Selfpropelling

Self-propelling refers to motion generated by an object using energy carried or harvested by the object itself, rather than relying on external forces. It covers systems and organisms that produce thrust or propulsion through internal energy conversion or interactions with their surroundings, in accordance with Newton’s third law.

In nature, self-propelled movement is ubiquitous. Bacteria and algae move with rotating flagella powered by chemical energy, while many cells migrate by reorganizing their internal cytoskeleton. Animals such as snails, fish, and birds use metabolic energy to drive muscles that push against a surrounding medium or air to produce forward motion.

In engineering, self-propulsion includes devices that move under their own power, from rockets and jet engines to electric vehicles and autonomous robots. Propulsion mechanisms range from expelling mass (chemical rockets or jet propulsion) to converting electrical energy into mechanical work via motors that drive wheels or propellers. At small scales, self-propulsion also encompasses micro- and nano-scale swimmers that harness chemical reactions or external fields to generate motion.

Key considerations for self-propulsion involve efficiency, controllability, and the source of energy. The performance of self-propelled systems depends on the medium (air, water, or other fluids), energy density, and the ability to direct thrust. At nanoscale, viscous forces and thermal fluctuations pose additional design challenges.