A mass-spring system comes into free vibration if the mass is once displaced from its resting position and then allowed to move on its own; forced vibration takes place if the system is continuously kept in motion by an external force. In Fig. 1.13, we see the difference between a free and

Consider the resultant wave at the points x = 0 m, 3 m, 6 m, 9 m, 12 m, 15 m and notice that the resultant wave always equals zero at these points, no matter what the time is. These points are known as fixed points (nodes).