Frank LoPresti

Fly casting is easily described in the language of physics if you understand how Newton’s first two Laws of Physics and the Laws of Energy Conservation relate to the casting of a fly line with a fly rod. Let’s look at each of these immutable laws and simply apply them to the casting of a fly line with the fly rod.

NEWTON’S FIRST LAW states that an object at rest will remain at rest; an object in motion will continue in motion with constant velocity, unless acted upon by an external force. The three primary considerations that pertain to Newton’s first law are Inertia, Force and Mass.

INERTIA is simply the tendency of a body to remain at rest or if in motion in a continuous motion. In fly casting it is the fly rod and fly line that are “at rest,” until a FORCE of some type, is initiated upon the fly rod and fly line.

Generally speaking FORCE is simply a push or pull that will change the velocity, (speed and direction), of an object. In fly casting when we accelerate the fly rod we are applying a force. The force that we apply to the rod comes from the action of the casting arm. Without force the rod and fly line will remain inert or at rest. Generally speaking the control of the fly rod and line is totally dependant on the action of the caster. As Vince Marinaro pointed out in 1976, in his book, “In The Ring of the Rise,” “The rod cannot make a cast any more than a baseball bat can hit a home run by itself. It is the casters own muscle and energy that makes the cast.”

Finally, an objects MASS is measured by its inertia. In this instance force is applied to the butt of the fly rod by the caster and this becomes the action that in turn overcomes the inertness of the fly rod’s mass and ultimately fly line’s mass. Objects with greater mass have more inertia to overcome. This is why a 10 weight outfit requires more applied force to cast as efficiently as a 4 weight outfit.

NEWTON’S SECOND LAW basically states that the more force we apply to an object the more it will accelerate. Essentially Force (F) = Mass (M) X Acceleration (A). The more massive it is the more it resists acceleration.

Before we examine the Law of Energy Conservation it is important to note two additional energies or properties that are potentially present in all fly rods and fly lines, that relate to the understanding of how fly casting works.

When the caster exerts a force upon the fly rod it will bend. The bending or loading of the fly rod is really a storing up of energy known as POTENTIAL ENERGY. We call it potential because it will later be transferred to the fly line as Kinetic Energy.

When the rod unbends it transfers the stored energy to the fly line in the form KINETIC ENERGY. Essentially all objects in motion possess kinetic energy. So to, the fly line moves with Kinetic Energy . The equation for kinetic energy is K = 1/2M x V2 where M = mass and V = velocity. In his book “Modern Fly Lines” Bruce Richards states that,” The relationship between the whip and the fly line is obvious. During a cast, a fly line unrolls just as a whip does. The bottom part of the fly line ‘loop’, (the U shape of the line), becomes stationary and the top leg of the loop accelerates. As the mass of the moving part of the line,(the top leg), decreases, it accelerates. As the line accelerates, wind resistance increases geometrically, and the energy of the cast dissipates, slowing the line and making possible a delicate delivery. It seems odd that what allows a fly line to deliver a fly gently is actually a very rapid acceleration of the line.”

Last but not least fly casting is governed by the Law of Energy Conservation that states; Energy can neither be created nor destroyed, but only converted from one form to another. Basically energy from the caster is applied to the rod. The fly rod in turn, because it is flexible third class lever stores energy by bending at the tip first, in resistance to the inertness or mass of the fly line holding it back. As acceleration increases the resistance of the fly line will cause the rod to bend further down from the tip and store more energy, until we stop applying force to the rod. Once the caster stops applying force to the rod it unbends or unloads and transfers the stored energy of the rod to the now kinetically charged energy of the fly line as it flips over the rod tip and begins to form the loop, a standing wave propagation if you will, something like the waves created by a wave machine at a water park only much more fun with the fly rod. Well at least to me it is.

So there you have it, the basic physics and principals behind fly casting. While this information has no real practical use for teaching someone to fly cast you may find an inquisitive mind out there one day who just had to know how it all comes together.

P.S Our longest casts would be in a vacuum. Who said vacuums weren’t any fun?

Simple Definitions

1. Force – Agency of change.

2. Energy- A measure of change imparted to a system. The capacity to do work

3. Velocity – Speed with direction.

4. Motion – Velocity

5. Inertia- resistance to motion
6. Acceleration – The speed of the speed.

7. Mass- the amount of matter of an object as determined by the objects inertia.

8. Power- The rate of energy transfer.

9. Work – A necessary evil to be avoided at all cost.