Forces Q (1 Viewer)

[krypticlemonjuice]

Can someone please explain why the normal force is 0 when she enters the pipe and then back to 0 at the other side??

 

[carrotsss]

theoretically she is vertical at the start/end, so the gravity force is perpendicular to the hypothetical normal force, and hence there is none

 

[howcanibesmarter]

remember the normal force is always perpendicular to the surfaces in contact. In this case, the only other force present is gravity, acting vertically downwards. Since normal is supposed to be perpendicular to the surface, i.e. its horizontal, non of the gravity is in the component of the normal, hence why its zero. Same could be said at the otherside.

 

[wizzkids]

The other factor to consider is centripetal acceleration. The skateboarder is not just falling vertically under gravity, but executing a circular path.
However, the velocity of the skateboarder is zero at point A and also diametrically opposite point A (maybe we should refer to as point C) Therefore the centripetal acceleration at these two points is also zero, and therefore centripetal acceleration does not contribute to the normal force.