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Bifilar ballistic pendulum and accelerometer
Cool  one more experiment...

For bifilar ballistic pendulum (the geometry presented in most textbooks) it is possible to use an accelerometer for graphical visualization of its oscillations upon a bullet impact and, thus, determination of the bullet speed. The sensor does not rotate and its measurements of the horizontal acceleration are not affected by the gravitation. Another advantage of the bifilar pendulum is independence of the result from the exact location of the bullet impact.

The photo shows the pendulum (a carton tube with double-V suspension to a support), the children spring gun and the projectiles. The sensor tag CC2650STK of Texas Instruments is visible inside the pendulum bob at the left. The projectiles are captured at the right.

The given phyphox program isolates the acceleration measurements after the impact and determines the acceleration amplitude and the pendulum frequency. Using the equation shown in the final graph (Python) it calculates the bullet speed.

The result is verified by a slightly modified version of the phyphox acoustic stopwatch which measures the time between the shot and the impact to a target placed at 3 m from the gun. The accordance is reasonable.

There is A PROBLEM: phyphox does not have a fitting block and FFT is used to determine the oscillation frequency. The result is not very precise even if a lot of oscillations are registered. To the contrary, Python fit by a sinusoidal function can determine the frequency just from some swings with a very high precision (the last figure).

More details can be found in a following paper. Any comments are welcome as usually.


More attachments

Attached Files
.phyphox   Ballistic pendulum 3.phyphox (Size: 15.56 KB / Downloads: 401)
.phyphox   Velocity using Acoustic Stopwatch.phyphox (Size: 69.1 KB / Downloads: 353)

Messages In This Thread
Bifilar ballistic pendulum and accelerometer - by solid - 06-14-2020, 07:38 AM

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