老师
同学们大家好，今天我们一起学习简单机械中的杠杆。首先我们来一起看一下学习目标。一、知道什么是杠杆儿，知道杠杆儿的五个要素，会画出已知动力或阻力的动力臂或阻力臂。二、通过探究杠杆的平衡条件，会用杠杆平衡条件解决简单的实际问题。三、会根据杠杆平衡条件判断生活中杠杆的类型。下面我们先来通过一段视频认识一下杠杆。
学生
A famous ancient Greek once said, give me a place to stand and I shall move the earth. But this wasn't some wizard claiming to perform impossible feats. It was the mathematician Archimedes describing the fundamental principle behind the lever. The idea of a person moving such a huge mass on their own might sound like magic, but chances are you've seen it in your everyday life. One of the best examples is something you might recognize from a childhood playground, a teeter totter or seesaw. Let's say you and a friend decide to hop on. If you both weigh about the same, you can totter back and forth pretty easily. But what happens if your friend weighs more? Suddenly, you're stuck up in the air. Fortunately, you probably know what to do. Just move back on the seesaw, and down you go. This may seem simple and intuitive, but what yo