带负电得小球静止在水平放置的平行板电容器两板间,距下板h=1cm,两板间的电势差为300V,如果两板间电势差减小到60V

U1=E1d;
E1q=mg;
m/q=E1/g;
U2=E2d;
am=mg-E2q;
2h=att;
tt=2h/a=2hm/(mg-E2q)=2hm/(E1-E2)q=2hE1/(E1-E2)g;