# Ball - sense that is falling

#### boriz

##### Senior Member
Init 6.

You use any units you like friend. I find it easier to write 1G than 9.80665 m/s/s.

â€œâ€¦reaching apogeeâ€¦falling â€¦â€ NO! NO! Thrice NO! - No matter how many times you say it, or wish for it. Itâ€™s the laws of physics. Nothing I can do about it Iâ€™m afraid.

Look. This is pretty basic stuff. You should know this from school!. Just wait â€˜till you get to the more interesting things, like space-time curvature, the fourth dimension, the finite size of space, the fact that your watch and mine can never be truly synchronised no matter how accurate they are, and that real cubes do not have 90 degree corners!

Iâ€™m sorry friend, but this is turning into a pantomime. â€œOh yes it isâ€ â€œOh no it isnâ€™tâ€. I canâ€™t help you if you are going to ignore my references. Go read some physics is my advice.

#### toxicmouse

##### Senior Member
hmmm, bit harsh telling someone to read some physics. i had a lecturer who claimed that the true test of understanding was if you could make your grandmother understand.

INIT 6: i think that your graph is a good starting point. the deceleration shown is very clear and i think that you reckon this is what the accelerometer in the juggling ball will show. well it is, but the balls travel slowly enough that the drag is too small to be accurately measured by the cheap accelerometers. what you will find is the squiggly lines shown around the 12 second to E (the rocket would be travelling at roughly the same velocity as the juggling balls would). only if you had a really high quality accelerometer and some awesome filtering then you would sense the deceleration due to drag at such low velocities.

#### init 6

##### New Member
the drag is too small to be accurately measured by the cheap accelerometers. .

It is helpful because you at least understand the limits. I think I put in my original post that the forces in play may be difficult to measure.
But, bear in mind that the force decelerating the object is not just drag, but also gravity. The original push is in the opposite direction to gravity. If it didn't decelerate then it would keep going up.
What I'm saying is that from the minute if leaves your hand it is decelerating until it reaches apogee. Then it accelerates under gravity. I belive that you can measure this force - there are accelerometers that have a ug (that's meant to be micro) resolution, and they're not that expensive.

Again I'm not sure if the picaxe is good enough to do the maths, especially when you add spin.

#### BeanieBots

##### Moderator
I promised myself that I'd keep out of this debate but I couldn't let this one go.

"What I'm saying is that from the minute if leaves your hand it is decelerating until it reaches apogee. Then it accelerates under gravity."

We are NOT leaving orbit with these balls! Gravity WILL be remaining a CONSTANT acceleration in these conditions.

This really IS schoolboy stuff. Poor old Newton will be turning in his grave

s=ut+1/2.at^2
v=u+at
v^2=u^2+2aS
F=ma

Einstein had a few issues with those equations but we're not getting into those realms with juggling balls either.

#### Tyro

##### Member
Boriz is absolutely correct in his understanding of the physics.

I believe that measuring the acceleration of the ball is a red herring, during the flight time the ball is ballistic, the ball is in free fall. What is required is a measurement of the vertical component of the ballâ€™s velocity.

After the ball leaves the hand, the ball will have a negative vertical velocity reducing to zero at its highest point. The velocity will then reverse and steadily increase. The only time that matters is when the velocity is zero so calibration will not be an issue.

Unfortunately, I do not know the best method to measure this velocity.

#### Tom2000

##### Senior Member
Unfortunately, I do not know the best method to measure this velocity.

Seriously, I don't have a clue on this one. I've been scratching my head over the problem for three or four days, and nothing -- NOTHING!! -- workable has come to mind. This is really a tough problem.

And when you solve it, I'm sure it will be with something simple and brilliant that will have all of us banging our heads into a brick wall.

Good luck!

Tom

#### Fowkc

##### Senior Member
I had an idea, but I don't know how successful it would be.

Start with three small tubes passing through the ball and meeting in the middle. While in motion there would be some movement of air through these tubes, which can be detected by some pressure sensor. When there is no movement of air, the ball is stationary (at apogee, one hopes). The three tubes (orthogonal to each other) would account for the ball spinning.

That's the best idea I can come up with that hasn't already been discussed. I was thinking about different ways of measuring velocity and tried to think of how to make a pitot tube for a juggling ball.

#### boriz

##### Senior Member
Toxicmouse.

You are right. I appologise Init 6. My frustration reached apogee.

#### moxhamj

##### New Member
"...the true test of understanding was if you could make your grandmother understand".

Ok, here's a challenge I can't resist. Grandma, I hope you are reading this.

First, it is quite ok not to understand the G forces intuitively. Our brain is not wired up to do this. Our brains are very good at modelling the parabolic trajectory of a rock because we will survive if we can avoid that rock. But we have little understanding of things outside our experience - like time slowing down at high speed or things getting heavier at speed or how tiny particles can be in two places at once. When would we ever experience the G forces involved here? Perhaps by jumping off a cliff, but that wipes one out of the gene pool. Perhaps being flung on a trebuchet, but ditto the outcome. Fairground rides, vomit comets and skydiving are relatively new experiences.

So let's leave aside maths and try to explain this with concepts. Consider a pendulum sitting in a car. Drive forward and the pendulum swings backwards. It is acting as an accellerometer. Now replace the pendulum with a 1Kg weight screwed onto a wooden ruler and screwed at the top to the inside roof of the car. This is stiffer than a pendulum but it will still move when the car moves off. Take our car with the pendulum inside and rotate 90 decrees, so the car is pointing straight up. If we went to drive straight up and then we stopped and came down, yes, the pendulum would move. But this is because the car is connected to the earth and is exerting a force on the earth.

