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Answers to Readers' Questions on Physics:

Pendulum

The following 37 comments and questions have been sent in. They are listed according to date.

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How does weight effect frequency of pendulum?

Question

February 25, 2010

How does the weight effect the frequency and duration of the pendulum?

Booby - USA

19290

Answer

Just as the rate an object falls is independent of its weight (assuming no air resistance), the frequency and duration of the pendulum swing is independent of the weight of the bob.

See:
http://www.school-for-champions.com/Science/pendulum_equations.htm

Weight or mass is not in the equations.

However, a very heavy bob may increase friction and a very light bob may be affected by air resistance.

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What is a bob on a pendulum?

Question

December 17, 2009

hi! i am just wondering ABOUT PENDULUM BOB'S HOW DO THEY WORK AND HOW DO YO U ASSEMBLE THEM????? WERE TO GET ONE AND HOW MUCH IT I PLEASE GET BACK TO ME PLEASE A.S.A.P. IF U GET BACK TO ME BY TOMORROW UR AWESOME! =]

bob - USA

18878

Answer

A pendulum consists of a wire or string with a weight tied to the end of it. For some reason, they call the weight at the end of a pendulum a "bob".

You can just take a string and tie a small weight to the end of it. Then either hold the other end of the string or tie it to something and you have a pendulum that you can do experiments with.

See: http://www.school-for-champions.com/science/pendulum.htm

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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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