Letting Students Discover Torque
After a couple of labs where the data was messy and students were unsure of how good their experimental technique was, it was time for a lab where things are more straight-forward and students could regain some confidence. I scheduled my "beam equilibrium lab". The set-up is simple. Take two force meters and lay a meter stick on top. A mass is then placed on the meter stick at various places and the forces are recorded.
I usually do this lab after having introduced the concept of torque and that a static situation means that the sum of the forces is zero and the sum of the torques is zero. Students make calculations based on the static conditions and confirm them with this experiment. This year, I changed things. Torque was not mentioned beforehand.
This year, I started by having my guys place the meter stick on the force meters at 20 and 80 cm, and then place a mass at 30, 40, 50, 60, and 70 cm. We then look for any patterns. Most see right away that the 30-70 readings and the 40-60 readings are reversed. Some will see that the close the mass is to a meter, the higher the recorded force, so it looks like how far the mass is from the support point is important. We then set the meters at 10 cm and 90 cm and place the mass at various places in between. Again I ask if they see any patterns. Some teams do, but I give the hint of pairing the force and distance measurements. After a few minutes, I can see "the light-bulb moment" for most. They are then to make predictions about what they would see when the meters are placed at 25 and 75 cm. Success! The class wraps up with a discussion of the concept of torque and that they discovered that for a system to be in static equilibrium, the sum of the torques has to be zero as well as the sum of the forces.
Sometimes, letting your students find things out themselves is easy to do.