## 3 thoughts on “3.5”

1. Sarah Yager says:

Maybe I'm not looking at the right problem, but the natural frequency solved for here is different than the answer in the back of the book. In the book, K1=10 kN/m, K2=15kN/m, and m=50kg. When I solve, I get .000707 rad/sec, and the book gets, like, 3.56 Hz, so I'm a little confused.

1. tutorpaul says:

Watch out for your units! To calculate omega_n our units need to be consistent which means our stiffnesses need to be in units of N/m rather than kN/m. When we use $k_1=10000$ N/m and $k_2=15000$ N/m we find our natural frequency to be 22.361 rad/sec just as I indicate in the video. If we convert that value to Hertz (by dividing the rad/sec quantity by $2\pi$ we find 3.559 Hz as the book indicates!

Always be careful of units, Dr Childs (as well as all the other professors for this course) likes to put inconsistent units on problem statements because it is an easy thing to overlook in our haste to solve the problem.

2. ledrone says:

This problem is the same as 3.5 in the 11th edition