阅读短文，下列问题

B

      Mark Twain was a great writer in America. He wrote many books. He liked telling jokes in his books. People liked reading them very much. And it was full of jokes in his life, too. Mark Twain liked reading. So he often borrowed books from other people. One day, he wanted to borrow a book from his neighbor. His neighbor was a mean man. He didn't want to lend it to Mark Twain. Then he had an idea. He said to Mark Twain, “I have a rule. You will have to read the book at my home. If you want to borrow it from me, you should never take it out." Mark Twain had to read that book at his neighbor's house.

     A month later, his neighbor went to borrow a cropper (割草机) from Mark Twain. Mark Twain said to him with a smile, "I make a rule, too. That is, you must use my cropper in my garden. If you borrow it, you can never take it out either."

— ___________?

— That sounds good!

请你设计一个比较两位同学从一楼上到三楼做功快慢的实验：
（1）已提供的器材是磅秤和皮尺，还需要的器材是____ ；
（2）实验需要测量的物理量是____ 、____ 和____ 。
（3）用所测量的物理量写出表达式____ .