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Answers to Readers' Questions on Physics:

Pendulum

The following 38 comments and questions have been sent in. They are listed according to date.

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What if pendulum is placed in a vacuum?

Question

August 19, 2008

if the pendulum is place in motion at the vacuum, is it will still swing for forever? is the rate will increase?

joe - Malaysia

16376

Answer

The pendulum will swing longer in a vacuum, but there still is some friction at the point where the string or wire is connected. Thus, it will ultimately slow down and finally stop.

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How to check pulse with pendulum

Question

June 15, 2008

Hello again, I just wanted to know whether the pendulum would actually stop since you mentioned that there still is friction resistance at the pivot point of the string or rod. Does it mean that it would somehow eventually stop due to this friction in a vacuum? So would the graph be directly proportional or inversely proportional? (if my graph is period against length). And about the pulse rate, how do we actually find our pulse rate using pendulum? How can it be carried out it?

Samantha - Malaysia

16091

Answer

The swing of a pendulum will slowly decrease in any situation. The more the resistance, the quicker the decrease. Toward the end, it will swing in extremely tiny amounts. This is called a damped vibration, which applies to pendulums and electronics. See: http://www.efunda.com/formulae/vibrations/sdof_free_damped.cfm for a picture of such a vibration.

To check your pulse with a pendulum, first find out how many times the pendulum goes back and forth in 1 minute. Suppose it goes back and forth 4 times in 5 seconds. Since 5 x 12 = 60, you multiply 4 x 12 = 48 swings in 60 sec or 48 swings/minute.

To check your pulse, you really need two people. You can count your own pulse, while the other person counts the swings of the pendulum. If your heart beats 7 times when the pendulum swings back and forth 4 times, you then multiply 7 x 12 = 84 beats per minute.

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Pendulum swinging in water

Question

June 13, 2008

Hello, i hope you could help me with some of these questions. It would be better if you include relevant explainations about these questions because it is an assignment given by my teacher and the presentation will be evaluated.

1.If a pendulum is made to swing in water, compare the time taken for this pendulum to come to a complete stop with the time taken by the pendulum swinging in air. Explain the difference.

2.Illustrate the motion of a simple pendulum swinging in water and in vacuum. What are the differences between these 2 graphs? (a question from me:will the pendulum continue swinging in a vacuum due to no air resistance?)

3.My teacher said that a pendulum can be used to measure time and it can used to measure one's pulse rate. So, how does it work anyway?

4.If a simple pendulum with a period of 1 second is set in motion on the moon, what is the change in the new period of this pendulum.

Samantha - Malaysia

16085

Answer

Friction from the string and air resistance slows down a pendulum. Otherwise, it would swing forever. Putting a pendulum in water greatly increases the resistance to motion as compared to air resistance. While a pendulum in air can swing for several minutes, up to many hours before it stops, one in water will only swing a few times before it stops.

In a vacuum, there is no air resistance, but there still is friction resistance at the pivot point of the string or rod.

The pendulum equations at http://www.school-for-champions.com/science/pendulum_equations.htm shows how to calculate the period or time it takes a given pendulum to swing back and forth. You could adjust the length of the string, so that it will swing back and forth once a second, thus making it useful for telling time. But since a pendulum will soon stop swinging, you could used a spring to power the swing. That is how many pendulum clocks work.

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Changing the weight of the bob in a pendulum

Question

May 12, 2008

Question
It is understood that the weight and/or mass of a pendulum bob has no bearing on rate of swing. With regard to a weight driven clock, if the "bob" is made lighter and the shaft length remains the same, may the driving weight be reduced? Example: My clock has a pendulum shaft length of 41.75 inches with a bob weight of 7 ozs. and a driving weight of 25 pounds. If I were to reduce the bob weight to "4" ozs. would I still need the 25 pounds of driving weight? If not, how much would you suggest I reduce the driving weight?
Santa Rosa, CA

Robert - USA

15935

Answer

It is true that the weight of the bob has no bearing on the rate of the pendulum swing. But since there is air resistance and friction involved, the rate is slowed slightly by those factors. Thus, a heavier bob will do a better job at negating those factors.

