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Answers to Readers' Questions on Physical Science:

Pendulum

The following 25 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Is the speed the same as if you dropped the object?
• Where is the period of a pendulum greater?
• How does length of pendulum affect period?
• Does the mass of the bob affect velocity?
• Kinetic and potential energy in a pendulum
• Why would gravity be different than 9.8?
• Using pendulum to find pulse
• What if pendulum is placed in a vacuum?
• How to check pulse with pendulum
• Pendulum swinging in water

Next 10

Is the speed the same as if you dropped the object?

Question

November 20, 2009

how can we calculate the highest speed of a pendulum ..at the lowest point of its path
is it the same speed we the same ball falls vertically from the same hight?

mustafa - USA

18710

Answer

Yes, the speed of the pendulum bob at the lowest point is the same as if you dropped the object from a given height. Of course, it will be going sideways, instead of downward.

The speed or velocity is v = SQRT(2gh), where h is the height from which you released the bob.

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Where is the period of a pendulum greater?

Question

October 10, 2009

where the period of a pendulum will be maximum??
pole or equator or any other place
please reply

ananda - India

18474

Answer

The value of the acceleration due to gravity "g" is greater at the equator, as well as on mountains. It is less near the poles.

When "g" is greater, the period of a pendulum is smaller and it moves back-and-forth faster.

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How does length of pendulum affect period?

Question

October 10, 2009

Question: If you increase length of a simple pendulum ,what will be the time period ??
Increase or Decrease

if i use equation T=2pi(l/g)^1/2 it found to increase but if i see that DEMO it seems to decrese please reply

- India

18471

Answer

The period T is the number of seconds for the pendulum to go back-and-forth. If you increase the length L, T is larger and the pendulum swings slower.

In the pendulum demonstration, increasing the relative length results in slowing down the motion. The default is 1. Increasing it to 2, results in a slower speed or greater period T. Putting the relative length to 0.5 increases the speed and decreases T or the time to swing back-and-forth.

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Does the mass of the bob affect velocity?

Question

September 28, 2009

If you are testing the velocity, will the a) height from which you drop the bob, b) the string length, or c) the mass in the bob directly affect the velocity? Since mass does not affect the frequency, is it wise not to include mass into this experiment?
How does one know which variable will affect the velocity the most?

Kara - USA

18401

Answer

The mass of the bob is not a factor in the velocity equation for the pendulum. However, a heavier bob will result in a better experiment, because air resistance will not be as much of a factor.

See Simple Pendulum Equations at:
http://www.school-for-champions.com/science/pendulum_equations.htm

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Kinetic and potential energy in a pendulum

Question

August 5, 2009

Hi,
I am a high school student who is studying Kinetic energy and Potential energy. and I have a couple of questions, if you could answer them fairly detailed that would be great.

1. Does the amount of weight affect the velocity of the pendulum why or why not?
2. What is a 'bob'?
3. What can be done to minimise the amount of friction and what variables could be changed in order to do so?
4. What roles does the Kinetic and potential play.
thanks sooo much

- Australia

18110

Answer

Since a pendulum is actually a falling weight that is controlled by a strained or wire, the mass of the bob or weight on the end of the string does not affect the velocity. It is similar to the fact that the mass or weight of an object does not affect its velocity when you drop the object. You can see the factors that affect a pendulum and its equation at:
http://www.school-for-champions.com/science/pendulum_equations.htm

There are some air resistance that slows down the movement of a pendulum. For example, a narrow wire would have less air resistance than a fat string holding the weight or bob. Also, the shape of the bob is a factor in air resistance.

but a bigger factor is the friction or resistance of the flexing of a wire or string at its pivot point at the top. Instead of having a strained firmly connected at the top, you could connect it to some sort of ring, that would reduce the friction.

Potential energy relates to the height you are holding the bob compared to the bottom of the swing. The kinetic energy is the moving energy that is related to the velocity of the bob.

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Why would gravity be different than 9.8?

Question

November 10, 2008

Hi
I'm in high school and Im doing a topic on Space and gravity. Recently we were given a practical assessment on Pendulums. The Aim of this Practical was To determine a value for acceleration due to gravity using pendulum motion and gather information to identify reasons for possible variations from the value 9.8. ms-2. My question is after performing this experiment how pendulums demonstrate acceleration due to gravity and why would the value of g be different than 9.8?

A - Australia

16759

Answer

The value of 9.8 m/s^2 for the acceleration due to gravity is really an average value. Gravity varies in different parts of the world, due to the shape of the Earth and amount of matter underneath.

Air resistance and friction at the pivot point can affect the measurements, reducing the rate of acceleration.

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Using pendulum to find pulse

Question

August 24, 2008

A simple pendulum can be used as a device to measure time. Describe how you can use to measure your pulse rate.

Nor Nazuha - Malaysia

16395

Answer

It would be difficult to use a pendulum to measure your pulse rate unless you had two people counting. One (probably you) would count your pulse beats and the other person would count the swings of the pendulum. If you know how many swings there are in 15 seconds and how many heart beats you counted during 15 sec, then just multiply by 4 to get your pulse rate.

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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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