スキーで雪に沈まないのはなぜ？船が水に浮かぶのはなぜ？これらの秘密は圧力と浮力にあるんだ。日常生活でよく見る現象の仕組みを理解すれば、テストの問題もスラスラ解けるようになるよ。
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Updated Apr 7, 2026
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圧力と浮力の基本
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圧力と浮力の基本概念
君が画びょうを指で押したとき、針の先だけが痛いよね？これが圧力の正体だ。同じ力でも、狭い面積に集中すると圧力が大きくなる。
圧力は1m²の面積あたりにかかる垂直な力の大きさのこと。単位はパスカル（Pa）で、公式はP = F/A（圧力 = 力 ÷ 面積）。この公式は絶対に覚えよう！
一方、浮力は水や空気の中で物体を上に押し上げる力だ。お風呂で体が軽く感じるのも浮力のおかげ。アルキメデスの原理によると、浮力の大きさは「物体が押しのけた液体の重さ」と等しくなる。
ポイント: 圧力の公式P = F/Aは必ずテストに出る！面積の単位はm²に変換することを忘れずに。
圧力が変わる条件としくみ
圧力の大きさは2つの要素で決まる。力が大きいほど圧力は大きく、面積が小さいほど圧力も大きくなる。
スキーが雪に沈まないのは、板が足の面積より大きいから圧力が小さくなるためだ。逆に包丁や針は面積を小さくして圧力を大きくし、よく切れるようになっている。
水圧も重要な概念だよ。水の中では深いほど水圧が大きくなり、あらゆる方向から同じ大きさで圧力がかかる。同じ深さなら、どこでも水圧は同じ大きさになる。
プールの底で耳が痛くなるのは、上にある水の重さによる水圧が原因なんだ。
テスト対策: スキー、スノーシュー、包丁、針の例は頻出問題！圧力を大きくする例と小さくする例を覚えておこう。
浮力が生まれる理由
浮力って実は水圧の差から生まれるんだ。水中の物体を見てみよう。物体の下面は上面より深い位置にあるため、下からの水圧の方が上からの水圧より大きい。
この水圧の差が物体を上に押し上げる力、つまり浮力を作り出している。物体の上面では小さな下向きの水圧、下面では大きな上向きの水圧がかかり、結果的に上向きの力が残るんだ。
物体が浮くか沈むかは、重力と浮力の大小関係で決まる。重力＞浮力なら沈み、重力＝浮力なら水中で静止、重力＜浮力なら浮き上がる。
空気中でも浮力は働いているけれど、空気の密度は水の約1/1000なので、普通は無視できるほど小さい。
理解のコツ: 浮力は「物体の上下での水圧の差」が原因。この仕組みを図で描けるようになると記述問題で点が取れる！
圧力の計算問題
計算問題では単位変換が最大のポイントだ。面積は必ずm²に直すこと！1m² = 10,000cm²だから注意しよう。
例題：質量1.2kgの直方体（底面積20cm×10cm）が机にかける圧力は？
- 力の大きさ：1.2kg = 1,200g → 1,200÷100 = 12N
- 面積：20cm×10cm = 0.2m×0.1m = 0.02m²
- 圧力：P = 12N ÷ 0.02m² = 600Pa
質量（g）と力（N）を混同しないことも重要。100g≒1Nの関係を使って変換しよう。
計算の流れは「力を求める→面積を求める→公式に代入」の3ステップ。面積をcmのまま計算してしまうミスが一番多いから気をつけて！
計算のコツ: 面積の単位変換でミスしやすい！cmをmに直すときは100で割ることを忘れずに。
浮力の計算問題
浮力の計算ではアルキメデスの原理を使う。「浮力 = 物体が押しのけた液体の重さ」が基本だ。
例題：1辺10cmの立方体（重さ20N）を完全に水に沈めたときの浮力は？
- 物体の体積：10×10×10 = 1,000cm³
- 押しのけた水の体積：1,000cm³（完全に沈んでいるため）
- 押しのけた水の質量：1,000cm³×1g/cm³ = 1,000g
- 水の重さ：1,000g ÷ 100 = 10N
- 浮力：10N
この物体は重力20N＞浮力10Nなので沈むことも分かる。物体が完全に沈んでいれば、物体の体積 = 押しのけた液体の体積になることを覚えておこう。
重要: 物体が一部だけ沈んでいる場合は、水面下の部分の体積だけが「押しのけた液体の体積」になる！
テスト対策と要点まとめ
テストでよく出るミスを避けるポイントをチェックしよう。単位変換は絶対に間違えないこと！面積はm²、圧力はPa、力はNを使う。
記述問題では理由もセットで書く練習をしておこう。「面積が小さいほど圧力は大きくなるから」のように、因果関係を明確に示すことが大切だ。
重要な公式と原理：
- 圧力の公式：P = F/A
- アルキメデスの原理：浮力 = 押しのけた液体の重さ
- 浮力の原因：物体の上面と下面での水圧の差
計算問題では、圧力は「力→面積→計算」、浮力は「体積→質量→重さ→浮力」の順で解いていこう。単位を間違えると全部間違いになるから、最初に単位変換を済ませるのがコツだよ。
最終チェック: 「力と圧力の違い」「浮力が生まれる理由」「アルキメデスの原理」この3つは必ず説明できるようにしておこう！
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content in 物理
Most popular content
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5
App Store
4.7/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
圧力と浮力の基本
スキーで雪に沈まないのはなぜ？船が水に浮かぶのはなぜ？これらの秘密は圧力と浮力にあるんだ。日常生活でよく見る現象の仕組みを理解すれば、テストの問題もスラスラ解けるようになるよ。
圧力と浮力の基本概念
