物理の基礎として学ぶ密度について、計算方法から実際の応用まで分かりやすく解説するよ。同じ大きさでも重さが違うのはなぜか、その謎を密度の概念で解き明かしていこう。
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Updated May 10, 2026
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密度とは何か: 基本の理解
1 / 6
密度の基本概念
発泡スチロールと鉄、同じ大きさなのに重さが全然違うよね。これは物質の混みぐあいが違うからなんだ。この物質そのものの混みぐあいを密度っていうよ。
密度を知ることで、その物質が何なのかを見分けることができるんだ。まさに物質の「指紋」みたいなもので、テストでもよく出題される重要な概念だよ。
覚えておきたい用語は3つ。質量は物質そのものの量（単位：g）、体積は物質が占める空間の大きさ（単位：cm³）、そして密度は単位体積あたりの質量のことだ。
重要公式： 密度 = 質量[g] ÷ 体積[cm³] この公式は絶対に覚えよう！
公式の覚え方と関係式
密度の公式は「し・み・た」の三角形で覚えると超便利だよ。上に「し」（質量）、左下に「み」（密度）、右下に「た」（体積）を書く。
求めたい値を指で隠すと、残りの計算式が見えるんだ。質量を求めたいなら「し」を隠すと「密度×体積」、密度なら「み」を隠すと「質量÷体積」って具合にね。
この方法を使えば、公式を忘れてもテスト中に思い出せるから安心だよ。計算問題で迷った時は、まずこの三角形を書いてみよう。
密度の性質と物質の見分け方
密度は物質の種類によって決まる値だから、どんなに大きさが変わっても同じ物質なら密度は一定なんだ。大きな鉄の塊も小さな鉄の釘も、密度は同じ7.87 g/cm³になるよ。
水に浮くか沈むかは、**水の密度（1.0 g/cm³）**と比べて決まる。物質の密度がこれより小さければ浮き、大きければ沈むんだ。
例えば氷の密度は0.92 g/cm³で水より小さいから浮くし、鉄は7.87 g/cm³で水より大きいから沈む。この「水の密度は1.0 g/cm³」は絶対に覚えておこう！
ポイント： 氷が水に浮くのは、固体になると密度が小さくなるからだよ。
計算問題の解き方（基本編）
実際の計算問題を解いてみよう。質量81g、体積30cm³のアルミニウムの密度を求める場合、まず公式「密度=質量÷体積」を書く。
数値を当てはめると「密度=81g÷30cm³=2.7g/cm³」になる。この値は水の密度より大きいから、アルミニウムは水に沈むことも分かるね。
不規則な形の物体の体積は、水の体積変化で測るよ。石をメスシリンダーに入れて水面が上がった分が、その石の体積なんだ。
テストのコツ： 単位を絶対に忘れないこと！計算の途中でも単位を書くクセをつけよう。
計算問題の解き方（応用編）
密度から質量や体積を求める問題も出るよ。「し・み・た」の三角形を使って公式を変形しよう。質量を求めるなら「質量=密度×体積」だね。
鉄の密度7.87 g/cm³で体積20cm³なら、質量は「7.87×20=157.4g」になる。逆算の問題も、この方法で簡単に解けるよ。
計算では小数第何位まで求めるか問題文をよく読んで、四捨五入も忘れずに。メスシリンダーの目盛りは、液面の一番低いところを水平に読むのがポイントだ。
重要： 密度は物質の量によらない！コップ半分の水もお風呂いっぱいの水も密度は同じだよ。
まとめとテスト対策
密度の学習で最も大切なのは、公式と水の密度をしっかり覚えることだよ。「密度=質量÷体積」と「水の密度1.0 g/cm³」、この2つは必須だ。
同じ物質なら大きさや形が変わっても密度は一定で、これが物質を見分ける手がかりになる。浮き沈みの判断も、水の密度との比較で簡単にできるね。
計算問題では単位を正確に書くこと、そして「質量」と「重さ」の違いも理解しておこう。理科では「質量」という用語を正しく使うことが大切だよ。
テスト必勝法： 「し・み・た」の三角形を問題用紙の端に書いて、公式を確認しながら解こう！
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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.