Lets put our car on a catapault and fling it straight up. After the car leaves the catapault any occupants of the car will be weightless. The pendulum will not move. Sure, there are external forces acting on the car from the gravitational field, but these forces apply equally to the car, the occupants and to the 1kg weight. If all the forces are acting equally, then there is no net difference between the car and the pendulum.

Lets take our car over to Jupiter where the gravity is much higher. We send it up on the catapault and it goes up and down much much faster. But, as it goes up and as it comes down, even though there is a really big gravitational field, the field still acts equally on the car, the pendulum and the occupants. Thus everything is still weightless inside the car as it goes up and as it comes down, and it is still impossible to detect the apogee.

Incidentally, the stiff beam with a weight on the end is exactly what an electronic accellerometer looks like on a microscopic scale.

http://www.rocktec.co.nz/links/rocktec_1_39.php Q: what forces would occupants of this car feel once it has left the trebuchet?

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#### sedeap

##### Senior Member
Issue research

*******************
Hi boys & girls...

In military industries, the ballistic projectiles can be sensed by themselves
the trick is figure how, so they build one special accelerometer merging different
technologies and sensors.
The key is around timing the whole test and partial "lapses".
To detect the falling part of the ballistic curve (after heyday, or apogee) they
use one altimeter (common in rocket model hobbyst) based on atmosferic pressure
and to meter the speed, they meter the air speed around, or do math calculations
based in altitude x timelapse x gravity at that ground height (or sea level)
In other words: distance x time x acceleration = speed (relative)
The air speed can be collected by one small fan or anemometer type, or, by one
kind of capacitive device, made with copper, ball shaped collector of static
electricity conected to one capacitor to accumulate it and "readed" by one MC
doing math calculations like timelapse x static / pattern (calibration)= speed

No further information can be researched because "Army Secret" flag "pop up".

But how can be do it with Picaxe...
Is another Story...

)

#### Jeremy Leach

##### Senior Member
I'm still wanting to know at what stage we deploy the chute on the juggling ball...

Just joking ...I've enjoyed reading this thread. I've been thinking of Boriz's looped wire solution and that it'd probably suffer from oscillation and false triggers. So began thinking of one sensitive electret mic on the inside surface of the ball (assuming it was something like a big ping-pong ball plastic etc). A noise gate could detect when it hits the hand, perhaps more reliably??

The real issue I can see is the size constraint. I'm no juggler but from what I've seen the ball could be around 5cm diameter??

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#### demonicpicaxeguy

##### Senior Member
i like how this thread exploded the way it did...
i think my solution using average up and down times was proabably the easiest....

#### Dippy

##### Moderator
Obviously.
I've done a new emoticon just for you DPAG....

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#### boriz

##### Senior Member
Since you donâ€™t need any detail or direction from your sensor just to detect a catch, a three axis accelerometer would be overkill. The looped wire, if not damped somehow, may indeed cause false triggering. Hereâ€™s another idea:

A piezo disk, like those found in musical greetings cards, fixed to the inside of the ball at one edge, with a machine screw or something similar fixed to the opposite edge. Any inertial forces will push against the head of the screw and bend or twist the disk no matter what direction they come from. A simple op-amp gain stage should suffice to detect even the softest catch.

#### toxicmouse

##### Senior Member
i agree with boriz that an accelerometer is overkill, and i like the simplicity of timing. maybe this could be a use for QTC- to detect the throw and catch?
http://www.tep.org.uk/millennium_smart_qtc.html

i have never used them for lack of application, but it doesnt look too difficult. Quantum juggling balls, that has got to sell well.

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#### boriz

##### Senior Member
Wow. That QTC stuff is great. It was not clear in the article, but does the quantum â€˜resistanceâ€™ exhibit the same kind of I^2*R losses to heat as an equivalent normal resistor?

#### demonicpicaxeguy

##### Senior Member
Obviously.
I've done a new emoticon just for you DPAG....
love it!.....

#### Tom2000

##### Senior Member
Obviously.
I've done a new emoticon just for you DPAG....
OUTSTANDING!

If you don't want to use that as an emoticon, it would make one of the coolest avatars I've ever seen!

#### D n T

##### Senior Member
This thread has got some balls

Unigamer, you could write a thesis on this and probably get a physics degree, its good to see the physics come out here.

While this is not an electronic solution, it would look very cool.
Get hold of some clear plastic balls,
Set up a strobe and have it at your 4 o'clock with an opaque screen at your three o'clock screening the strobe unit.
Have the audience at your 12 o'clock.
Turn the lights off and the strobe on., although you could just turn them down a little.
The plastic would reflect and refract the strobe nicely
Adjust thew strobe speed and keep an eye out for seizures

Sorry, no PICAXE

#### unigamer

##### Member
Unigamer, you could write a thesis on this and probably get a physics degree, its good to see the physics come out here.

While this is not an electronic solution, it would look very cool.
Get hold of some clear plastic balls,
Set up a strobe and have it at your 4 o'clock with an opaque screen at your three o'clock screening the strobe unit.
Have the audience at your 12 o'clock.
Turn the lights off and the strobe on., although you could just turn them down a little.
The plastic would reflect and refract the strobe nicely
Adjust thew strobe speed and keep an eye out for seizures

Sorry, no PICAXE

I'm just surprised at the level of interest! I will feel guilty if I don't do something with all the knowledge that has been put into this.

I am doing Advanced Higher Physics (Scottish 6th Year) and part of this includes an investigation on a chosen topic. I am going to have a think about what I could do from this.

You mention strobe lights and I have actually built something that would effectively do this.

http://www.eccentricgenius.com/wp/2006/08/10/stopping-time-visually/
(note that's not me, but I used that design)

Sources of the right kind of plastic balls are going to be interesting to find as I'd only settle for perfection because of the money spent on the other parts. It's really nice juggling with exactly the right kind of balls but I digress.