Now let’s use the scientific method to discover a couple of things about pendulums. Before we start, I need to tell you two new terms. One is constant variable and the other is changing variable. A variable is a part of your experiment, like the coin in the Underwater Presidents experiment. If it is a constant variable, it stays the same for every trial of that experiment.


For example, we always used the same penny in the Underwater Presidents. Those variables never changed. A changing variable is what you change for each trial. It is often what you are testing for; “If I change this, what happens to that?”


For example, in the Underwater Presidents experiment, if we tried water in the dropper, then we tried vegetable oil, then corn syrup; the changing variable would be the liquid we are using in the droppers. When you do an experiment you have to try very hard to keep all variables constant except for the one you are testing for. If you don’t keep all but one variable constant, you won’t know why you are getting the results you’re getting. If you change the size of the coin, and the type of liquid with the Underwater Presidents experiment, you will have a hard time knowing if it’s the change of coin or the type of liquid that’s causing more or fewer drops on the coin. Let’s try the following experiment and see if this becomes clearer.


What you need:


  • String
  • Weight of some sort
  • Tape
  • Timer (or a watch with a second hand)

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First of all, you have to make your pendulum. A pendulum is really nothing more than a weight at the end of something that can swing back and forth. The easiest way to make one is to get a string and tape it to the edge of a table. The string should be long enough so that it swings fairly close to the ground. Tie a weight of some sort (a washer, a watch, your dog (just kidding, live things make poor pendulums)) to the bottom of your string and you’ve got a pendulum. Now, for this experiment the changing variable is going to be the length of string. In each trial you will be changing the length of the string. The rest of the variables will be constant. The weight at the end of the string, the string itself, the time you will be letting it swing will be the same for every trial. Getting the hang of constant and changing variables now? Okay so here’s what you want to do:


1.      Make an observation. Play with the pendulum a bit and see how it behaves.


2.      Make a hypothesis. How will the length of string effect the number of swings in 10 seconds? Will there be more, less, or no change in the number of swings as the string gets shorter.


3.      Set a timer for 10 seconds or get someone who has a watch with a second hand to tell you when 10 seconds are up.


4.      Now for the test. Pull the pendulum back as far as you’d like (the pendulum swings smoother if you don’t lift the weight higher than the top of the string).


5.      Start the timer and let go of the weight at the same time.


6.      Count the swings the pendulum makes in 10 seconds.


7.      Write down what you found (collect the data). This works well if you make a chart with two columns, one for length of the string, and one for number of swings.


8.      Do two more trials with the string at that same length.


9.      Now change the changing variable. In other words, shorten the string. I would recommend shortening it at least an inch.


10.  Redo steps 3 through 9.


11.  Continue shortening the string and doing trials until you get at least five trials with five different lengths of string.


12.  Now report your results. Take a look at your data and see if you find a trend. Do you get more swings as the string shortens, less swings, or does the length of the string matter? Something interesting to notice is that at a certain length you will get 10 swings in 10 seconds or a swing a second. This is why pendulums are used in grandfather clocks. They keep good time!


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Comments

5 Responses to “The Size of the Swing”

  1. tabbi8407 says:

    We had an unexpected changing variable, CAT! The kids and the cats really loved this experiment.

  2. Colleen LaGrange says:

    awesome science experiment. We had fun!

    Thanks,
    Rachel and Judah with Dad

  3. Yanina Noble says:

    Hello Aurora,

    We are doing some research on pendulums and learned that mechanical energy = potential energy + kinetic energy. And so when the string with weight is half way in it’s swing (vertical or perpendicular to the ground), PE = KE. We also know understand KE and PE and when each are occurring. Also, we learned that without gravitational force and air resistance, this pendulum action would continue infinitely. What else is there to understand from this experiment (aside from the changing and constant variables you speak of here)? Also, in all our research, we can’t figure out an explanation for the moment where there’s a swing every second, hence why pendulum keep good time. Is there a relation between length of string and weight? Please help.

    Yanina and her children

  4. Sounds like it worked just as it should for you! Remember to have fun while you’re doing this, and don’t stress over the details now.. just enjoy the learning process. You’ll get better fine-tuning the details as you go along.

  5. Hamish McGillivray says:

    Hello?

    We enjoyed this experiment immensely but felt that we weren’t able to really keep all constant variables constant. The position of launching the pendulum, the counting of seconds (no stop watch), and evaluating where the pendulum was in its swing at 10 seconds might leave something to be desired and certainly open to question.

    Having said that we did get a solid trend in our results, so whilst not brilliantly accurate it gave us the idea.

    Thank you.