Then for a small area d A on the container wall, a particle with speed v at angle θ θ from the normal of the area d A , will collide with the area within time interval d t , if it is within the distance v d t from the area d A . Therefore, all the particles with speed v at angle θ θ from the normal that can reach area d A within time interval d t are contained in the tilted pipe with a height of v cos ⁡ ⁡ ( θ θ ) d t and a volume of v cos ⁡ ⁡ ( θ θ ) d A d t .