Answer:

Explanation:
We know that for a capacitor
, where <em>Q </em>is the charge of one plate, <em>C</em> the capacitance and <em>V</em> the potential between the plates.
We also know that
, since
is the surface charge density and <em>A</em> the area of the plate (both equal in our case).
Putting all together:

Which for our values is:

Where we notice that the S.I. units combination FV/C must not have units (we can verify it directly from their definitions or we notice that
is enough to describe an area).
Force of gravity on an object is the weight of the object and is given by mass times accerelation due to gravity. The accerelation due to gravity is the accerelation of an object in free fall and is given by 9.8m/s^2. Given that the force of gravity acting on a child's mass on earth is 490 newtns, i.e. F = mg which means that 490 newtons = 9.8 times mass. Therefore, mass of the child is 490 / 9.8 = 50 kg.
For a sound wave traveling through air, the vibrations of the particles are best described as longitudinal. Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport. They move the same direction as the wave.
Need more than that to answer this question