球

In a sentence

• 我今天下午要去打篮球。

  This afternoon, I will go play basketball.

  Here, the word '球' is used as part of the compound noun '篮球', which means 'basketball', referring to the sport.

• 弟弟在公园里踢皮球玩。

  My younger brother is playing with a rubber ball in the park.

  In this sentence, '球' refers to a ball as an object (specifically a rubber ball, '皮球'), used for recreational activities.

• 地球是太阳系的一部分。

  The Earth is part of the solar system.

  Here, '球' is part of the term '地球', meaning 'Earth', describing the shape of our planet as a sphere.