Final answer:
Heavy crates on concrete demonstrate the difference between static and kinetic friction, with static requiring a higher force to overcome. Adding mass increases both types of friction, while lubrication reduces them.
Step-by-step explanation:
Attempting to slide a heavy crate across a concrete floor illustrates the principles of static and kinetic friction. Initially, the force of static friction increases to match your pushing force, preventing movement. Once this threshold is exceeded, the crate moves, and the resistance you feel drops to the level of kinetic friction, which is lower than static friction. Adding mass to the crate increases the static and kinetic friction, requiring more force to both start and maintain movement. Conversely, lubricating the surface with oil reduces both static and kinetic friction, making it easier to move the crate.