根据短文内容判断正误。

    Tom has a strong and faithful dog, Bruce. The dog likes playing with Tom. Sometimes he also follows Tom's father to work.
One morning when Tom's father worked in the field, he took off his coat and put it on the ground under a big tree. He turned to the dog and said,“Watch my coat, Bruce!”
   Bruce sat down on the coat. Later Tom's father remembered he had to buy some food. So he hurried to the shop. After shopping, he went home directly and forgot all about his coat and the dog.
   Late in the evening Tom looked for his dog everywhere in the house, but he couldn't find him. At this time Tom's father suddenly remembered his coat and the dog. He went back to the tree at once. He found that Bruce was still sitting on the coat!

在水平面上放着两个质量分别为2 kg和3kg的小铁块m和M，它们之间用一原长为10 cm，劲度系数为100 N／m的轻弹簧相连，铁块与水平面之间的动摩擦因数均为0.2。铁块M受到一大小为20 N的恒定水平外力F，两个铁块一起向右做匀加速直线运动，如图所示。这时两铁块之间弹簧的长度应为（重力加速度g取10 m／s²）(   )