In a rollercoaster, the first hill is always the tallest. Subsequent hills are never as tall as the first. Why? What would happen to the rollercoaster cart
if the second, third, etc. hills were taller?

diagram is missing, but i will try to explain your doubt.

answer:

amplitude is the maximum displacement or distance moved on a point of a vibrating body or wave measured from its equilibrium position.it is equal to half the length of vibration path.

no. bullets are macro objects. wave nature is for light waves, extremely tiny particles. if a light particle was the size of your finger, the bullet would be the size of the universe. so basically, that huge difference in size makes the wave nature of matter insignificant.