Scientists measure the velocity of projectiles, such as bullets, hurled rocks and javelins, using a device called a ballistic pendulum. A box on the ballistic pendulum absorbs the impact of the projectile and marks the amount of movement the projectile causes it to swing. Because the weight of the projectile and the weight of the box are predetermined, the amount of movement the projectile creates becomes the final variable. The three factors are then entered into a formula that, when solved, provides the velocity. The practice is useful for firearms marketing and a replica can be constructed at home, using common materials.
Things You'll Need
- 12-by-12-by-18-inch corrugated-cardboard box
- Duct tape
- Felt-tip marker
- 4-by-4-foot piece of plywood or thin cardboard
- 4-by-4-foot piece of white paper
Cut a 9-by-9-inch hole in one of the short ends of the cardboard box. Place duct tape across each corner of the box to strengthen it.
Place the corrugated-cardboard box on the scale and fill it with rocks until the scale reads exactly 4 lbs. Fill the remaining portion of the box with crumpled newspaper to act as a cushion when the box is struck by the projectile.
Secure one eye-hook in the top of each corner of the box in such a way that the eye-hooks will hold the box upright and steady. Cut the string into four 3-foot lengths and tie one to each eye-hook.
Poke a hole in one long side of the box large enough to push the felt-tip marker through but small enough to hold it firmly.
Tape the 4-by-4-foot sheet of white paper to the 4-by-4-inch plywood or thin cardboard, covering it completely. Mount the plywood or thin cardboard to a vertical surface, such as the studs inside of an unfinished garage. Hang the box from a horizontal surface, such as the rafters inside an unfinished garage, so that the felt-tip marker just reaches the paper when the box hangs steadily. You can adjust the marker as needed by pushing it farther in or out of the hole in the box.
Tips & Warnings
- To determine the velocity of a projectile, multiply the vertical rise of the box (measured in inches) by 43.9 and find the square root of the answer. Then divide 4 by the weight of the projectile (in pounds) and add 1. Multiply the two answers, and the velocity will be determined in miles per hour.
- Throw a potato or other harmless object at the box to test the ballistic pendulum. Homemade ballistic pendulums are intended for instructional use only and are not meant to actually measure the velocity of bullets fired from firearms. Also, do not throw edged blades or other dangerous objects at the box to avoid injury to any bystanders or damage to the ballistic pendulum.