新增有关物理的英文词汇以及英文解释

参照下面的格式，给出中学物理涉及到的词汇以及单词的英文解释，英文解释要能够尽量使得学生容易理解和记忆

```json
[
{
"word": "hello",
"grade": "1st-grade",
"definitions": [
"A greeting used when meeting someone",
"A word said to start a conversation",
"An expression used to say hi"
]
}
]
```

Start with the JSON structure shown in issue #1 and identify where this learning material belongs in the repository. Add middle-school physics vocabulary with simple English definitions that follow the example format; done means the requested terms and explanations are complete and valid JSON.

Written by the indexing model from the issue text.