Is Normal Force Relevant in Frictionless Scenarios?

[rygza]

My instructor told the class to keep in mind that we should only account for normal force when friction is involved in the problem. In my book there is a problem in which friction is negligible and they account for normal force, but ultimately the normal force has no effect on the conclusion of the problem.

Should I not include normal force in my diagrams if there is no friction?

 

[tiny-tim]

hi rygza!

if the problem is about things moving on a plane (horizontal or sloping), then (if there's no friction) there's no movement, and therefore no acceleration, perpendicular to the plane, and so the perpendicular forces are irrelevant (ie your instructor is correct)

but if the surface is curved, then even though there's no movement perpendicular to the plane, there is a centripetal acceleration perpendicular to it, and so any perpendicular forces (including the normal forces) are relevant

Should I not include normal force in my diagrams if there is no friction?

If you're only interested in forces and acceleration along the surface, there's no need to clutter the diagram with perpendicular forces