Achild on a sled come flying over the crest of a small hill. his sled does not leave the grounf, but he feels the normal force between his chest and the sled decrease as he goes over the hill. explain the decrease using newtons second law

 Let Fn = normal force 
When the sled is moving on level ground then 
Fn = mg (on level ground)(1) 

The crest of a hill is rounded. Therefore the child experiences downward centripetal acceleration. 
Let centripetal acceleration = a 
The two forces acting on the child are 
1. Weight mg downward 
2. Fn upward 
Acceleration = a (downward) 
Therefore mg - Fn = ma 
Fn = mg - ma (on crest)(2) 

From (1) and (2), 
Fn on crest is less than that on level ground. 

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