Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,2015/1/29,#,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 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,F,c,=F,g,+F,N,.,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 of gravity,（引力）,as the accelerating force?,Suppose,the,car moves more slowly,so that,F,c,=F,g,.,The,track,doesnt exert,F,N,on the car to keep it moving on the track.,What if the car moves too slowly?,The,F,c,F,g,The car will fall down.,Minimum speed,（最小速度）,around a loop,The slowest,any,car can go around the track,is a speed that,requires a centripetal force that has a magnitude equal to gravity.,F,c,=,F,g,What about,tension,（张力）,?,Top of Circle,Which forces are centripetal?,F,g,and F,T,.,So,:,F,c,=F,g,+F,T,Sides of the circle,Which forces are centripetal?,Only F,T,is F,c,.,So,F,c,=F,T,Which forces are centripetal?,F,T,and F,g,Now,Fg isnt directed towards the centre,but its parallel in the opposite direction.,F,c,=,F,T,-,F,g,Centripetal Force(,F=ma,),Newtons Second Law of,Moti,on,牛顿第二定律,F,c,=mv,2,/r,F,c,=mr,2,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 0.75 m and the speed of the,bucket is,3.00,ms,-1,.,What is the tension on the rope in position,C?,