added lab 3 to main repo

README.md

@@ -25,6 +25,24 @@ then follow the link to the class server.*
 
 # ME 3263 Introduction to Sensors and Data Analysis (Fall 2018)
 
+## Lab #3 Measuring Natural Frequencies
+
+### What are natural frequencies
+
+In free vibration (i.e., no external forcing), structural components
+oscillate at specified frequencies or combinations of frequencies. Since
+these vibrations are unforced, the associated frequencies are referred
+to as natural frequencies; it's how the system vibrates if left to
+behave on its own. In contrast, driven linear systems vibrate at the
+driving frequency. An amplification of the response (called resonance)
+occurs when the driving frequency coincides with one of the natural
+frequencies. In short, the system is driven at a frequency at which it
+likes to vibrate. Large amplitude oscillations are the result. So it is
+important to know what the natural frequencies are *a priori* so you can
+avoid driving the system into resonance.
+
+[Lab 3 github files](https://github.uconn.edu/rcc02007/ME3263-Lab_03.git)
+
 # Lab #2 - Static beam deflections with strain gage
 
 ## What is a Strain Gage?