The solution at revision #20660 of , by Valter. This is not the current version.

Statement

3.4.19. The graph of coordinate versus time for a motion that is the sum of two harmonic oscillations is shown in the figure. Use it to determine the amplitudes and frequencies of these oscillations.

For problem $3.4.19$
For problem

Solution

1. Extracting Amplitudes from the Graphical Envelope
The given graph illustrates a classic beating phenomenon resulting from the linear superposition of two harmonic oscillations with nearly equal amplitudes and slightly different frequencies ().
The general equation describing the displacement of such a combined system is:

Looking closely at the graph parameters:
— Maximum Envelope Peak (): This occurs when the two individual oscillations interfere constructively:

— Minimum Envelope Neck (): This occurs when the two components interfere destructively, opposing each other:

We can solve this simple algebraic system to isolate the distinct amplitudes and (assuming ):

2. Extracting Frequencies from the Graphical Periods
The graph defines two specific time intervals on the horizontal axis:
— (Period of Fast Oscillations): This corresponds to the time it takes to complete one full cycle of the rapid inner wave. It is directly tied to the average carrier frequency:

— (Beat Period): This represents the time interval between two consecutive minimums (necks) of the slow amplitude envelope. The beat frequency is related to the difference between the two frequencies:

3. Mathematical Combination to Isolate and
From our period relations, we can construct expressions for the sum and difference of the target frequencies:

To isolate , we add the two equations and divide by 2:

To isolate , we subtract the second equation from the first and divide by 2:

Converting to cyclical frequency yields:

Answer