# Talk:A bug in my high school physics intuition

Rocket ships are weirder than bicycles most of the energy you expend in accellerating isn't your kinetic energy, but the kinetic energy of your propellant: $E = p^2 / 2m$, $p^2$ is the same for the rocket and the propellant, but pointing oppositely, and for small thrust the propellant mass is much smaller. (For things like orbital inserters, this isn't true anymore; one needs Tsiolkovski's rocket equation; [[1]] e.g.). On a bicyle, your "propellant" is actually the earth, which means most of the energy expense *is* your own kinetic energy (because the earth is much heavier than you)