ing形式的语法疑问

题目

ing形式的语法疑问
句子是这样的
their intensities must be within the working limits of the materials composing the surfaces.
materials composing the surfaces 这个帮我分析下,ing放后面表示什么,如何理解,是宾补吗?请说得详细些,
must be within 为什么这中间要加个be啊?

答案

of the materials （WHICH) composing the surfaces
composing the surfaces做materials的定语,解释说明materials.
within是介词而非动词,而must后需be动词或实义动词,介词前用be动词.