What force is primarily responsible for the decrease in mechanical energy of the block?

The force primarily responsible for the decrease in mechanical energy of the block is friction. When an object, such as a block, moves along a surface, friction acts opposite to its direction of motion. This force converts mechanical energy into thermal energy, leading to a loss in kinetic energy as the block slows down.

Friction occurs due to the interactions between the surfaces in contact and is often dependent on the nature of the materials involved and the normal force acting on the block. As friction opposes the motion, it does work on the block that results in energy being dissipated, which manifests as heat. This is why, in scenarios involving moving objects, friction plays a crucial role in reducing the total mechanical energy available to the system.

The other forces, such as gravity, inertia, and elasticity, do influence the motion and behavior of the block but do not directly account for the energy loss in the same way that friction does during sliding or rolling motion. Gravity affects the potential energy, while inertia relates to the resistance of an object to changes in its state of motion, and elasticity pertains to the ability of a material to return to its original shape after deformation. None of these directly dissipate energy like friction does.