Gas expansion reduces piston collision frequency

Steps:

• Heating increases air temperature, causing expansion at constant pressure, so volume rises and piston moves right.
• Kinetic theory states collision rate with piston equals number density times average speed.
• Average speed rises with sqrt(T), but density falls as 1/V and V proportional to T, so density scales as 1/T.
• Net collision rate scales as (1/T) * sqrt(T) = 1/sqrt(T), which decreases.

Why D is correct:

• Collision frequency decreases per kinetic theory, as density reduction outpaces speed increase during isobaric expansion.

Why the others are wrong:

• A: Force per collision rises with particle speed squared, increasing with temperature.
• B: Average speed increases proportionally to sqrt(T).
• C: Mass remains constant in sealed cylinder.

Final answer: D