Why is the amplitude of the radio wave in figure 1 varied?

This is an example of amplitude modulation which is a method to transmit analog signals via radio-spectrum waves. A carrier sine wave (in the image, the one with the higher frequency) is modulated (in amplitude) by another signal (the target of transmission). The carrier due to its appropriate frequency enables successful transmission of a signal that would otherwise be difficult to transmit (due to its too low frequencies) and also enables transmission of multiple channels at different carrier frequencies.

Classic example: AM radio.

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.