You just managed to land your plane on a deserted island. The field is so
short that you were barely able to bring the plane to a stop without
running into the trees. Now you wonder whether you will be able to leave.
Is this field long enough to take off?  You know precisely how much runway
is needed for a takeoff with this plane, but you don't have a measuring
tape or a yard stick to measure the field. 

After a while, you realize that you do have a piece of string and a stop
watch. You tie a small object to the end of the string and make yourself a
pendulum. You know that on earth the gravitational acceleration, g, doesn't
vary much and that 9.8 m/s^2 is a fairly accurate value of g for
intermediate latitudes. Now you can find the length of your string and then
use the string to measure the field! 

To simulate this situation I have hidden the vertical scale and I will also
change that scale and the pendulum each time you do the assignment. Measure
the period carefully, remembering that the simple pendulum formula is very
accurate as long as you keep the oscillation amplitude smaller than about
20 degrees. Calculate the length, check your measurement and your
calculations and then press F2 to enter your answer.