# Center of Mass Activity – Free Printable Lesson

To most people, the Center of Mass seems like a concept only high school and college-aged students could understand. But my son (who’s just entering second grade) recently developed an experiment that introduced him to the concept of the center of mass.

He recently bought himself a light-up fidget spinner with replaceable colorful lights that blink after being pressed. However, being able to remove the three segments opened an opportunity for me to teach my son about the center of mass and its effect on rotational movement.

Once at home, he started playing with the fidget spinner and noticed his new toy acted strangely when he took the inserts out of the spinner. He showed me, and asked me what was going on. I told him I’d explain it to him while he did a little experiment!

**It’s important to use a fidget spinner with removable inserts. This set is what we used.** (aff link)

You will need:

- This fidget spinner set or a fidget spinner where the inserts easily pop out.
- Stopwatch
- Pencil
- Printable chart you can request below

Before testing, let’s talk about the force used to make the fidget spinner spin. This force is called torque.

Torque is influenced by the force’s strength, its distance from the axis of rotation, and the component of the force that acts perpendicular to the spinner’s arm. So, it’s important to hold the fidget spinner in the same way with teach test spin. Plus, we want to try to apply as close to the same amount of force each time too.

**Step 1:**** ** We started our experiment by seeing how the spinner spun with all the weights in. His spinner spun just as it always, smoothly.

** Step 2:** Next, we removed one of the weights and watched how the spinner changed. We noticed two things:

- The spinner “wobbled” as it spun.
- The spinner no longer spun as it slowed down but swung back and forth.

I told him the change was due to moving the **center of mass** (COM) of the spinner. When he took out one of the weights, the COM moved from the middle of the spinner to a new spot closer to the other two lights.

* Step 3: * He removed a second weight and spun his fidget spinner.

By removing another weight, he moved the center of mass closer to the remaining weights. He noticed his fidget spinner continued to wobble when it spun and swung back and forth as it slowed down.

* Step 4: *Lastly, he removed the remaining weight from his fidget spinner.

He noticed his toy no longer wobbled or swung back and forth when it slowed down! I told him he had returned the center of mass to the middle of the spinner, so it acted as if all the weights were back in place!

## Center of Mass Activity Videos

Start the videos together and compare how each spins from start to stop.

We wrote down how long it took for each spinner to stop. We did this by watching the time-lapse on the video and noting when the fidget spinner started and stopped spinning.

As you can see, when the fidget spinner had three weight in, it spun for over a minute. With two weights in the spinner, it only spun for 30 seconds, and with only one weight, it spun for 22 seconds. Once all the weights were removed from the fidget spinner, it only spun for 17 seconds.

My son continued to play with his spinner, taking the weights in and out and exploring how his fidget spinner changed. He noticed that with all 3 weights in or out of the spinner, the toy spun around the center until it stopped. When he had only 1 or 2 weights in the spinner, it would wobble and swing back and forth when it slowed down.

**Explaining Our Center of Mass Activity**

Think about a seesaw at the playground. The seesaw is perfectly balanced if you have two people of the same weight on each end. If one of those two people moves closer or farther from the center. What happens?

The seesaw tips, right? The point where it would balance perfectly if the people were evenly spread out is like the ‘center of mass.’

The center of mass is like the point where something would perfectly balance. It’s the average position of everything that makes up an object.

A good demonstration is to have your child balance a ruler on their finger without it tipping in any direction. Next, have your child balance different pens and pencils on their finger. If a pencil has a heavy eraser on the end, the center of mass will be different than the same type of pencil where the eraser has been removed.

The center of mass is like finding an object’s ‘balance point.’

If an object has a regular shape, like a square or a circle, the center of mass is right at the middle, at the geometric center. But, if it’s a weird shape, like our fidget spinner, or has more stuff on one side, the center of mass moves closer to that side.

This was demonstrated in our fidget spinner experiment. The center of mass changed as we moved inserts in and out, changing where more/less mass was located on the spinner.

## Law of Inertia & Fidget Spinners

While we were doing this experiment, we were also playing with another physics concept, the **Law of Inertia**. Inertia is an object’s resistance to changing its state of motion. For example, if a ball is rolling on a surface with ZERO friction (a force that causes moving things to stop), the inertia of the ball would keep it moving at the same speed forever! But in the real world, we have friction, so the inertia of an object is its resistance to being stopped.

In our center of mass experiment, we discovered removing the fidget spinner inserts one at a time affected how the fidget spinner spun. But as we reviewed the videos of each spinner, we also noticed something else changing. That’s when I realized my son was also doing a fidget spinner science experiment about the **Law of Inertia**!

During our center of mass experiment, we wrote down how long it took for each spinner to stop. We did this by watching the time-lapse on the video, and noting when the fidget spinner started and stopped spinning.

As you can see, when the fidget spinner had three inserts in, it spun for over a minute. With two inserts in the spinner, it only spun for 30 seconds, and with only one insert, it spun for 22 seconds. Once all the inserts were removed from the fidget spinner, it only spun for 17 seconds.

Let’s start by looking at the equation for** torque,** the force my son applied to the fidget spinner to make it spin.

Torque is a measure of the tendency of a force to rotate an object around an axis or pivot point. It is the product of the force applied to an object and the perpendicular distance from the axis of rotation to the line of action of the force–that’s the more technical and advanced definition.

However, for younger students, we can explain that torque is a twisting or turning force that makes things rotate or spin. Imagine trying to open a door – the force you use to turn the doorknob is like torque. It depends on how hard you push (force) and where you push (distance from the hinge or pivot point). The longer the distance and the stronger the push, the more torque you have. So, torque is what makes something twist or rotate!

Whoa! That was a lot of math! But we’re just interested in how time (t) relates to mass (m). Looking at our equation above we see that time and mass are *directly* related, meaning as **mass gets bigger, the longer the fidget spinner takes to stop!** This is exactly what we saw happen as we removed inserts (and therefore weight) from our spinner. The more inserts that were in the spinner, the longer it spun!

Center of Mass and the Law of Inertia can be difficult to grasp when learning about it through a textbook, but when it involves a fun toy, it is so much easier to understand!

## Request the Experiment Lesson Printables

I hold a master’s degree in child development and early education and am working on a post-baccalaureate in biology. I spent 15 years working for a biotechnology company developing IT systems in DNA testing laboratories across the US. I taught K4 in a private school, homeschooled my children, and have taught on the mission field in southern Asia. For 4 years, I served on our state’s FIRST Lego League tournament Board and served as the Judging Director. I own thehomeschoolscientist and also write a regular science column for Homeschooling Today Magazine. You’ll also find my writings on the CTCMath blog. Through this site, I have authored over 50 math and science resources.