如图，静静同学手中握着一个圆形饮料瓶（瓶重忽略不计），在空中静止，杯中装有重2N的饮料，瓶子受到的摩擦力是____ N；当她喝掉一半饮料后用力握瓶子，瓶子仍在空中静止，此时瓶子受到的摩擦力是____ N.

从下面方框中选择正确的单词并用其适当形式填空，使短文意思完整、通顺。

Dear Jenny,

    My name is Alan. I'm a 12-year- old girl. I'm the only ____ in my family. My birthday is on ____ 1st. It's the ____ day of the sixth month. It's also the Children's Day(儿童节). My parents ____ a party for me on that day. My friends ____ to the party. They bring(带来) me many birthday presents(礼物), and they sing Happy Birthday to me. ____ eat some strawberries, oranges and ____. We eat chicken and cakes ____ dinner. We also play games. We have a ____ time! How old are you? ____ is your birthday? Can you tell me?

                                                                                                                                                                                      Alan

已知a，b是平面内两个互相垂直的单位向量，若向量c满足（a-c）$·$（b一c）=0，则|c|的最大值是(  )