在正负电子“湮灭”实验中,产生的两个光子的总能量为多少?(已知me=0.000 549u) 答案:解析:在应用质能方程解题时,应注意分析在此过程中的质量亏损,以免漏掉亏损质量.正、负电子“湮灭”造成质量亏损,生成核能,以光子的形式存在,由质能方程可求出总能量.核反应方程为:,反应前,正、负电子的总质量为5.49×10-4 u×2=1.098×10-3 u,反应后生成物是两个光子,它们的质量为零,则
We ________ three films so far this year and this is the best film that we ________ in the past three years.
[ ]
A.
saw; had ever seen
B.
have seen; have never seen
C.
will see; had ever seen
D.
have seen; have ever seen