Hi, First-timer here! I'm designing a physics project that involves pumping a certain pressure of air into an empty bottle. SETUP: The bottom of the bottle has a small hole through which a thumb tack fits tightly; the top (cap-side) has a valve through which the air is inserted. The bottle is then laid on its side lengthwise, and attached to a small toy car--removing the thumb tack releases a stream of concentrated air from the orifice, which acts as a force of thrust propels the car. All in all, the setup is pretty similar to that seen here. I'm trying to find the velocity of the air at the orifice as well as within the bottle, assuming bottle pressure po and standard atmospheric pressure outside. I also wanted to determine whether or not the velocity at the orifice would be constant over a period of time given the release of air and loss of pressure. I've researched the topic but what I've found seems to apply more to actual rockets with defined nozzles and throats. What equations would I use? I'm only in high school, so most of this except the basic motion/forces/thrust equations is entirely new to me. Thanks again for the help!