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.