Improving on the tug of war
For a few years now, I’ve done the tug of war activity to help students explore Newton’s Third Law. It’s one of my favorite lessons. Mostly, I followed the plan from what I did last year, but came across a few key insights this year that I want to make sure to record.
- We decided the name Tug of War is a misnomer: since the tension in the rope is always the same, pulling harder on the rope really isn’t a great strategy for winning. I asked students to rename the content, and they decided it should be called friction battle.
- A new twist on setting up experiments: I asked students to think about how we could most clearly demonstrate that friction determines the winner of this contest, and we tried to set up contests that would clearly demonstrate this. The first idea was to create a team with the strongest students in the class, but have them wear socks, and then watch as they lose to the other team. From there we explore all sorts of other ways of manipulating the frictional force assuming everyone is wearing similar shoes, and this leads to one team on grass, and the other on concrete.
- The Normal Force changes things up: finally students say that you can rig the contest by holding it on a hill. This time the frictional force is the same on both teams, but now the normal force acts in the direction of victory for the team at the bottom of the hill.
- What happens when you let go of the rope?: Pretty soon, the teams decide they don’t like setting up on the losing side of a rigged battle, and so inevitably, someone decides to convince the whole team to let go of the rope. And when the other team falls over, we think about what force caused this—the frictional force of the earth.
Finally, I show students a real National Tug of War Championship, and we marvel at these matches that drag on for 5 minutes, and discuss the physics of why one team can’t just walk the other back, why the competitors then choose to lean back, and what the purpose of digging in one’s heels would do.