Chapter 4 - Graphs of the Circular Functions - Section 4.1 Graphs of the Sine and Cosine Functions - 4.1 Exercises - Page 146: 55a

Amplitude$=5$ Period$=\frac{1}{60}$

For the graph of $y=a\cos x$, the amplitude is $|a|$. Therefore, for the case of $E=5\cos 120\pi t$, the amplitude is $|5|=5$. In addition, the period of the graph $y=\cos bx$ is $\frac{2\pi}{b}$. Therefore, for the case of $E=5\cos 120\pi t$, the period is $\frac{2\pi}{120\pi}=\frac{2}{120}=\frac{1}{60}$.