Introduction
A wide variety of everyday activities involuntarily applies fundamental laws of physics. While fishing, from casting to adding bait, physics is always at work. Even when fish latches on to the bait, it makes use of the water currents to its advantage to get away. Thus, pulling a fish in against the current requires a certain tension on the string to overcome the drag exerted by the fish, a particular tensile strength to ensure the string does not break, etc. The Laws of Motion, Newtonian Mechanics, etc. are continuously at work during fishing. This paper takes an intricate look at the physics behind the different aspects of fishing such as the fishing pole, the downrigger, clips, and eventually bringing in the fish.
The Constituents
The downrigger is a simple machine which consists of an extended arm, a hand crank and a pulley. When the handle is cranked in a circular motion, the wire is spun into the spool with the help of the pulley. The wire consists of a piece of deadweight which is used to guide the bait down to an optimal depth. A fishing pole is similar to a downrigger with a spool, a hand crank and an extended arm. The rod is flexible and is able to bend to different degrees of stiffness. This mechanics of the pole is what differentiates it from the downrigger. The clips are added to the end of the line and are held to the fishing line through clamps.
Usually while fishing out in a lake or a large water body, the boat is always in motion with the help of a motor. The drag caused by the motor propellers through the water causes the bait to move in a particular way which attracts fish. The piece of lead deadweight attached to the end of wire helps in determining the depth through which the wire has travelled. The clip holds the bait which, in turn, holds the fish. Once the fish takes the bait, the rod and the downrigger are no longer interdependent. While the rod deals with the lead tip, the downrigger is used to reel in the fish.
The system consisting of the fishing pole, the wire and the bait are initially in a state of equilibrium until the fish takes the bait and tugs the wire causing an external force to act on the system. Consider the lead deadweight attached to the end of the wire. This deadweight is immersed into the liquid and experiences a drag force. There is no spin on the ball because of the reaction forces. The drag acts opposite to the velocity of the boat and the force of gravity acts vertically downwards.
The Downrigger
When the system is in equilibrium, the tension on the wire acts at an angle to the level. Thus, the tension has both a horizontal and a vertical component. The vertical component counteracts the force of graity while the horizontal component is responsible for moving the system forward. The force of gravity acting on the ball is given ball is given by Newton’s Second Law as F=m×a where m is the mass of the ball and a is the acceleration due to gravity which is 9.8m/s2. The force exerted on the ball due drag is given as F=K×v2 where K is the drag constant the value of which is obtained experimentally, and v is the velocity. Theoretically, the value of K is directly proportional to the surface area of the lead deadweight and the density of fluid (Robson 1990).
The Rod and Reel System
The rod and the reel, however, do not experience any drag since they are above water. The rod and reel system is, usually, analysed when the downrigger is not taken into consideration since the downrigger is a rigid body. Since the rod is flexible, it can be considered to be a type of spring, giving it a few significant features. It is important to reel the sack while setting up the system from the tip of the rod to the clip beneath so that the tension on the string is maximized. It is imperative to limit the slack. The elasticity of the fishing line and the flexibility of the rod help in this process.
Let us assume that the system is initially in equilibrium. However, when the fly is cast, the rod assumes a bent, parabolic shape. Thus, since the rod is disturbed from its own equilibrium, it possesses a potential energy which is enhanced by the tension in the fishing line. Besides the potential energy, an external force is also exerted on the fishing line. The potential energy is given as
PE= 12 ×k ×x2 and the external force is given as F= -k ×x where k is the spring constant which is evaluated experimentally and x is the displacement from the equilibrium position. For the rod, this value is the distance the tip travels to make the rod bent. Even though fishing rods are not springs, they can be modelled on similar terms because of many similar features.
Different rods have different rigidities. The extent through which the rod bends depends on its stiffness and the weight attached to the end of the line. In the system under consideration, the rod applies an upward force, while the fishing line exerts a tension which is in the downward direction. The rod is not solely responsible for counteracting the tension on the string. The reel also shares this responsibility. It is important to note that the rod also acts like a pulley system that redirects the tension on the wire into the reel (Spolek 1986). This tension creates a torque on the wheel given as:
=r ×Fwhere r is the perpendicular distance from the force to the center of the couple. And F is the tension on the line. The torque is a vector quantity obtained out of the cross product of r and F. Since there is no net movement of any component of the fishing system, the system is still in equilibrium and the net torque is zero.
The Fishing Line and Bait
Besides the rod and the reel set up, an important constituent of the fishing system is the fishing line and the bait. The line is made of monofilament line which is elastic and hence spring-like which fits the equations cited earlier. The line is not affected by the fluid drag. However, since the bait is attached to the clip onto which the deadweight is attached, it does experience the drag. In fact, anything that is below the liquid surface will experience a drag (Gatti-Bono et al. 2004).
The tension in the line above the clip is not the same as the tension on the line below the clip. This is because above the clip, the reel has cranked down on the line and is a contributor to the tension on the line while after the clip, the tension on the line is solely due to the drag acting on the bait in the opposite direction. However, it is important to note that since the system is in equilibrium, the total tension on the line and the potential energy stored in the line are equal and oppositely directed.
Taking the Bait
What happens when a fish eventually takes the bait? The system gets disturbed and thrown out of equilibrium. A force is applied on the bait in the opposite direction which increases the tension on the string. When the tension on the line exceeds the maximum force of the clip, the tension on the fishing line goes to zero, the fishing line shortens its length due to its elasticity, and the downrigger gets detached. This is when a fisherman reels the fish in. This process starts because the potential energy stored in the system and now the hard part of reeling the fish in begins.
Conclusions
Every time you go fishing, you use physics. From casting out a line which uses principles of projectile motion, to the forces of gravity acting on the system that gives the line both its horizontal and vertical components of velocity and the acceleration due to gravity which pulls the weight down, there is physics always being applied. Tensile forces on the fishing line, buoyancy which determines whether the bait sinks or floats, the dependency of the deadweight on the currents in the fluid. Another important physical tool that is used in understanding different concepts of velocity, direction and the different kinds of forces is vector mathematics. Vector cross product that determines torque, vector addition which determines the direction of the boat when there are undercurrents, resolution of the velocities into both horizontal and vertical components, etc. all apply basic postulates of physics.
References
Robson, John M. "The physics of fly casting." American Journal of Physics58.3 (1990): 234-240.
Spolek, Graig A. "The mechanics of flycasting: The flyline." American Journal of Physics 54.9 (1986): 832-836.
Gatti-Bono, Caroline, and N. C. Perkins. "Effect of loop shape on the drag-induced lift of fly line." Journal of applied mechanics 71.5 (2004): 745-747.
