# Need some help with shear stress

Just started getting into aerodyanamics/fluid dynamics and came across this paragraph.

In it, it explains how layer a exerts a frictional force on layer b due to a velocity gradient but it also says that we have to imagine an area dA that is perpendicular to the y-axis. I was wondering what exactly this meant or whether it matters at all i.e. does it mean that the area is facing in the direction of the y-axis?

I thought that didn't make sense because there would be no friction between the layers as the area of concern is flat so I was wondering if it meant that the direction that the area is facing is perpendicular to the y-axis.

I would appreciate any help! Thanks!

• Yes, what they said is correct. What area did you think the tangential frictional force was acting on? Jun 22, 2020 at 11:35
• An area that is perpendicular to the y direction is tangential to the surface of the body. Jun 22, 2020 at 11:47
• Oh ok. I thought that the frictional force was acting on the side of the fluid element that is parallel to y but this area was so small that it could be regarded as being tangential to the streamline.
– Ewan
Jun 22, 2020 at 13:56
• y is perpendicular to a streamline, so an area can't be perpendicular to both y and perpendicular to a streamline. Jun 22, 2020 at 20:28