Final answer:
The interaction between a movable piston and gas involves forces exerted by both the gas and the piston. The kinetic energy of the gas molecules causes them to collide with the piston and exert pressure, thus moving the piston, and vice versa when the piston compresses the gas.
Step-by-step explanation:
The interaction between a movable piston and the gas in a container can be best described as both a) and b) since the gas and piston exert forces on each other. According to the kinetic theory of gases, gas molecules in constant motion collide with the walls of the container (which includes the piston when it is part of the container wall), exerting a force distributed over the area, causing pressure. This pressure is what moves the piston depending on the balance of forces. When the gas is heated, it expands, and as the gas molecules gain more kinetic energy, they exert a greater force on the piston, causing it to move outward, resulting in an increase in volume.
On the flip side, if the piston is pushed inward, the gas molecules are compressed, resulting in an increase in pressure if the temperature is kept constant, following Boyle's Law. This is how the piston exerts a force on the gas, causing it to compress. Therefore, the gas and the piston are in a dynamic interaction, where the movement of either affects the state of the other.