【Science】Grade 5: Pendulum Motion 〜Master the Rules and Become an Expert!〜

Hello everyone! Have you ever been on a "swing" at the park? Believe it or not, a swing is actually a type of "pendulum" that we study in science. Have you ever noticed that the rhythm feels the same whether you swing high or low?
In this chapter, we will explore what affects the "time it takes for one full swing" through experiments. It might seem tricky at first, but the rules are very simple. Let’s learn together and have some fun!

1. Let's Learn the Basic Pendulum Terms

First, let’s check the terms we often use in pendulum experiments. Knowing these will make solving problems much easier!

  • Length of the pendulum: The distance from the point where the string is attached (the pivot point) to the center of the weight (bob).
  • Amplitude (swing width): The angle or distance of how far the weight moves from the center.
  • One full swing: The motion from the moment you release the weight, to the opposite side, and back to the starting position.
  • Time of one full swing: The time it takes for the weight to complete one full swing.

Tip: A "full swing" means "going there and coming back." Be careful not to just count the trip "there"!

【Common Mistake!】
Some people measure the "length of the pendulum" only up to the knot in the string. Make sure you always measure to the center (middle) of the weight. This alone can improve your test scores!

2. What Determines the Time of One Full Swing? (Experiment Tips)

What changes the "time of one full swing"? In science, to find the answer, there is a rule: "Keep everything the same except for the factor you want to investigate." This is called a "controlled experiment."

① When changing the weight

If you increase the number of weights from one to two, what happens to the time of one full swing?
Example: Compare a 10g weight with a 20g weight.

Result: The time for one full swing does not change.
Whether the weight is heavy or light, the time is the same. Surprising, right?

② When changing the amplitude (angle)

Does the time change if you release the weight from a small distance versus a large distance?
Example: Compare an amplitude of 10 degrees with 30 degrees.

Result: The time for one full swing does not change (as long as the amplitude is not too large).
Even if you swing it further, the speed increases, so the total time taken remains the same.

③ When changing the length of the pendulum

What happens if you make the string longer or shorter?
Example: Try changing the length from 25cm to 50cm.

Result: The time for one full swing does change!
When the pendulum length increases, the time increases (it moves slower), and when it decreases, the time decreases (it moves faster).

★ Summary (This is the most important part!)
The time for one full swing is determined only by the "length of the pendulum"!
The "weight" and "amplitude" have no effect.

【Memorization Trick】
"The length of the pendulum is the only thing that changes the time of the pendulum."
Try memorizing that like a magic spell!

3. The Mysterious Relationship Between Length and Time

There is an interesting relationship between the length of a pendulum and its time. While you won't need to do complex calculations in 5th grade, it’s cool to know this!

Fun Fact: The 1-second pendulum
If you set the length of a pendulum to about 25cm, one full swing takes about 1 second.
So, how much would you need to increase the length to double the time (to 2 seconds)?
Actually, it’s not "two times" the length—you need to make it "four times" longer, which is about 100cm (1m)!

If you have the capacity, keep in the back of your mind that there is a relationship where \( \text{If the length is 4 times, the time becomes 2 times} \).

4. Steps to a Successful Experiment

When conducting experiments at school, following these steps will help you collect accurate data.

Step 1: Measure the length accurately

Use a ruler to carefully measure from the pivot point to the center of the weight.

Step 2: Measure the time for 10 full swings

The time for one swing is very short, so it's hard to measure with a stopwatch. The scientific way to do this is to "measure the time for 10 full swings and then divide by 10".
\( \text{Time for 1 full swing} = \text{Time for 10 full swings} \div 10 \)

Step 3: Measure multiple times

If you only measure once, you might make a mistake in timing your button press. If you measure about three times and take the average, you will get a more accurate result.

【Words of Encouragement】
You might think at first that "a heavier weight should make it move faster..." But the fun of science is discovering that "Oh, the weight really doesn't matter!" It’s okay if the results are different from your prediction. Trust your experimental data and keep going!

5. Chapter Summary (Key Takeaways)

Finally, let's review the important points that will be on the test!

  • Changing the weight does not change the time of one full swing.
  • Changing the amplitude does not change the time of one full swing.
  • Increasing the length of the pendulum makes the time of one full swing longer.
  • Decreasing the length of the pendulum makes the time of one full swing shorter.
  • To measure accurately, measure the time for 10 full swings and divide by 10.

Now you've got the basics of "Pendulum Motion" down! Use some string and a weight you have lying around (like an eraser) and try the experiment at home too!