# Need help using PocketLab as projectile

I'm new to the PocketLab and am struggling to get the data I want from it. My students are creating catapults and we want to use the PocketLab to record the muzzle velocity (the speed in the direction of travel) at the moment the PocketLab leaves the machine. Alternatively, knowing the speed in all three (x, y, z) directions would also work. It's not clear what I'm getting when I use the Projectile Speed graph, but it doesn't seem to be what I want. Any help, advice, or sharing of resources would be great.

Original Post

Hi Beth,

Thanks so much for your question and your support of PocketLab!

The Projectile Speed graph can be a little tricky. Basically it calculates speed from the accelerometer. The PocketLab calculates speed using just the accelerometer and the application is meant for projectile motion experiments. If you place the PocketLab sensor inside a ball and throw it up in the air, you can approximately measure the projectile speed. You can combine the calculated speed graph with the altitude and accelerometer graphs to compare altitude vs. time, speed vs. time, and acceleration vs. time.

Many of our beta tester teachers really wanted a way to illustrate the connection between position, speed, and acceleration. So we implemented a simple way to calculate speed for the specific use case of projectile motion. Here is a youtube video of an example of the speed graph being used: http://support.thepocketlab.co...-motion-of-an-object

In other applications, the speed graph will most likely drift significantly because of readings in the accelerometer.

In your use case, it does seem like there would be too much drift to calculate the velocity of the muzzle at launch. The speed graph would be better for approximating the speed of the projectile itself as it travels through the air. You could quantify the muzzle at launch through just the acceleration graph. That would also give you a way to quantify direction (x, y, z).

I'll post a more thorough explanation from our CEO on the difficulties in calculating speed.

Let me know if you have any additional questions.

Thanks!

Robby

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Hi Beth,

There is actually one other thing you could try to get the linear velocity of the muzzle at launch. If you place the PocketLab on the basket of the catapult and then measure the angular velocity during the launch you can derive the linear velocity if you know the radius (length of the catapult arm).

You would just use the equation v = ω r where v is linear velocity, ω = angular velocity, and r = radius.

Let me know if you have any other questions.

Thanks!

Robby

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Hi Beth,

One last thing. If you are finding the linear velocity from the angular velocity like I described above, you would need to make sure you are using radians to measure angular velocity.

Right now, PocketLab just uses degrees/second and RPMs for angular velocity units, so you would need to first convert the data to radians before plugging into v = ω r.

To convert from degrees to radians, multiply the degrees by pi/180.

Let me know if you have other questions.

Thanks again!

Robby

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