# Measuring speed of falling object

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?

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

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)

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.

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

tempalte:
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 8) ) 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

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.

wind may be a major factor here hitting the target from more than a few feet may be nigh on impossible

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).

I would think a wireless message from top to bottom would be easier to implement. RF travels at speed of light too.

The easiset way is a good camcorder and video tape it. If you use DV you can get 1/30th second resolution.

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?

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.

Lefty

retrolefty: 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.

PeterH:

retrolefty: 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.

Lefty

The air resistance would be the same for a same size ping pong ball no matter what weight difference.

It’s not correct, air resistance/drag also depends on speed,

retrolefty:
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.

The aerodynamic drag would be the same, but the impact of this on the acceleration would differ. If the ball is very light then the drag would be very significant relative to the weight and the ball would reach terminal velocity at quite low speed. If the ball was filled with water the drag would be the same (at a given speed) but would be less significant relative to the weight. Just imagine the two balls dropped from an infinite height and imagine the speed/time curve as they each accelerated up to their respective terminal velocity. The empty ping pong would end up going very slowly, and the filled one would be going much faster.

The classic ‘cannon ball’ experiment only works when air resistance is negligible. It is negligible for iron balls dropped a few tens of feet, but it’s not negligible for ping pong balls dropped a similar distance.

Magician:

The air resistance would be the same for a same size ping pong ball no matter what weight difference.

It's not correct, air resistance/drag also depends on speed,

But what does that have to due with a objects weight? Two ping pong balls will drop at the same speed regardless of weight difference so their air resistance/drag AND speed will be the same.

Isaac Newton would be doing a face palm if he was following this thread. ;)

Lefty

retrolefty: Two ping pong balls will drop at the same speed regardless of weight difference so their air resistance/drag AND speed will be the same.

That's simply not true.

The nett downward acceleration is weight minus drag divided by mass. If drag is negligibly small (relative to weight) then it can be ignored and then numerator and denominator are proportional to mass so the acceleration is independent of mass. This is the classic 'falling cannon ball' experiment.

Change the circumstances so that drag becomes significant (relative to weight) and the numerator is no longer proportional to mass and acceleration is no longer independent of mass. The change could be either making the speeds higher (drop the objects from a greater height) or make the mass smaller (so that the drag becomes more significant at the speeds encountered in the experiment) or change the medium so that the drag is higher (for a given shape/speed).

Are you familiar with the concept of terminal velocity? An empty ping pong and one the same size filled with water have different terminal velocities, because of the effects of drag.

Wasn't the feather VS Hammer experiment confirmed on the moon. No wind resistance caused both the feather and Hammer to fall and land at exactly the same rate and time regardless of their weight. Same thing happens in a vacuum. No drag.

PeterH:

retrolefty: Two ping pong balls will drop at the same speed regardless of weight difference so their air resistance/drag AND speed will be the same.

That's simply not true.

The nett downward acceleration is weight minus drag divided by mass. If drag is negligibly small (relative to weight) then it can be ignored and then numerator and denominator are proportional to mass so the acceleration is independent of mass. This is the classic 'falling cannon ball' experiment.

Which is what the OP is wishing to demonstrate with his 40ft dropped ping pong ball experiment and that is the context for my comments to the OP.

Change the circumstances so that drag becomes significant (relative to weight) and the numerator is no longer proportional to mass and acceleration is no longer independent of mass. The change could be either making the speeds higher (drop the objects from a greater height) or make the mass smaller (so that the drag becomes more significant at the speeds encountered in the experiment) or change the medium so that the drag is higher (for a given shape/speed).

Are you familiar with the concept of terminal velocity? An empty ping pong and one the same size filled with water have different terminal velocities, because of the effects of drag.

And ping pong balls falling from 40 feet will reach terminal velocity? Again my point is to link Sir Isaac Newton's claim to the experiment that the OP is wishing to reconstruct. Why must you change the context to try and show different results for a different experiment?