Jumping Jellyfish |
Building a background
Jellyfish
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Jellyfish, of the phylum Cnidaria, have floated through the Earth's oceans since prehistoric times. Such longevity is not caused by the creatures' anatomy, but rather in spite of it. In essence , a jellyfish is little more than a membrane and a singular gut. In the absence of internal organs, the gut functions as the center of all digestive, waste, and reproductive processes. The membranous "bell" of a jellyfish contains two layers of cells: the gastrodermis and the epidermis. Nutrients and oxygen pass through the gastrodermis, while the epidermis hosts an extremely basic network of nerves. The jellyfish is held together by mesoglea, a water-based gel containing muscle cells and structural proteins. Anatomically, jellyfish are categorized as having either a medusa body plan, in which there is an inverted bell with tentacles attached inside, or a polyp body plan, in which the mouth and tentacles flow up and away from the bell.
Jellyfish are often content to utilize passive motion, in which they are moved throughout the ocean by underwater currents. Such movement, however, is not controlled by the jellyfish; the speed with which, and direction in which, the jellyfish moves is left entirely to chance. While pursuing food sources, such as krill and crustaceans, avoiding predators, or purposefully migrating, jellyfish need an alternate mechanism of motion. The solution is an aquatic form of jet propulsion, or simply "jetting". Jetting relies on the principles of Newtonian Mechanics, as is investigated throughout this site.
Jellyfish are often content to utilize passive motion, in which they are moved throughout the ocean by underwater currents. Such movement, however, is not controlled by the jellyfish; the speed with which, and direction in which, the jellyfish moves is left entirely to chance. While pursuing food sources, such as krill and crustaceans, avoiding predators, or purposefully migrating, jellyfish need an alternate mechanism of motion. The solution is an aquatic form of jet propulsion, or simply "jetting". Jetting relies on the principles of Newtonian Mechanics, as is investigated throughout this site.
Physics Concepts
Fundamental to the field of physics are Sir Isaac Newton's Laws of Motion. In general, the Laws of Motion describe the relationships that exist between mass, acceleration, and force.
- First Law: Unless an unbalanced force is applied, an object at rest will stay at rest, and an object in motion will stay in motion. The law dictates the concept of inertia, a mass-dependent tendency of an object to resist a change in its motion. From the First Law of Motion, it can be determined that, when all forces acting on an object are balanced, the object either remains at rest or remains in motion at a constant velocity.
- Second Law: When a force acts on a mass, the object accelerates. Acceleration is directly proportional to force and indirectly proportional to mass. Mathematically, this Law can be stated as Force = Mass x Acceleration.
- Third Law: For every action, there is an equal and opposite counter-action. Forces exist in pairs that are equal in magnitude and opposite in direction. Colloquially, the object upon which one pushes pushes back with the same force.
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Propulsion, which refers simply to the act of pushing something forward, is an essential application of Newton's Third Law. An object is propelled when the surface upon which it exerts force exerts an equal and opposite force on the object.
In realistic application to the material world, propulsion systems are necessary to catalyze the force pair that will move an object forward. Such systems generate thrust, or the mechanical force in the direction of desired motion that acts on an object to propel it forward. For example, jet engines, which epitomize propulsion systems, accelerate, pressurize, and emit gases in the direction opposite to the desired motion direction. Considered in terms of two dimensions, an airplane that is flying in the right-hand direction emits jets of accelerated gas to the left. Conversely, an airplane that is moving in the left-hand direction emits jets of accelerated gas to the right. The gas jets, considered part of the airplane, exert a force on the air particles, which, in turn, exert an equal force on the airplane and propel it forward in the direction of motion. Airplanes and rockets utilize propulsion and propulsion systems to move forward and upward, respectively.
In relation to Newton's First Law of Motion, jet propulsion at constant velocities relies on an equilibrium of forces. The aforementioned thrust force must balance drag, its counterpart. Quite simply, drag is the force that acts in the direction opposite to that of motion. Regarding the common propulsion systems of jet and rocket engines, drag is primarily the result of friction amongst air particles and changes in air pressure. When a propelled object is moving at a constant velocity, the thrust and drag forces are balanced. Neither has a greater magnitude than the other, so the object does not accelerate in either the forward or backward direction.
At certain points in object's propelled motion, it is necessary for the thrust force in the direction of motion to overcome, or exceed the magnitude of, drag. Airplanes in the takeoff stage, for example, must overcome the drag of air resistance, while rockets must overcome the downward drag from gravity. Likewise, during periods of slowing or stopping, drag must be greater than thrust. According to Newton's Second Law of Motion, the propelled object will accelerate in the direction of the net force acting on it. Therefore, the propelled object accelerates when the net force is in the direction of thrust and decelerates when the net force is in the direction of drag.
In realistic application to the material world, propulsion systems are necessary to catalyze the force pair that will move an object forward. Such systems generate thrust, or the mechanical force in the direction of desired motion that acts on an object to propel it forward. For example, jet engines, which epitomize propulsion systems, accelerate, pressurize, and emit gases in the direction opposite to the desired motion direction. Considered in terms of two dimensions, an airplane that is flying in the right-hand direction emits jets of accelerated gas to the left. Conversely, an airplane that is moving in the left-hand direction emits jets of accelerated gas to the right. The gas jets, considered part of the airplane, exert a force on the air particles, which, in turn, exert an equal force on the airplane and propel it forward in the direction of motion. Airplanes and rockets utilize propulsion and propulsion systems to move forward and upward, respectively.
