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### Topic: Measuring speed of falling object (Read 5379 times)previous topic - next topic

#### PeterH

#15
##### Jan 07, 2013, 05:40 pm

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.
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#### retrolefty

#16
##### Jan 07, 2013, 05:42 pm

Quote
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

#### PeterH

#17
##### Jan 07, 2013, 05:53 pmLast Edit: Jan 07, 2013, 05:55 pm by PeterH Reason: 1

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.
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#### HazardsMind

#18
##### Jan 07, 2013, 05:54 pmLast Edit: Jan 07, 2013, 05:57 pm by HazardsMind Reason: 1
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.
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#### retrolefty

#19
##### Jan 07, 2013, 06:00 pmLast Edit: Jan 07, 2013, 06:14 pm by retrolefty Reason: 1

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?

#### PeterH

#20
##### Jan 07, 2013, 06:27 pmLast Edit: Jan 07, 2013, 06:29 pm by PeterH Reason: 1

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?

I don't know what the terminal speed of a ping pong ball is or how close it would get to that in 40 feet, but I expect that the speed will be high enough for the effects of drag to be significant.

Quote
The Terminal Velocity is about 9.5 m/s

98% of which is attained after falling 12.5 m

I'm NOT changing the context. The effects I'm describing, which I believe will manifest in this experiment as described, and which you say do not exist, are precisely what the OP is trying to demonstrate. The experiment is NOT trying to prove Sir Isaac's claim. If anything, it is disproving it.
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#### retrolefty

#21
##### Jan 07, 2013, 06:30 pm

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?

I don't know what the terminal speed of a ping pong ball is or how close it would get to that in 40 feet, but I expect that the speed will be high enough for the effects of drag to be significant.

Quote
The Terminal Velocity is about 9.5 m/s

98% of which is attained after falling 12.5 m

I'm NOT changing the context. The effects I'm describing, which I believe will manifest in this experiment as described, and which you say do not exist, are precisely what the OP is trying to demonstrate. The experiment is NOT trying to prove Sir Isaac's claim. If anything, it is disproving it.

Then I guess we will both be interested in reading what the final results and findings of the OP's experiment are.

Lefty

#### terryking228

#22
##### Jan 07, 2013, 11:51 pm
I helped a kid do a similar but different falling-object project: (Early, with a IBM PC, Parallel Port driving IR LED and IR Phototransistor)

- 4 inch plastic drainage pipe about 6 feet long, bottom end 2 feet from floor, pillow on floor.

- Drop different-length 2x4 inch wood blocks from the top

- I-R Beam sees leading and trailing edge of falling block

- Do the math and display how long the block is.

It was correct and repeatable to about 1/4 inch as I recall.

Doubt I could resurrect  the Turbo Pascal code after 20+ years...
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#### PaulS

#23
##### Jan 08, 2013, 04:44 pm
Quote
Same thing happens in a vacuum. No drag.

It's hard to breathe in a vacuum. That's a drag.

#24
##### Jan 08, 2013, 10:24 pm
My first thought was that two balls identical apart from mass would take the same time to fall.
Thinking about it a bit more I would put my money on the heavy ball falling faster.

Here is how I would explain it;

Newtons Second Law gives F=ma
where
F is the force on the body
m is the mass of the body
a is the acceleration of the body in the direction of the force

If we take the two balls and drop them, at the same time, from the same height, in a vacuum they will both hit the ground at the same time.

The masses of the balls were different, but their acceleration was the same (they hit the ground together), so the gravitational force on the balls must have been different.

Now lets play a mind game and suppose that gravity would exert the same force on each ball, if we ran our experiment again what would happen?

In this case because F=ma (but we have said the force on both balls is the same) the ball with the smaller mass must accelerate faster and so would hit the ground first. In other words the same force accelerates the smaller mass more.

Now lets consider dropping the balls in air (with normal gravity).
The dimensions of the balls are identical so the air resistance on both balls, even though they have different masses, will be the same and will just depend on the speed of the ball.

However if both balls accelerate at the same rate they will be traveling at the same speed but that means both balls will experience the same resistive force and the resistive force will affect the smaller mass more. As a result the lighter ball will hit the ground last.

I would go for photosensors to detect the the start and end.
On earth the acceleration due to gravity is approximately 10m/s/s.
As a result a drop of 5m would take 1s.

Six stories seems a bit high.
What would be great though would be a sealed 5m perspex tube attached to a vacuum pump.
You could do the experiment with and without air.
An electromagnet could be used to drop the balls inside the tube.

#### Henry_Best

#25
##### Jan 16, 2013, 06:21 am

I helped a kid do a similar but different falling-object project: (Early, with a IBM PC, Parallel Port driving IR LED and IR Phototransistor)
...
Doubt I could resurrect  the Turbo Pascal code after 20+ years...

Thanks for trying to get this thread back on track, Terry, so that the kid can prove Isaac Newton right or wrong.

What I think he needs is a bit of code to measure the time, in milliseconds, between two events, however he detects them.

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