But also, air resistance and friction slowly reduces the amplitude of the swing, such that the pendulum will soon stop. That is why a weight or spring is used to maintain the swing or to drive the pendulum.

If you reduced the weight of the bob, it would have less inertia and less driving force would be needed. But I believe that accuracy of the clock might suffer. But also, the friction of the clock movement would be a factor.

Optimizing the configuration would usually be done by trial-and-error experimentation, as opposed to using some equation.

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How does weight effect a pendulum?

Question

August 28, 2007

How does weight effect a pendulum?

Cynthia - USA

14276

Answer

The lesson states that the frequency is independent of the mass of the bob. That means that weight does not influence the motion of a pendulum.

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Need a plan on pendulums

Question

June 6, 2007

hey my name is yalda and i am 14 years old i have a coursework assighnment on PENDULUMS unfortunatly and i only know a few things about them :(
but then i saw your website and thought it was really cool and i learnt alot about it (pendulums).
could you please help me on......
how to plan a pendulum practical plan- please. ( fair test, wat 2 change , and how to do it)
i would be very grateful if you help me.
this is no near to cheating but ur just helping me.
thank you,

yalda - UK

13886

Answer

I am not sure what you mean by a pendulum practical plan. But if this is a plan to do experiments with pendulums, what you need to do is to make a pendulum with a length of string and a weight. Then measure the angle at which you pull it to the side and have a way to measure the time it takes to go back and forth. I think this is what you are looking for. I hope it helps.

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Calculating gravity from pendulum measurements

Question

September 28, 2006

I'm a high school student. I got all your questions correct.

Does the formula above include air friction or neglect air friction? If I measure the period and length very acculately, will it be possible to get 9.81 for the acceleration due to gravity?

Masashi - Canada

12033

Answer

The pendulum equation is an approximation. If the angle is small and the bob is small and heavy, you will come close to the calculation of g = 9.8. Air resistance and friction will also affect the results, but not enough to notice.

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Does damping have an impact on the frequency?

Question

May 23, 2006

Does damping have an impact on the frequency of a pendulum? For example would a pendulum in water (with greater damping) have a higher, lower or equal frequency to one in air?

james - UK

11167

Answer

Yes, damping affected the frequency of a pendulum. You can see this as the motion slows until the pendulum stops, at which time the frequency = 0.

A pendulum in water will have a greater rate of decrease in frequency than one in air.

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Does pendulum go forever?

Question

March 28, 2006

sir how r u . my question is that pendulam swings it self when we just move it from mean position to extream position we apply force once and it swing for a long time in this condition can we say that it give more output then input.for example if we move it from mean position to extream position. we do a work on it .then due to gravity it moves down word and not stop at mean position due to inertia it moves on the othere extream position and cycle repeats many times.if collect the all force it is more then input if we feed back some part of this force in to the pedulam then its runs for ever what u say.

ghulam - Pakistan

10646

Answer

Yes, when you move the pendulum to an extreme position, you are doing work against gravity, such that the pendulum now has a potential energy PE = mgh, where h is the height it has been moved. By the law of inertia, the pendulum will swing back and forth forever, but there is friction from air resistance and in the string that will gradually slow it down.

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Phenomenon of pendulum at the poles

Question

February 27, 2006

You might wonder how much more clearer can you make it, but I would like you to be a little more explicit? I didn't really understand exactly what you ment about the different in the swing of the pendulum on the south pole and the equator. Is there a different between the south pole and the north pole? (Am I right? the shorter the string, the faster the swing and the longer he string the slower it seems to swing... chuck.

Charles - USA

10246

Answer

The axis of rotation of the Earth goes through the poles. If a pendulum was placed exactly over the North Pole or the South Pole, it would swing back and forth toward some point in space as the Earth slowly rotated under it. To an observer at the pole, it would appear that the pendulum was changing directions.

I made no mention of the Equator, but the phenomenon probably would not work there. It is not easy to visualize except at the poles.

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