4. Real case for fluid approaching a body All the fluid particles are in random motion, and has a average velocity. During their motion, they collide with each other. The molecules strike on to the solid surface will be rebounded and these rebounded molecules will make collision to other molecules. This process transfers the message of the existence of the body and most of the particles will go other round After the collision between fluid particles and solid surface, the momentum change of the particles is in the perpendicular direction of the surface4. Real case for fluid approaching a body All the fluid particles are in random motion, and has a average velocity. During their motion, they collide with each other. The molecules strike on to the solid surface will be rebounded, and these rebounded molecules will make collision to other molecules. This process transfers the message of the existence of the body, and most of the particles will go other round. After the collision between fluid particles and solid surface, the momentum change of the particles is in the perpendicular direction of the surface