[Luke Wren] just wrote in to tell us about his new science blog called Wren’s Tech — it’s only a few days old, but he’s already got some pretty cool science experiments written up! Like how to estimate the muzzle velocity of a BB gun using just a voice recorder, and a curtain!
There are many different ways you could do this. One of the easiest is using a high-speed camera with a known grid or pattern as the background — like how Mythbusters does it. Unfortunately, high-speed cameras are usually out of reach for most hobbyists. [Luke] explains a rather cool system you can build with some electronics, whereby you have two thin wires a known distance apart — run current through both and use a circuit that can detect the interrupt as your projectile breaks the wires — or, you can use a voice recorder.
His method is by far the easiest and can be done by pretty much anyone. By using sound, you can record the shooting of the BB gun at a known distance into a curtain, or wall, or anything really. Take the sound clip and measure the time between the two sounds — the firing and the impact. It’s not perfectly accurate but it’s a pretty good estimation of the average speed of the projectile: 33.6-38.3m/s.
Unfortunately, that’s just the average — not muzzle velocity. By making a few assumptions about drag force and doing a few calculations, [Luke] was able to calculate the approximate muzzle velocity of 63m/s, with the kinetic energy of about 0.22 Joules. While that may seem fast, by the time the BB hits the curtain 4 meters away, it’ll have a mere 1/9th of its initial energy. Ouch.