邢秀娜竖直方向圆周运动中的向心力.ppt

1、Centripetal Force向心力,Circles at amusement parks 游乐场中的竖直圆周运动.,理综组邢秀娜,Uniform Circular Motion,Uniform circular motion （匀速圆周运动）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.,知识回顾（匀速圆周运动）,why,演示 线的一端系一小球，使小球在光滑水平面上作匀速圆周运动uniform circular motion。,一、什么叫向心力Centripetal Force ？,观察并思考： 1、小球的受力情况如何？(draw free body diagram) 2、线的拉力方向(direction)有何特点？,想一想：向心力的方向direction 是否改变？,迫使物体做匀速圆周运动，方向总是沿半径指向圆心的力叫向心力。,注意：向心力是从力的作用效果命名的,O,F,F,F,V,V,V,O,分析：,结论：向心力的方向(direction) 是在不断变

3、化(change)的,Non-Uniform Motion非匀速圆周运动（重点）,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.,谁来提供向心力？,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（张力）?,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 牛顿第二定律,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