君が画びょうを指で押したとき、針の先だけが痛いよね？これが圧力の正体だ。同じ力でも、狭い面積に集中すると圧力が大きくなる。
圧力は1m²の面積あたりにかかる垂直な力の大きさのこと。単位はパスカル（Pa）で、公式はP = F/A（圧力 = 力 ÷ 面積）。この公式は絶対に覚えよう！
一方、浮力は水や空気の中で物体を上に押し上げる力だ。お風呂で体が軽く感じるのも浮力のおかげ。アルキメデスの原理によると、浮力の大きさは「物体が押しのけた液体の重さ」と等しくなる。
ポイント: 圧力の公式P = F/Aは必ずテストに出る！面積の単位はm²に変換することを忘れずに。
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圧力が変わる条件としくみ
圧力の大きさは2つの要素で決まる。力が大きいほど圧力は大きく、面積が小さいほど圧力も大きくなる。
スキーが雪に沈まないのは、板が足の面積より大きいから圧力が小さくなるためだ。逆に包丁や針は面積を小さくして圧力を大きくし、よく切れるようになっている。
水圧も重要な概念だよ。水の中では深いほど水圧が大きくなり、あらゆる方向から同じ大きさで圧力がかかる。同じ深さなら、どこでも水圧は同じ大きさになる。
プールの底で耳が痛くなるのは、上にある水の重さによる水圧が原因なんだ。
テスト対策: スキー、スノーシュー、包丁、針の例は頻出問題！圧力を大きくする例と小さくする例を覚えておこう。
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
浮力が生まれる理由
浮力って実は水圧の差から生まれるんだ。水中の物体を見てみよう。物体の下面は上面より深い位置にあるため、下からの水圧の方が上からの水圧より大きい。
この水圧の差が物体を上に押し上げる力、つまり浮力を作り出している。物体の上面では小さな下向きの水圧、下面では大きな上向きの水圧がかかり、結果的に上向きの力が残るんだ。
物体が浮くか沈むかは、重力と浮力の大小関係で決まる。重力＞浮力なら沈み、重力＝浮力なら水中で静止、重力＜浮力なら浮き上がる。
空気中でも浮力は働いているけれど、空気の密度は水の約1/1000なので、普通は無視できるほど小さい。
理解のコツ: 浮力は「物体の上下での水圧の差」が原因。この仕組みを図で描けるようになると記述問題で点が取れる！
Sign up to see the contentIt's free!
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圧力の計算問題
計算問題では単位変換が最大のポイントだ。面積は必ずm²に直すこと！1m² = 10,000cm²だから注意しよう。
例題：質量1.2kgの直方体（底面積20cm×10cm）が机にかける圧力は？
- 力の大きさ：1.2kg = 1,200g → 1,200÷100 = 12N
- 面積：20cm×10cm = 0.2m×0.1m = 0.02m²
- 圧力：P = 12N ÷ 0.02m² = 600Pa
質量（g）と力（N）を混同しないことも重要。100g≒1Nの関係を使って変換しよう。
計算の流れは「力を求める→面積を求める→公式に代入」の3ステップ。面積をcmのまま計算してしまうミスが一番多いから気をつけて！
計算のコツ: 面積の単位変換でミスしやすい！cmをmに直すときは100で割ることを忘れずに。
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
浮力の計算問題
浮力の計算ではアルキメデスの原理を使う。「浮力 = 物体が押しのけた液体の重さ」が基本だ。
例題：1辺10cmの立方体（重さ20N）を完全に水に沈めたときの浮力は？
- 物体の体積：10×10×10 = 1,000cm³
- 押しのけた水の体積：1,000cm³（完全に沈んでいるため）
- 押しのけた水の質量：1,000cm³×1g/cm³ = 1,000g
- 水の重さ：1,000g ÷ 100 = 10N
- 浮力：10N
この物体は重力20N＞浮力10Nなので沈むことも分かる。物体が完全に沈んでいれば、物体の体積 = 押しのけた液体の体積になることを覚えておこう。
重要: 物体が一部だけ沈んでいる場合は、水面下の部分の体積だけが「押しのけた液体の体積」になる！
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
テスト対策と要点まとめ
テストでよく出るミスを避けるポイントをチェックしよう。単位変換は絶対に間違えないこと！面積はm²、圧力はPa、力はNを使う。
記述問題では理由もセットで書く練習をしておこう。「面積が小さいほど圧力は大きくなるから」のように、因果関係を明確に示すことが大切だ。
重要な公式と原理：
- 圧力の公式：P = F/A
- アルキメデスの原理：浮力 = 押しのけた液体の重さ
- 浮力の原因：物体の上面と下面での水圧の差
計算問題では、圧力は「力→面積→計算」、浮力は「体積→質量→重さ→浮力」の順で解いていこう。単位を間違えると全部間違いになるから、最初に単位変換を済ませるのがコツだよ。
最終チェック: 「力と圧力の違い」「浮力が生まれる理由」「アルキメデスの原理」この3つは必ず説明できるようにしておこう！
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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Most popular content in 物理
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Students love us — and so will you.
4.6/5
App Store
4.7/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user