BR Physics I pg. 147

[Sonyfan08]

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#9: http://tinypic.com/r/2co0f11/7

If the cart is barely able to complete the loop, what is its speed at point P, the top of the loop?

The answer explanation states that the total force is normal force + mg = centripetal force. Why is there normal force? It says both the normal force + weight points down. Also, it says that because the cart barely makes it around the loop, normal force is 0. Why is that?

 

[ruchir]

#9: http://tinypic.com/r/2co0f11/7

If the cart is barely able to complete the loop, what is its speed at point P, the top of the loop?

The answer explanation states that the total force is normal force + mg = centripetal force. Why is there normal force? It says both the normal force + weight points down. Also, it says that because the cart barely makes it around the loop, normal force is 0. Why is that?

Normal force exists always when something is on a surface, whether it is facing up or "down". At the top both the weight and normal force pull down because weight is always down and the normal is also down since the object's surface is above it.