Boyle’s Law

The pressure of a fixed mass of gas is inversely proportional to its volume when kept at a constant temperature.

for constant T

Think about it…

If temperature is constant this means that the ball is travelling at a fixed, constant speed. If we increase the size of the box it makes fewer collisions in the same time because it has to travel further before it collides with the side. If we make the box smaller the ball will collide with the box more often since it has less distance to travel.