1、Centripetal Force向心力,Circles at amusement parks 游樂場中的豎直圓周運動.,理綜組邢秀娜,Uniform Circular Motion,Uniform circular motion （勻速圓周運動）occurs when the speed （速率)of an object experiencing circular motion remains constant. Is velocity（速度） constant for uniform circular motion? No. The velocity is always changing

2、direction.,知識回顧（勻速圓周運動）,why,演示 線的一端系一小球，使小球在光滑水平面上作勻速圓周運動uniform circular motion。,一、什么叫向心力Centripetal Force ？,觀察并思考： 1、小球的受力情況如何？(draw free body diagram) 2、線的拉力方向(direction)有何特點？,想一想：向心力的方向direction 是否改變？,迫使物體做勻速圓周運動，方向總是沿半徑指向圓心的力叫向心力。,注意：向心力是從力的作用效果命名的,O,F,F,F,V,V,V,O,分析：,結(jié)論：向心力的方向(direction) 是在不斷變

3、化(change)的,Non-Uniform Motion非勻速圓周運動（重點）,Accelerating （加速）into the loop the first time.,At the top of the loop, the car will experience Fc = Fg + FN. Which forces are centripetal forces? Normal force （正交反作用力） and weight引力.,誰來提供向心力？,What if the car goes around a second time?,And we only use the force

4、 of gravity（引力） as the accelerating force?,Suppose the car moves more slowly, so that Fc = Fg. The track doesnt exert FN on the car to keep it moving on the track.,What if the car moves too slowly?,The Fc Fg The car will fall down.,Minimum speed（最小速度） around a loop,The slowest any car can go around

5、the track is a speed that requires a centripetal force that has a magnitude equal to gravity. Fc=Fg,What about tension（張力）?,Top of Circle,Which forces are centripetal? Fg and FT. So: Fc = Fg + FT,Sides of the circle,Which forces are centripetal? Only FT is Fc. So, Fc = FT,Which forces are centripeta

6、l? FT and Fg Now, Fg isnt directed towards the centre, but its parallel in the opposite direction. Fc = FT -Fg,Centripetal Force (F = ma),Newtons Second Law of Motion 牛頓第二定律,Fc = mv2/r,Fc = mr2,Practice 1,A bucket of water with a mass of 1.5 kg is spun in a vertical circle on a rope. The radius of the circle is 0