Final answer:
Mass movements are influenced by both external and internal forces, which can include gravity, water content, and particle cohesion, among others, affecting their behavior and motion.
Step-by-step explanation:
Mass movements that behave like fluids and are dominated by a slip surface can be influenced by external forces, such as gravity, and also internal forces within the material, like water content and particle cohesion. Examples of these forces include a person standing up from a seated position, a car accelerating while driving, and various other scenarios where motion is induced by external or internal sources. In physics, specifically when discussing Newton's laws of motion, it's essential to differentiate between these forces to understand the dynamic behavior of objects.