that played by gravity in surfing.
So, we’re back on the street and once again we’re facing the police officer with the fire hose, but this time you’re standing in front of a bunch of your friends and as the jet hits you they hold you up. In your pain, you notice that as the water hits you square in your chest it sprays off in all directions but for the most part right back at the officer (like before, just prior to your being knocked on your ass.) Again, undeterred, you manage to angle your chest to the jet’s direction, but now in order to keep providing you with support your (so called) friends find they have to move a little sideways. The police (also undeterred) follow your sideways movement keeping the hose blast on you, in the process you and your friends continue to move sideways, the police following. (One of your well protected friends is heard to utter, ‘Dude, we’re surfing.’)
In a nutshell, your presence in the path of the jet served to change the momentum of the jet of water, you managed this by capturing a little of the jet’s momentum for yourself (you went flying back, or off to the side.) Your momentum was changed from zero to some non-zero value, and the jet’s momentum was changed from whatever it was prior to impacting your body, to an amount just that much less (an amount equal to the amount you gained.) The jet applied the force necessary to change your momentum, and you (and your friends) applied an equal and opposite force to the jet changing its momentum by a corresponding amount.
(If at this point you’re wondering if I’ve introduced a new concept, you’d be right - Newton’s Third Law, ‘for every action there is an equal and opposite reaction.’ My apologies, but leading with Newton’s Third Law in any discussion always seems to lead to trouble. Its always best to let it reveal itself. Newton’s Third Law, of course address force. But there are two other laws which sort of have a similar flavor; that of conservation of momentum, and conservation of energy. They’re basically book-keeping laws. The similarity of these laws is that they tend to say everything has a price…or nothing is for free… push something here, something pops out over there.)
So we have our model, your chest is the bottom of the surfboard, you’re friends are gravity and the jet of water coming from the fire hose held by the police is the wave. You may seriously question if surfing can really be summed up by jet of water hitting some obstruction, it seems all to simplistic. I agree, it is simplistic, but its not an unreasonable illustration of the forces which are at play during surfing.
Surfing is a trade-off, a continual trade-off. The trade-off is between the momentum of the fluid rushing up the face of the wave and the momentum of the surfer/surfboard. You change something here, and something over there changes, you go faster, something goes slower, etc. To a large extent the surfer has control over some of parameters of this trade-off and exercises that control by changing the presentation of the bottom of his board to the flow. Nevertheless, the important point is that everything has a cost.
Money for nothing and chicks for free? I wish.