Resumen de Students Learning Physics While Lifting Themselves: A Simple Analysis of a Scissors Jack
Every time I have to jack up my car, I am a bit surprised by how slowly the scissors jack works the higher I raise it, and close to maximum height I need very little force to turn the crank. This agrees well with the principle of simple machines. Since I have to jack up my car at least twice a year to change between winter tires and summer tires, I thought it was time to take a closer look at the physics behind the process. And like most physics teachers, I am always looking for new ideas for my teaching. In this note I will present a few ideas on how a jack can be a topic in physics teaching.
If the class is not too large, I think the best idea is to let some of the students one by one try the jack while the rest of the class is closely watching. They start by sitting on the jack in order to raise their own body weight, as shown in Fig. 1. Instead of the crank handle I have mounted a short handle on the screw so the student sitting on the jack can comfortably raise herself. I also mounted a wooden stopper near the end of the screw so students can start from the same low position every time. The person sitting on the jack is urged to notice how the lifting process develops and whether he can feel any change. Typically, students remark that both the speed and the force change. To focus on speed, a little competition can be created by recording the time each person takes to reach maximum height. At the start it seems very easy to beat the preceding person. But as the height increases there is the feeling of slowing down and losing a lot of time. The other students standing around can observe the working principle of the jack and begin to hypothesize why it works so slowly as the height increases.
