本题目来源于试卷: Space Time & Motion A.2 Forces & Momentum，类别为 IB物理学
[单选题]
A roller coaster passes a vertical loop in 29fox-+zuthx lbg) 1m;k1r i-s wa5nq a constant gravitational field. Fr c7 ),fkpaol 1dd5bo1uiction and air resistance are neglig dda k )cbpuf5o1,o l17ible.
How do the kinetic energy of the roller coaster and the normal reaction force acting on the roller coaster compare at point X and Y?
A. kinetic energy is greater at X than Y; normal reaction force is greater at Y than X
B. kinetic energy is greater at Y than X; normal reaction force is greater at X than Y
C. kinetic energy is greater at Y than X; normal reaction force is greater at Y than X
D. kinetic energy is greater at X than Y; normal reaction force is greater at X than Y
参考答案: D
本题详细解析:
Explanation:
By conservation of energy, the roller coaster has greater kinetic energy at the bottom of the loop (X), then kinetic energy is converted to gravitational potential energy at the top of the loop (Y). Hence B and C are rejected.
The normal reaction force by the track contributes to the centripetal force needed for the roller coaster to move along the loop. Since centripetal force increases with speed, the normal reaction force is larger at the bottom of the loop (X), when speed is greater. At the bottom of the loop, the normal reaction force is also the sum of the centripetal force and the weight of the roller coaster, while at the top of the loop the centripetal force is normal force is equal to the centripetal force minus the gravitational force.
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