In a radio, amplitude refers to encoding a wave with information by:

a) adjusting the height of the waves
b) adjusting how many waves are produced per second
c) turning the wave on and off
d) changing the medium in which the waves travel

In radio communication, "amplitude modulation" refers to encoding a wave with information by adjusting the height (amplitude) of the waves. (a)

The vertical axis represents the velocity of the object. This probably sounds obvious, but be forewarned—velocity graphs are notoriously difficult to interpret. People get so used to finding velocity by determining the slope—as would be done with a position graph—they forget that for velocity graphs the value of the vertical axis is giving the velocity.

Try sliding the dot horizontally on the example graph below to choose different times and see how the velocity changes.

Explanation: