The process that involves physical forces that break rock into smaller pieces is called ?

The process that involves physical forces that break down rock into smaller pieces is called mechanical weathering. 

Weathering and erosion.
Explained
Weathering is the process where rock is dissolved, worn away or broken down into smaller and smaller pieces. There are mechanical, chemical and organic weathering processes. Organic weathering happens when plants break up rocks with their growing roots or plant acids help dissolve rock.