One day, a teacher was giving a speech to his students .He held up（举起）a glass of water and asked the class. "How heavy do you think this glass of water is ?"
The students’ answers ranged（范围） from 20g to 500g .
"It does not matter on the weight itself .It depends on how long you hold it .If I hold it for a minute ,it is OK. If I hold it for an hour ,I will have an ache in my right arm .If I hold it for a day ,you will have to call an ambulance（救护车）.It is exactly the same weight ,but the longer I hold it ,the heavier it becomes."
"If we carry our burdens （负担）all the time, sooner or later ,we will not be able to carry on ,the burden becoming increasingly heavier ."
"What you have to do is to put the glass down ,rest for a while before holding it up again ."
We have to put down the burden sometimes ,so that we can be refreshed and are able to carry on .
So before you return home from work tonight ,put the burden of work down .Don’t carry it back home. You can pick it up tomorrow .
Whatever burdens you are having now on your shoulders, let it down for a moment if you can .
，求乙车速度的算式是（ ）。
