Understanding Inertia in PE

Why Bodies Keep Moving (or Stay Still)

� Biomechanics — Understanding Inertia in PE: Why Bodies Keep Moving (or Stay Still) and How to Use It in Class

Hello, this is Young-in teacher! :)

In a running lesson, you blow the whistle and shout, “Stop!”
But even after the signal, students’ bodies keep moving forward for several steps.

It’s the same principle as when a bus suddenly brakes and our bodies are thrown forward.

Behind this phenomenon is a kind of “stubbornness” in objects that want to keep doing whatever they’re already doing — this is inertia.

Today, let’s look at what inertia is and how we can use it wisely in PE lessons!

1️⃣ What is Inertia?

Inertia is an object’s tendency to continue in its current state of motion.
(This is Newton’s First Law of Motion.)

An object at rest: wants to stay at rest. (inertia of rest)

An object in motion: wants to keep moving in the same direction and speed. (inertia of motion)

To change this stubborn state, we must apply a force.

For students, you can explain it like this:

“Our body is a bit stubborn. If it’s standing still, it wants to keep staying still. If it’s running, it wants to keep running.”

2️⃣ Everyday Examples of Inertia in Elementary PE

Many things we see in PE can be explained by inertia.

� Why starting to run feels so hard (Inertia of rest)

When students go from standing still to a full sprint, they’re changing from a resting state to a moving state.

Their leg muscles must apply a big force to overcome the inertia of rest.

That’s why the first few steps feel the heaviest.

� Why bodies pitch forward when stopping

When students try to stop quickly after running:

Their feet act like brakes and stop first,

But the upper body, because of inertia, still wants to keep going forward.

That’s why they pitch forward, lose balance, or even fall.

⚽ When balls collide with the body

If a fast-moving ball hits a student’s chest:

The ball “wants” to keep going forward due to inertia,

That momentum creates a strong impact on the body.

3️⃣ Connecting Inertia to Safety & Tactics

Once students understand inertia, they can see why teachers give certain safety rules and tactical advice.

� Safety (Running & Tug-of-War)

When students sprint at full speed and then try to stop suddenly at the finish line:

Their bodies are full of inertia of motion,

It’s very hard to control their movement in an instant,

They can easily crash into others or fall over.

So you can explain why we need a “buffer zone”:

“We don’t stop like a wall. We slowly reduce speed and then come to a stop, so that our body’s inertia doesn’t make us crash.”

�️ Defensive positioning (Strategy)

If an attacker is running fast to the left:

Because of inertia, it’s hard for them to suddenly cut sharply to the right.

If defenders think:

“Their body also has inertia pushing them that way,”

they can stand in the lane ahead of the attacker,
cutting off the path more intelligently instead of just chasing the ball.

� Classroom Application – Young-in Teacher’s TIP! (Use it right away)

✅ Tip ① “Freeze vs Soft Stop” — Feeling the body’s stubbornness

In the gym, have students walk or lightly jog in lines. Then give a signal:

Freeze on the spot when the teacher shouts “Stop!” Students will feel their body lurch forward and wobble.

Stop within 3 steps after the “Stop!” signal (gradual slowing). Students slow down and stop safely with balance.

Compare the two and then explain:

“Because of inertia, your body wants to keep going. If you try to stop all at once, it can be dangerous. That’s why we slow down gradually when we stop.”

✅ Tip ② Explaining inertia through catching (linked to impulse)

Try this in a dodgeball or catching lesson:

A. Standing stiff and catching in place: The ball’s inertia is transferred directly to the hands and arms. The impact feels strong and painful.

B. Stepping back slightly while catching: You move a little in the same direction as the ball. The ball slows down over a longer time, and the impact is softer.

Then say:

“The ball also has a stubbornness (inertia) that makes it want to keep going forward. If we step back a little as we catch, it doesn’t hurt as much because we’re helping the ball slow down.”

� In Closing

“Start powerfully, stop gently.”

This is one of the most basic principles of controlling inertia in movement.

In your next PE lesson,let your students feel the “stubbornness” of their bodies (inertia) and teach them how to manage it wisely.Their movements will become safer, smarter, and more controlled.