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How to Calculate the FPE

FPE, or foot pounds of energy, is a means of measuring and calculating the power of a projectile, such as a dart, airsoft pellet, paintball or bullet, at any given point on its path. Calculating FPE is not difficult, and the method can even be used to the calculate faster-than-the-eye objects such as bullets. The key to the process is deriving a velocity by defining a constant distance and timing the projectile from its exit to the target. Once the velocity is found, calculating the FPE of a projectile is virtually a plug-in-the-numbers problem.

Things You'll Need

Airsoft gun/dart/slingshot
Ammunition/projectiles
Tape measure
Stopwatch
Calculator
Electronic scale

Instructions

- 1
  Start with a slower moving, easier to measure projectile.
- 2
  Measure a certain distance (use feet) from the target.
- 3
  Mark the distance with tape or a marker so it is a measured constant.
- 4
  Count down before each shot to synchronize the start of the stopwatch.
- 5
  Stop the watch when the projectile reaches its target, either by watching the projectile or listening for the sound of impact. Use the stopwatch to time at least 5 different shots.
- 6
  Add the times and divide by the number of shots to produce an average time in seconds.
- 7
  Divide the distance traveled by the time to get feet per second. This is the velocity of the projectile. So, a dart that travels 15 feet in 3 seconds has a velocity of 15 / 3 = 5 feet per second.
- 8
  Weigh at least 5 of the projectiles. Add the weights together and divide by the number of projectiles weighed for an average mass in grams.
- 9
  Plug the values for mass and velocity into the equation: ((Mass) x (Velocity)^2) / 450240 (the gravitational constant) to calculate the FPE of the projectile. So, a projectile with a mass of 2 grams traveling at a velocity of 5 feet per second would equal (2 x 5^2) / 450240 = (2 x 25) / 450240 = 50 / 450240 = .000111 foot pounds of energy.

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