Physics of Dipnetting


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The Physics of Dip Netting

Introduction to Dip Netting

Dip netting for salmon in the Copper River is a lot of fun but is also a lot of work.� During the summer months salmon will leave their home in the ocean and travel upstream in the Copper River to spawn in its many tributaries.� Near the small town of Chitna the Copper River flows through a narrow canyon which greatly increases the speed of the river.� This makes it harder for the salmon to swim upstream.� However the canyon also creates back eddies near the shore in which the river will actually flow the opposite direction.� This is good and bad news for the salmon.� Good news because the back eddies are flowing the direction the salmon want to go which makes their trip a lot easier.� And bad news (from the salmons point of view of course), it makes the salmon easier to catch because the water is flowing the ideal direction for dip netters as shown in the pictures below.

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Notice that the back eddie makes it really easy for the dip netter.� If there was no back eddie the current would push the net the other direction, which makes dip netting a lot harder.

The Physics

The physics of dip netting is really quite simple.� All a person has to do is find a back eddie with a nice constant current and hold the net underwater in the hopes a salmon will swim into it.� The physics then becomes a static equilibrium problem which means that none of the parts are moving in any way either in translation or in rotation (applies only to reference frame used) (Halliday 307).� This is illustrated in the picture below.



The dip net pole can be compared to a lever of class 1 and the lever principle can be applied, similar to the applet at http://www.walter-fendt.de/ph11e/lever.htm.� As stated in the applet from the Contemporary College Physics Simulation Library a lever is in balance if the total left side torque is equal to the total right side torque.� Applying that statement to the picture above the person must apply a much greater force on the pole in order to maintain torque equilibrium because the distance from the pivot point is much less than the distance from the force of the current to the pivot point.� This can be expressed mathematically.

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F1D1 = F2D2.������� �(where F is each force, and D is the distance each force is from the pivot point)

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So what this equation basically says is the guy in the picture is going to have to pull really hard on the pole to keep equilibrium even if the force of the current is not very strong because D1 is so much larger than D2.� Also notice all of the forces act on a perpendicular angle to the pole so there are no sines or cosines in the equation.� This is why I usually turn lazy after like 30 seconds of dip netting and try to wedge my pole on a rock which makes the difference between the D1 and D2 values smaller or even better yet the D2 value bigger than the D1 value.�



Another common method of dip netting is to let the bottom of the net rest on the bottom of the river. ��This creates another torque around the axis of the dip net handle as shown in the picture below.� The torque created by letting the dip net rest on the bottom is relatively large compared to the amount of torque a person can comfortably apply to the handle alone; so a cheater bar is attached to the pole which allows the person to easily maintain a balance of torques.� All the cheater bar does is increase the distance from the axis of rotation which in turn lowers the amount of force a person has to apply.� As seen in the equation

���� ������Torque = Force * Radius� ����������(Where Radius is the length of the cheater bar)

It�s the same with screwdrivers.� The nice big fat ones are like the pole with a cheater bar, and the tiny little ones are like the pole without a cheater bar. �And that�s pretty much all the physics involved with dip netting.



Bibliography



Fendt, Walter. "Lever Principle." �Contemporary College Physics Simulation Library. 27 Dec. 2002. 5

Nov. 2007 .



Halliday, and Resnick. "12-2 Equilibrium." Fundamentals of Physics. 8th ed. 2008. 306.



Janes, Jeffrey, Kendra Penton, and Kyle Hiscock. "Classes of Levers - First, Second and Third."

MCA. 5 Nov. 2007 .



"Lever." Wikipedia, The Free Encyclopedia. 5 Nov 2007, 17:41 UTC. Wikimedia Foundation, Inc. 6

���� Nov. 2007 .


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