4th grade is studying electric motors. To break down the concept to first principles we start with an electromagnet: a coil of wire wound around a metal cylinder. As electricity flows through the coil a magnetic field develops around the metal core. We can attract or repel any other metal that contains iron by controlling the amount of electric current in the coil. Most people think about motors as something that makes a shaft or an axle turn, but motors quite often need to stop and hold in a certain position. Examples of those motors are servos and stepper motors and we build machines around those in 7th grade. Think about this project as a single step stepper motor: It only holds position when it is on and lets a weight drop when power turns off.
We took our strongest 4th grader and had him apply full force to pull the metal plate with the handle on the bottom away from the silver electromagnetic coil attached to the wooden dowel on top. The student was able to break the magnet's 24volts/3amp grip by applying more than 72 watts of downward pressure to the handle