In relation to Newton's First Law of Motion, jet propulsion at constant velocities relies on an equilibrium of forces. The aforementioned thrust force must balance drag, its counterpart. Quite simply, drag is the force that acts in the direction opposite to that of motion. Regarding the common propulsion systems of jet and rocket engines, drag is primarily the result of friction amongst air particles and changes in air pressure. When a propelled object is moving at a constant velocity, the thrust and drag forces are balanced. Neither has a greater magnitude than the other, so the object does not accelerate in either the forward or backward direction.
At certain points in object's propelled motion, it is necessary for the thrust force in the direction of motion to overcome, or exceed the magnitude of, drag. Airplanes in the takeoff stage, for example, must overcome the drag of air resistance, while rockets must overcome the downward drag from gravity. Likewise, during periods of slowing or stopping, drag must be greater than thrust. According to Newton's Second Law of Motion, the propelled object will accelerate in the direction of the net force acting on it. Therefore, the propelled object accelerates when the net force is in the direction of thrust and decelerates when the net force is in the direction of drag.
Marrying the Two Together
Much like airplanes, jellyfish travel in a horizontal plane of motion using a form of jet propulsion. This horizontal motion, or motion along an x-axis, is made successful by the jellyfish's ability to use its simple body structure as a propulsion system. While "jetting", the jellyfish contracts its water-filled bell. As the bell contracts, the pressure within it increases. The increase in pressure accelerates the water out of the bell, thus forming a water jet that is pushed out in the direction opposite to the jellyfish's desired motion direction. As explained by Newton's Third Law of Motion, the jellyfish's jet of fluid exerts a certain force on the water particles behind it. In turn, the water exerts a force equal in magnitude and opposite in direction to that which was applied to it on the jellyfish. The jellyfish is propelled forward. The force of the jellyfish on the water and the force of the water on the jellyfish are a force pair, as they are equal and opposite to each other.
The thrust of the jellyfish's jet propulsion is generated by the bell's pressurization and acceleration of the water it contains. The drag that affects the jellyfish's propulsion system is in the form of resistance from the surrounding water in which the jellyfish is immersed. In general terms, every pulsation of the bell causes a 'motion arc'. The jellyfish transitions from acceleration to constant velocity to deceleration, eventually coming to a stop. At the stopping point, the jellyfish contracts its bell once again, and the motion cycle begins anew. When the jellyfish first begins its active motion, its thrust must overcome the drag acting on it as a still body. Accelerated movement forward occurs when the thrust produced by the pressurizing bell is greater than the drag of the surrounding water. According to Newton's Second Law of Motion, the net force, and therefore the acceleration, is in the direction of motion. Conversely, deceleration occurs when the drag force of the water exceeds the thrust., and the net force acts in the opposite direction of motion.
Both passive and active jellyfish motion can be related to Newton's First Law of Motion. During passive motion, as mentioned previously, a jellyfish simply remains immersed in the water and allows itself to move with the surrounding water. Disregarding currents for the purpose of this investigation, the jellyfish generally remains at rest. The only forces acting on the jellyfish in the rest stage are gravity and buoyant force. In the absence of a solid surface upon which to rest and the related normal force, buoyant force caused by the water displaced by the jellyfish pushes the jellyfish upward. The buoyant and gravitational forces balance each other. As there is no motion in the vertical, or y-axis, direction regardless of the jellyfish's motion, whether it be at rest, accelerating, decelerating, or at a constant velocity, the gravitational and buoyant forces remain at equilibrium. When the jellyfish reaches a constant velocity in the middle of its motion 'arc', the forces impacting horizontal, or x-axis motion, are also at equilibrium. The force of drag and the force of thrust balance each other.
The thrust of the jellyfish's jet propulsion is generated by the bell's pressurization and acceleration of the water it contains. The drag that affects the jellyfish's propulsion system is in the form of resistance from the surrounding water in which the jellyfish is immersed. In general terms, every pulsation of the bell causes a 'motion arc'. The jellyfish transitions from acceleration to constant velocity to deceleration, eventually coming to a stop. At the stopping point, the jellyfish contracts its bell once again, and the motion cycle begins anew. When the jellyfish first begins its active motion, its thrust must overcome the drag acting on it as a still body. Accelerated movement forward occurs when the thrust produced by the pressurizing bell is greater than the drag of the surrounding water. According to Newton's Second Law of Motion, the net force, and therefore the acceleration, is in the direction of motion. Conversely, deceleration occurs when the drag force of the water exceeds the thrust., and the net force acts in the opposite direction of motion.
Both passive and active jellyfish motion can be related to Newton's First Law of Motion. During passive motion, as mentioned previously, a jellyfish simply remains immersed in the water and allows itself to move with the surrounding water. Disregarding currents for the purpose of this investigation, the jellyfish generally remains at rest. The only forces acting on the jellyfish in the rest stage are gravity and buoyant force. In the absence of a solid surface upon which to rest and the related normal force, buoyant force caused by the water displaced by the jellyfish pushes the jellyfish upward. The buoyant and gravitational forces balance each other. As there is no motion in the vertical, or y-axis, direction regardless of the jellyfish's motion, whether it be at rest, accelerating, decelerating, or at a constant velocity, the gravitational and buoyant forces remain at equilibrium. When the jellyfish reaches a constant velocity in the middle of its motion 'arc', the forces impacting horizontal, or x-axis motion, are also at equilibrium. The force of drag and the force of thrust balance each other.
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Above: Footage of jellyfish motion captured with laser technology and a discussion of the implications of such technology (New Science Magazine)
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Above: Motion captured particle-footage demonstrating the contraction-expulsion propulsion system and locomotion of a moon jellyfish (Marine Biological Laboratory at the Woods Hole Oceanographic Institution)
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