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 Author Topic: Measuring speed of falling object  (Read 677 times) 0 Members and 1 Guest are viewing this topic.
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 « on: January 07, 2013, 01:23:05 am » Bigger Smaller Reset

Hi,

My son has to do a science project for 6th grade.  We are thinking about dropping weighted ping pong balls from about a 6 six story height.  We would measure how fast each ball fell.  (air resistance would cause the lighter balls to fall slower).

Any ideas on how we could time the ball drop?

I am thinking about using two arduinos.  One would be at the top.  The ball would fall through a tube and I would use a photo sensor to detect when the ball dropped.  The top arduino would then fire a laser pointer to signal the bottom arduino to start a timer clock.  The bottom arduino would have an accelerometer attached to a table.  When the ball lands on the table, the accelerometer senses the impact and stops the clock.  I then have the start/stop time sent back to my laptop.

Anyone think this would work or have a better idea?  Would an accelerometer be able to sense this type of thing?

thanks
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 « Reply #1 on: January 07, 2013, 01:32:25 am » Bigger Smaller Reset

use one arduino cause they are more than capible and 2 optical sensors at the bottom

so one arduino, one trigger switch at the top, two "magic eyes" at the bottom for each tube and your done
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 « Reply #2 on: January 07, 2013, 01:37:08 am » Bigger Smaller Reset

Hmmmm you could probably use one Arduino with two microswitches like these to stop and start the clock.

But by measuring the time, you're not going to measure the speed as such... you'll have to infer the speed from the Equations of Motion in which you would need to assume a value of g. (Or measure g in another experiment, like measuring the period of a pendulum.)

(Bit unclear as to your objective... your subject says speed, but your opening para says how fast it fell, which could be just the elapsed time? Time's one thing, measuring the terminal velocity is another)
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 « Reply #3 on: January 07, 2013, 01:44:08 am » Bigger Smaller Reset

His teacher wants him to do a repeatable experiment and show effects of changing one variable.  So we want to show that changing the wait, changes how long it takes the ball to fall a specific distance.  So not really needing to measure the speed, just the time to fall over a specific distance.

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 « Reply #4 on: January 07, 2013, 01:50:49 am » Bigger Smaller Reset

You might be able to use a sound sensor to detect when the ball hits the ground/table/etc.
http://www.dfrobot.com/wiki/index.php?title=Analog_Sound_Sensor_%28SKU:_DFR0034%29
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 « Reply #5 on: January 07, 2013, 02:58:56 am » Bigger Smaller Reset

So we want to show that changing the wait, changes how long it takes the ball to fall a specific distance.

Well in theory, changing the weight (which is what you meant I guess   ) won't change the time, although with "floaty" objects it might.

Here's a much simpler experiment and involves nothing but the stop watch on your cellphone. Set up a simple pendulum, and show that the mass doesn't affect the time of the swing, only the length does.

Edit.... and while you're at it, verify that T = 2 pi (l / g)1/2 or conversely rearrange and use it as a means of showing what g is
 « Last Edit: January 07, 2013, 03:10:33 am by JimboZA » Logged

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 « Reply #6 on: January 07, 2013, 08:44:37 am » Bigger Smaller Reset

Go with Osgeld's idea, because not only can that way calculate fall time, but it can calculate the speed as well. The only downside is that you need to keep the PP ball within a decent range of the sensors, otherwise the data will be off.
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 « Reply #7 on: January 07, 2013, 09:13:33 am » Bigger Smaller Reset

wind may be a major factor here
hitting the target from more than a few feet may be nigh on impossible
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 « Reply #8 on: January 07, 2013, 11:08:38 am » Bigger Smaller Reset

If only average speed has to be determined, know height and time, only time should be measured accurately. I'd put arduino (smallest one?) inside a ball,  with two buttons, one to be released by operator when dropping a ball, second one 'd be pressed automatically at falling time ( feather tail would sure ball always fall on the same side).
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 « Reply #9 on: January 07, 2013, 11:10:46 am » Bigger Smaller Reset

I would think a wireless message from top to bottom would be easier to implement.
RF travels at speed of light too.
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 « Reply #10 on: January 07, 2013, 11:17:45 am » Bigger Smaller Reset

The easiset way is a good camcorder and video tape it. If you use DV you can get 1/30th second resolution.
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Measurement changes behavior
 « Reply #11 on: January 07, 2013, 11:27:40 am » Bigger Smaller Reset

Quote
My son has to do a science project for 6th grade.  We are thinking about dropping weighted ping pong balls from about a 6 six story height.  We would measure how fast each ball fell.  (air resistance would cause the lighter balls to fall slower).

No it wouldn't. The air resistance would be the same for a same size ping pong ball no matter what weight difference. Didn't Sir Isaac Newton already demonstrate that weight is not a factor in a falling object?

Quote
Any ideas on how we could time the ball drop?

Sure, optical sensors/emitters on the outside of a plastic tube or pipe that has a ID diameter just a little larger then a ping pong ball that is 'looking through' the pipe, using one sensor/emitter at near the top of the tube and one near the bottom of the tube. Arduino would time interval between start and stop, speed is just a math equation based on that elapsed speed.

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 « Last Edit: January 07, 2013, 11:30:13 am by retrolefty » Logged

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 « Reply #12 on: January 07, 2013, 11:31:38 am » Bigger Smaller Reset

No it wouldn't. The air resistance would be the same for a same size ping pong ball no matter what weight difference. Didn't Sir Isaac Newton demonstrate that weight is not a factor in a falling object?

I'm very surprised to see that from you retrolefty. Sir Isaac Newton's experiment only works when the effects of air resistance can be ignored. If you drop a feather and a cannon ball, the feather falls more slowly.

If the balls are being dropped through a pipe that is anywhere near the size of the balls then the aerodynamic blockage will cause a much higher drag than you'd seen in fresh air. This would have a much greater effect on very light balls such as a ping pong ball. You may also see mechanical drag if the balls start bouncing off the sides of the tube.
 « Last Edit: January 07, 2013, 11:34:46 am by PeterH » Logged

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Measurement changes behavior
 « Reply #13 on: January 07, 2013, 11:34:01 am » Bigger Smaller Reset

No it wouldn't. The air resistance would be the same for a same size ping pong ball no matter what weight difference. Didn't Sir Isaac Newton demonstrate that weight is not a factor in a falling object?

I'm very surprised to see that from you retrolefty. Sir Isaac Newton's experiment only works when the effects of air resistance can be ignored. If you drop a feather and a cannon ball, the feather falls more slowly.

Well I'm very surprised at you, don't two ping pong balls have the same wind resistance even if one is hollow and the other filled with water? Read the actual experiment the OP is proposing.

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