[kitty123000]

hi everyone~~ it's my first time here^^
I Need help for homework~~! :confused:

I just did a lab which uses the Vernier Motion Detector to find the terminal velocity of a filter when it is released from the rest right below the detector to the ground, and here's the question: why does the motion detector has a limited range from 0.5 to 4.5m? And what's the reason for each end of the range?
guys thank you so much for helping me!! ;)

[asterisk_man]

I honestly can't say for sure so use your best judgement.
From what I've seen online Vernier Motion Detector uses ultrasonic pulses to measure distance. The sound frequence is about 40kHz. Speed=frequency*wavelength and speed of sound in air is about 343m/s so the wavelength is about 0.008575m. Therefore the wavelength is clearly not the reason for the minimum range. I'd say the reason for the minimum range is that the detector hardware can not measure the time between the transmission and receipt of the pulse any faster. At 0.5m the pulse will travel 1m total. At 343m/s that's 2.9ms. Its clearly a limitation of the hardware, not of physics.

My guess for the max range is that the sound will attenuate after 4.5m*2 to a level which is too low for the detector to detect. Again, a limitation of the hardware, not of physics.

I'm hoping that others here can post their thoughts since mine are based on hunches.

good luck!

[Capuchin]

I believe this is all to do with attenuation.

if the "signal" at 0.5m is 10, then the signal at 4.5m is



i.e.. Over 80 times smaller, and this doesn't even take into account attenuation of the signal, just the spherical spreading.

I believe that the issue is that the device has to be sensitive for the far away measurements, but would be flooded out at the closest distance. This puts a cap on the maximum and minimum distance (they would likely need to balance it out to find a reasonable range, and stick with it).

I found this device: http://www2.vernier.com/booklets/md-btd.pdf

This has a larger range - 15cm to 6m, in the data sheet it states that they change sensitivity over time so that they can intercept both the smaller and larger signals. So when it first sends out the ultrasound, it would have minimum sensitivity because it would expect a large signal, then as time passes it would become more and more sensitive, because a small signal would be expected.

This would seem to agree with my analysis above.

Hope this helps.

[kitty123000]

ohhhhh, thank you so much for helping me out! And that makes sense!
also I found another device that make help to have a deeper understanding of how the motion detector works, take a look at it if any of you guys are interested:
http://www2.vernier.com/pdf/polaroid.pdf

also, I have another question, do you know what would be the curve looks like in the acceleration versus velocity graph? (including the air resistance when an object is falling)

Thanks again^_^

[Capuchin]

Wow great research :)