When you fire a 9 millimeter handgun

When you fire a 9 millimeter handgun straight into the air, its bullet can leave the barrel at around 350 m/s, carrying about 500 joules of kinetic energy. As it rises, gravity and air resistance slow it down, so its kinetic energy keeps decreasing. At its highest point, the bullet briefly reaches 0 m/s, meaning its kinetic energy is 0 joules. But because it is high above the ground, it now stores gravitational potential energy. Gravity then accelerates the bullet downward, converting that potential energy back into kinetic energy. Air resistance limits its falling speed, typically to around 50 to 70 meters per second. Ignoring air resistance, gravity accelerates objects at 9.81 meters per second squared, so after one second the bullet would be moving about 10 meters per second, after two seconds about 20 meters per second, and after 3 seconds about 30 meters per second. But in reality, as the bullet gets faster air resistance gets stronger, so each second adds less speed until it approaches its terminal velocity. At around 50 to 70 meters per second, a falling bullet can still cause a fatal injury if it strikes someone.