Windshield wipers.
a grin
A swing, a pendulum, or a conversation are some examples of things that go back and forth constantly.
Rocking back and forth can be a self-soothing behavior that helps with stress or anxiety. It can also be a sign of autism or sensory processing disorder. If someone is constantly rocking back and forth and it is interfering with their daily life, it may be helpful to seek advice from a healthcare professional.
A rally, is when the ball constantly goes back and forth, between the two teams.
a grin
Particles in matter move back and forth due to thermal energy, which causes them to vibrate. This vibration is a result of the kinetic energy of the particles constantly changing direction.
"Flip back and forth" means to alternate or switch quickly and repeatedly between two options, ideas, or actions. It can also refer to constantly changing one's mind or position on a topic.
You can attach any object - a small stone, for example - to a piece of string.
The same way you find any shiny pokemon, either constantly run back and forth in grass that you want to encounter as a shiny, or constantly breed the pokemon until you hatch a shiny.
Greg, most of the time, doesn't get along well at all with his brother, and they have a series of dramatic events constantly going back and forth between them
In a swinging pendulum, energy is constantly being converted between potential energy (at the highest point of the swing) and kinetic energy (at the lowest point of the swing). The total energy remains constant, but it changes form as the pendulum moves back and forth.