I've looked at the following problem several times and have gotten stuck. The Problem ----------- An earthquake-produced suface wave can be approximated by a sinusoidal transverse wave. Assuming a frequency of 0.50 Hz (typical of of earthquakes, which actually include a mixture of frequencies), what amplitude is needed so that objects begin to leave contact with the ground? My Thoughts So Far ------------------ To leave the ground acceleration must be 9.8 m/s^2. The equation of motion is x = A cos 2 Pi f t, where A is amplitude, f is frequency and t is time. I'm not sure how go from this point to get the equation for acceleration. What's the next step?