Hello, I'm Jack. I live in a house. My address is 246 Second Street. It's far from the school. So I go to school by bus every day. But my house is near the park. So I often go to the park on foot. Tom is my friend. He lives in a house, too. It is near the school. So he goes to school on foot every day.

一根弹簧的劲度系数为K=500N/m，竖直悬挂在天花板上，下端有一质量为5Kg的物体（在弹性限度内），处于静止状态，则这时弹簧的伸长量为（   ）