A constant variable is one that does not change from trial to trial. A changing variable is the one variable you are testing for. It does change from trial to trial. One of the most difficult things to do in scientific research is to know what all of your variables are and to keep all but one variable constant.

In these pendulum experiments other variables were the temperature of the room, the humidity, the spin of the Earth, the design of the pendulum, etc., etc. We made an assumption that all those variables remained constant and didn’t really matter to our experiments. In this case, that’s a safe assumption but sometimes you can’t be too sure!

Constant and changing variables are around you all the time. What would be some variables in your breakfast? Which ones change from morning to morning? Which ones stay the same? What about some variables in the car? Which are constant and which are changing?

Let’s take a look at how to handle these questions:

What you need:

• String
• Several weights of some sort (a bunch of the same kind of washer works very well)
• Tape
• Scale (optional)
• Timer (or a watch with a second hand)
• Use the same pendulum set up you used for “The Size of the Swing” experiment.

### 5 Responses to “A Weighty Problem”

1. janelle_martin says:

We are really enjoying this curriculum so far! Where is the next lesson where you discuss gravity?

2. Vera Christian says:

We did this experiment as part of the scientific method set of experiments. We’ll need to go back and learn about force, mass, acceleration, amplitude, and frequency! We were asking mainly because it tells us in step 12 that we will learn about why in the next lesson, which didn’t seem to make sense. But your answer does explain why! Thank you!

3. You’re right – it the weight doesn’t affect the period of the pendulum at all! Baffling, isn’t it? Remember that the period depends on the length of the pendulum as well as gravity. Your experiment told you that adding extra weight or removing weight didn’t change the period at all. Surprise!

The more a pendulum weighs, the higher the force it feels. But the more a pendulum weighs, the higher the force needed for a given acceleration. These two effects exactly cancel.

A lighter mass would swing faster and with higher acceleration, but frequency will be the same, because the higher force results with higher amplitude of oscillations, but the frequency remains the same. Does that help?

4. Vera Christian says:

Hi,
We just finished this experiment and was wondering if you could direct us to the page that explains why the results are constant. We couldn’t figure out where it was. Thanks!