problem finding torque?

Question

A fishing pole is 2 m long and inclined to a horizontal at an angle of 20 degrees. What's the torque (t) exerted by the fish about an axis perpendicular to the page and passing through the hand of the person holding the pole?

Answer 1

The units for torque are a force times a lenght(lb-ft, newton-meters). This gives a clue of how to calculate it. You always think of torque as a rod attatched at one end to a rotating axis, with some kind of force(weight) pulling at a right angle to the other end of the rod, in the direction the axis will rotate. If the rod is a foot long and the weight pulling on the rod is 1lb at a right angle to the rod the torque is 1lb-ft. If the rod is 2 feet long and the force is one pound then the torque is 2lb-ft.
If the force is at an angle that is different from 90 degrees or is not exactly pulling in the direction of rotation then you have to figure out the portion of the force that is pulling in the right direction to make torque, which is at a right angle to the rod and in the same plane as the circle made by the rotating rod. Draw right triangles and use sines and cosines to figure this out. If the total force is not in the direction of rotation the torque force will always be somewhat less than the total force. This will give you some idea if you've done your math correctly.
In this case the axis of rotation is kind of tricky. Is the axis of rotation the end of the rod or the persons wrist? If they don't give you any dimensions for the person's hand then you would have to assume the axis is at the end of the fishing pole.

Answer 2

Need more information. How much force is the fish pulling, i.e. what is the tension in the line? At what angle is the fishing line from the end of the pole?

Answer 3

First you need to do some trig to figure the effective radius of the fishing line versus the center of rotation of the rod. With that you have the radius. Torque is force times radius.