Answer
0.85 kg
Work Step by Step
The spring constant stays the same.
Use equation 11-6b to relate frequency, mass, and spring constant.
$$f=\frac{1}{2 \pi}\sqrt{\frac{k}{m}}$$
$$k=4 \pi^2f^2m $$
$$ (0.83Hz)^2(m \;kg) = (0.60Hz)^2(m+0.780kg)$$
Solve for m = 0.85 kg.
