的小球在水中受的浮力为常力.ppt

1、Forces Application of Newtons Laws,1. Forces,There are four fundamental forces : gravitational force , electromagnetic force , strong force , weak force .,P195,An object moving through a fluid experiences a force of viscosity .,When the object moves at a relatively low velocity the viscous force may

2、 be approximated by assuming that it is proportional to the velocity .,Where is the coefficient of viscosity . K depends on the shape of the object .,2. Viscous force,The viscous force acting on a sphere takes the simple form :,P202,P233 - 234,3. Application of Newtons laws,Example 1 A 3.0 kg monkey

3、 holds on to a light rope , which passes over a frictionless pulley and is attached to a 4.0 kg bunch of bananas . What is the acceleration of the system ?,P206,Example 2 A puck of mass m on a frictionless table is attached to a mass M by a light string which passes through a hole in the table . Of

4、the many types of motion possible in this situation , we are interested in the case in which the puck , having been given an initial push , is found moving in a circle of radius r , while the mass M remains at rest . We want to know the speed of the puck in this case .,P207,Example 3 A bug walks rad

5、ially outward on a record that is rotating at an angular speed =45rpm ( revolution per minute ) . If the coefficient of static friction between the bug and the record is 0.08 , how far can it walk before it slips ?,P212 - 213,Example 4 Find the terminal speed of a raindrop , assuming it to have a ra

6、dius of 0.2 mm and using the density of water to be 1.0103 kg/m3 . Compare the value with a speed of a raindrop falling in a vacuum from 1.6 km . (=1.810-5 Ns/m2),When,the raindrop falls with a constant speed , which is called terminal speed vL .,P234 - 235,If there were no air resistance , the spee

7、d of the raindrop may be :,Heavier bodies fall faster !,How long does it take to reach the terminal speed ?,P235 - 237,By Newtons second law , we have :,Let,the solution is :,t0 is called characteristic time . It governs the rate at which speed tends to terminal speed .,Why heavier bodies fall faster ?,例2、质量为 m 的小球在水中受的浮力为常力 F,当它静止开始沉降时,受到水的沾滞阻力为 f = kv（ k为常数 ）,求小球在水中竖直沉降的速度 v 与时间 t 的关系。,解: 小球受力如图,根据牛顿第