Never Miss Your Target! The Strange Law That Guarantees a Hit—The Monkey Hunting Game Explained
I am Science Trainer Ken Kuwako. For me, every day is an experiment.
Imagine a slightly timid monkey perched high in a tree. He is so sensitive to loud noises that the moment he hears a “Bang!”, he let go of the branch and starts to fall. Now, imagine you are aiming a ball at him from the ground. The second you fire with a “Bang!”, the monkey begins his descent. It sounds like hitting a moving target would be nearly impossible, doesn’t it? But actually, there is a bit of physics magic hidden in the laws of motion. Today, I want to introduce you to the fascinating world of Monkey Hunting, one of the most popular topics in high school physics!
The Mystery of the Monkey Hunter Problem
“Monkey Hunting” is a world-famous thought experiment in the realm of physics.
The puzzle is simple: for a monkey that starts a free fall with zero initial velocity from a tree, what angle and what speed should you fire the projectile at to ensure a hit?
If the target stayed still, it would be easy. But since the monkey drops the very instant you fire, should you aim a little lower? Or is there a different secret? (I have included a challenge at the end of this post for those who want to solve it mathematically!)
Try it Yourself with This Simulation!
Before we get into the theory, the best way to understand is to try it! In the program I built, I have placed the monkey at a 45-degree angle. At what angle should you aim to hit him? Give it a shot here!
Press the “GO” button to fire the ball. You can adjust the angle and speed using the buttons in the top-left corner. Try out various combinations! For example, what happens if you fire at 45 degrees with a speed of 17? If you cannot view it in your browser, check out this video:
How did it go? Believe it or not, there is a surprising answer.
A Physics Miracle! The Law of the Guaranteed Hit
Here is the secret: As long as you aim your launcher directly at the monkey’s starting position, you will hit him every single time—regardless of the projectile’s speed! You might be thinking, “Wait, it hits even if the speed changes?” It sounds hard to believe, right?
Take a look at this real-life experiment video. You can see the power of physics in action immediately.
Why does this happen? It is because both the projectile and the monkey are being pulled by the exact same gravity. If there were no gravity, the ball would travel in a straight line and hit the monkey. In reality, gravity pulls both of them downward at the same rate. This means that at any given moment, the ball drops from its “straight-line path” by the exact same distance the monkey has fallen from the branch. They collide in mid-air with a perfect strike!
If you fire faster, they collide higher up. If you fire slower, they collide lower down. Of course, if it is too slow, the ball hits the ground first—but theoretically, if the ground were not there, a hit is guaranteed!
Take the Monkey Hunter Challenge!
I invite you to prove this phenomenon using math! For students, thinking through this is a great way to get a head start on high school physics.
Problem: At time t=0, a projectile A is fired from origin O with speed v0 at an angle θ from the x-axis. A monkey B at coordinates (a, b) is startled by the sound and begins a free fall with zero initial velocity at the same moment. Let g be the acceleration due to gravity. Show that if the launcher is aimed at a specific angle θ, the projectile will hit the monkey regardless of v0, and find that angle θ.
Download the problem sheet here!
Some Wonderful Feedback!
I received a lovely comment from a parent who played with this simulator with their child:
“My son absolutely loved the challenge of hitting the monkey! He does not understand the physics behind it yet, but he was experimenting in his own way, saying things like ‘Maybe it will hit if I aim higher?’ or ‘Maybe if I go faster, it will work!’ He was even shouting ‘GO GO GO!’ every time he pressed the button.”
Physics can seem intimidating, but seeing how curiosity grows through play like this is truly moving. It is the perfect way to spark a lifelong love for science!
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２月のイチオシ実験！梱包材で遊ぼう！
- 静電気の時期になってきました。子供と一緒に梱包材で盛り上がろう！→ やめられなくなる！静電気実験２０
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- 「月曜から夜更かし」（日本テレビ）にて科学監修・出演しました。
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書籍のお知らせ
- 1/27 『見えない力と遊ぼう！電気・磁石・熱の実験』（工学社）を執筆しました。
- サクセス15 ２月号にて「浸透圧」に関する科学記事を執筆しました。
- 『大人のための高校物理復習帳』（講談社）…一般向けに日常の物理について公式を元に紐解きました。特設サイトでは実験を多数紹介しています。※増刷がかかり６刷となりました（2026/02/01）
- 『きめる!共通テスト 物理基礎 改訂版』（学研）… 高校物理の参考書です。イラストを多くしてイメージが持てるように描きました。授業についていけない、物理が苦手、そんな生徒におすすめです。特設サイトはこちら。
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- 3/20（金） 日本理科教育学会オンライン全国大会2026「慣性の法則の概念形成を目指した探究的な学びの実践」について発表します。B会場 第３セッション： 学習指導・教材（中学校）③ 11:20-12:20
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