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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
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Best app on earth! no words because it’s too good
Thomas R
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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.
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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!
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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.
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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
密度とは何か: 基本の理解
物理の基礎として学ぶ密度について、計算方法から実際の応用まで分かりやすく解説するよ。同じ大きさでも重さが違うのはなぜか、その謎を密度の概念で解き明かしていこう。
密度の基本概念
発泡スチロールと鉄、同じ大きさなのに重さが全然違うよね。これは物質の混みぐあいが違うからなんだ。この物質そのものの混みぐあいを密度っていうよ。
密度を知ることで、その物質が何なのかを見分けることができるんだ。まさに物質の「指紋」みたいなもので、テストでもよく出題される重要な概念だよ。
覚えておきたい用語は3つ。質量は物質そのものの量（単位：g）、体積は物質が占める空間の大きさ（単位：cm³）、そして密度は単位体積あたりの質量のことだ。
重要公式： 密度 = 質量[g] ÷ 体積[cm³] この公式は絶対に覚えよう！
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公式の覚え方と関係式
密度の公式は「し・み・た」の三角形で覚えると超便利だよ。上に「し」（質量）、左下に「み」（密度）、右下に「た」（体積）を書く。
求めたい値を指で隠すと、残りの計算式が見えるんだ。質量を求めたいなら「し」を隠すと「密度×体積」、密度なら「み」を隠すと「質量÷体積」って具合にね。
この方法を使えば、公式を忘れてもテスト中に思い出せるから安心だよ。計算問題で迷った時は、まずこの三角形を書いてみよう。
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密度の性質と物質の見分け方
密度は物質の種類によって決まる値だから、どんなに大きさが変わっても同じ物質なら密度は一定なんだ。大きな鉄の塊も小さな鉄の釘も、密度は同じ7.87 g/cm³になるよ。
水に浮くか沈むかは、**水の密度（1.0 g/cm³）**と比べて決まる。物質の密度がこれより小さければ浮き、大きければ沈むんだ。
例えば氷の密度は0.92 g/cm³で水より小さいから浮くし、鉄は7.87 g/cm³で水より大きいから沈む。この「水の密度は1.0 g/cm³」は絶対に覚えておこう！
ポイント： 氷が水に浮くのは、固体になると密度が小さくなるからだよ。
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計算問題の解き方（基本編）
実際の計算問題を解いてみよう。質量81g、体積30cm³のアルミニウムの密度を求める場合、まず公式「密度=質量÷体積」を書く。
数値を当てはめると「密度=81g÷30cm³=2.7g/cm³」になる。この値は水の密度より大きいから、アルミニウムは水に沈むことも分かるね。
不規則な形の物体の体積は、水の体積変化で測るよ。石をメスシリンダーに入れて水面が上がった分が、その石の体積なんだ。
テストのコツ： 単位を絶対に忘れないこと！計算の途中でも単位を書くクセをつけよう。
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計算問題の解き方（応用編）
密度から質量や体積を求める問題も出るよ。「し・み・た」の三角形を使って公式を変形しよう。質量を求めるなら「質量=密度×体積」だね。
鉄の密度7.87 g/cm³で体積20cm³なら、質量は「7.87×20=157.4g」になる。逆算の問題も、この方法で簡単に解けるよ。
計算では小数第何位まで求めるか問題文をよく読んで、四捨五入も忘れずに。メスシリンダーの目盛りは、液面の一番低いところを水平に読むのがポイントだ。
重要： 密度は物質の量によらない！コップ半分の水もお風呂いっぱいの水も密度は同じだよ。
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
まとめとテスト対策
密度の学習で最も大切なのは、公式と水の密度をしっかり覚えることだよ。「密度=質量÷体積」と「水の密度1.0 g/cm³」、この2つは必須だ。
同じ物質なら大きさや形が変わっても密度は一定で、これが物質を見分ける手がかりになる。浮き沈みの判断も、水の密度との比較で簡単にできるね。
計算問題では単位を正確に書くこと、そして「質量」と「重さ」の違いも理解しておこう。理科では「質量」という用語を正しく使うことが大切だよ。
テスト必勝法： 「し・み・た」の三角形を問題用紙の端に書いて、公式を確認しながら解こう！
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.
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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