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### Topic: pendulum on a string (Read 1 time)previous topic - next topic

#### treebykooba

##### May 03, 2012, 05:49 pm
Hello,

I am interested in making a 6oz pendulum swing seamlessly (i.e - no jerky motion) from a string using a motor about 12ft above...so it looks like the pendulum is in perpetual motion. I was thinking of using a DC motor with perhaps some reed switches to change the polarity of the motor with relays and an arduino to control some delays. I do not know any physics and thinking about it more and more, this seems to be a difficult task. Is this possible to do with an arduino? Is it possible at all? I couldn't find much online of anyone doing much with a pendulum from a string and a microcontroller.

Any advice as to where to go with this would be much appreciated!

#### kf2qd

#1
##### May 03, 2012, 06:03 pm
Sounds like you are trying to make a Foccault Pendulum. Nice one in the Smithsonian. Demonstrated the rotation of the Earth. I believe that one has a switch and a magnetic coil that attracts a ring up near the pivot point.

Not much for an Arduino to do - the Pendulum will have a very predictable period and that will never change.

#### Yankee

#2
##### May 03, 2012, 06:07 pm
One way to do this would be to use an electromagnet below the pendulum bob which could provide an impulse when triggered by a hall sensor at the appropriate time in the pendulum's cycle. This would, of course, require a magnet to be embedded in the bob.
I know I've seen this arrangement somewhere on the web to power a clock pendulum so you might try googling it.

#### Grumpy_Mike

#3
##### May 03, 2012, 06:28 pm
Quote
I was thinking of using a DC motor

No that will not work, the speed of a pendulum is constantly changing and you do not have that degree of speed control with a DC motor. It might be possible to do it with a stepping motor or servo, but the impulse method described above is perhaps the best unless the period of the pendulum has to be  different from it's natural frequency.

#### treebykooba

#4
##### May 03, 2012, 09:14 pm
thanks so much for your help. I'm having a real hard time finding a suitable stepper motor for this....i got one from sparkFun awhile ago that has practically no torque. it's a bipolar and i'm using the easyDriver with it.

#5

#### winner10920

#6
##### May 04, 2012, 03:57 pm
All you need is a weak pulse, just basically replacing the energy lost in friction of the pendulum swinging
and if you know the weight and length of string you can calculate the period which wont change no matter how fast its swinging, if you had a stepper motor at the top and gave it a pulse every time its called for it'll work fine
id guess it'd be best to pulse it while its directly below to not notice any jerks or just as it starts to reverse

#### Grumpy_Mike

#7
##### May 04, 2012, 04:27 pm
Quote
if you know the weight and length of string you can calculate the period

Well it is only the length that determines the period. The overall mass dictates the amount of momentum or energy in the system and this contributes to how long it will swing before it is damped, as it is the energy reserve that is sapped by the friction in the joint.

#### winner10920

#8
##### May 04, 2012, 05:10 pm
Tru, its been 4 years since physics class I guess I might be starting to forget lol
I've seen the one in the smithsonian, pretty cool you can actually see how it moves over time

#### Grumpy_Mike

#9
##### May 04, 2012, 05:23 pm
Back in November 1983 I published a computerised pendulum project. It was simply a wire wound pot with a rod attached. The rod swing like a pendulum and you read the value of the pot through the analogue input port and plotted the values on the screen. You got a perfect sin wave that decayed. You could lengthen the rod to change the frequency.

#### MarkT

#10
##### May 05, 2012, 02:27 am

All you need is a weak pulse, just basically replacing the energy lost in friction of the pendulum swinging
and if you know the weight and length of string you can calculate the period which wont change no matter how fast its swinging, if you had a stepper motor at the top and gave it a pulse every time its called for it'll work fine
id guess it'd be best to pulse it while its directly below to not notice any jerks or just as it starts to reverse

Not quite true a pendulum's period does depend on its amplitude - most noticeable for larger angles of swing.  A pendulum is not a true example of SHM since the restoring force is not proportional to displacement, but to a trigonometric function of displacement.

The formula for the period of small amplitude oscillations depends on the length, the distribution of the mass along the pendulum and the acceleration due to gravity but not the total mass of the pendulum.
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