# Why is the sphere of influence of the molecule in mean free path a cylinder?

In my book for deriving mean free path a cylinder is taken to act as a sphere of influence of molecules with which it could bump into other molecules and its radius = diameter of the molecule we are considering with

Now why is this a sphere of influence considered cylinder? If its a moving sphere then it should be a capsule like shape , it doesn't have a flat projection in the start to give the cylinder like shape. Is it an approximation or something else ?

• if you consider only free path in one dimension the cylinder or circle will be the space. Feb 22 at 15:26
• Why is cylinder or circle the space though , if the molecule is a sphere and is moving in 1d then it doesn't necessarily create a path starting with a horizontal circle and then be a 3d cylinder , it starts with it being a sphere so sphere is constantly projected as time goes on not a circle like projection or doesn't be a cylinder right ? Feb 22 at 15:35