《动量》是力学部分的一个重点内容,也是一个难点。而动量定理和动量守恒定律又是这一章的主要内容。教材中有两个关于这两部分内容的例题笔者认为值得作适当的修改或补充说明。例1 一个质量为0.18kg的垒球,以25m/s的水平速度飞向球棒,被球棒打击后,反向水平飞回,速度大小为45m/s。设球棒与垒球的作用时间为0.01s,球棒对垒球的平均作用力有多大? 分析:球棒对垒球的作用力是变力,力的作用时间很短。在这段短时间内,力的大小先是急剧地增大,然后又急剧地减小为零。在冲击、碰撞一类问题中,相互作用的时间很短,力的变化都具有这个特点。动量定理适用于变力,因此,可以用动量定理求球棒对垒球的平均作用力。由题中所给的量可以算出垒球的初动量和末动量,由动量定理即可求出垒球所受的平均作用力。解:取垒球飞向球棒时的方向为正方向。垒球的初动量 “Momentum” is a key part of the mechanics and it is also a difficult point. The momentum theorem and the law of conservation of momentum are the main contents of this chapter. There are two examples of the content of these two parts of the textbook that I believe is worthy of appropriate changes or additional explanations. Example 1 A softball with a mass of 0.18 kg flew to the bat at a horizontal speed of 25 m/s. After being hit by the bat, it reversed and flew back at a speed of 45 m/s. Let’s set the time for the club and the softball to be 0.01s. What is the average force of the club on the softball? Analysis: The force of the bat on the softball is inflexible, and the force has a short acting time. In this short period of time, the magnitude of the force first increases sharply and then sharply decreases to zero. In the problems of impact and collision, the interaction time is very short, and the change of force has this characteristic. The momentum theorem applies to invariant forces. Therefore, the momentum theorem can be used to find the average force of the bat on the softball. From the amount given in the question, the initial and final momentum of the softball can be calculated. From the momentum theorem, the average force of the softball can be calculated. Solution: Take the softball to the direction of the bat when the direction is positive. Softball’s initial momentum