The ultrasonic is for us to hear it.

The ultrasonic wave it to high of a frequency  for us to hear it.

Explanation:

A ultrasonic wave is used for a ultrasound to look at a fetus. The wave is way up on the frequency level for hearing.

force is mass*acceleration.

we have a very small mass: 11.4*0.001 kg

if we want 0.035n of force, we need an acceleration:

0.035 = 11.4*0.001*a

a = 0.035/(11.4*0.001) m/s^2

not too sure thinking all accelerations are pointed down