How Fast Does It Go?

This page is reproduced from "Backyard Ballistics" by permission of William Gurstelle.

How Fast Does It Go?

You've probably noticed that many of the projects in this book involve a device that fires, shoots, hurls, or otherwise propels an object at a reasonably high velocity. After the novelty of the shooting experience wears off, inquiring boomers may want to experiment with the ballistic device called a "ballistic pendulum". With the pendulum, we can dig deeper into the mechanics of motion that govern all of the experiments listed in this book.

If you truly understand the mechanics of motion, you can answer all of the "how" questions -- how far will it fly?, how high will it go? how long will it travel?, and, most intruigingly, how fast does it go? Does the potato cannon shoot a spud faster than Randy Johnson throws a fastball? Faster than Tim Wakefield throws his knuckleball? Which is faster, a tennis ball shot from a mortar, or Bris Becker's crosscourt backhand? Does a walnut hurled from the tabletop catapult fly faster than the airspeed velocity of an unladen sparrow (African or European)?

The "how fast does it go?" question can be answered by using a ballistic pendulum. A ballistic pendulum uses two well-known Newtonian ideas -- the principle of conservation of momenturm and the principle of the conservation of energy -- to allow you to easily determine the velocity of almost any projectile. Ballistic pendulums measure the velocity of bullets, cannonballs, catapulted boulders, or just about any nonpowered flying item.