Science is hard enough. But when the textbooks and resources mess up it’s no wonder students get confused. These posts are not only for entertainment, but also to remind the curriculum developers amongst us about important principles of teaching and learning. Do contact us with any instructive mistakes you’ve found.
Bird-brain balanced forces
In the Balanced forces animation from Kerboodle (OUP), it shows a bird on the branch and says the forces are balanced. Then another bird comes, it says the forces are unbalanced, and the branch breaks. What’s wrong?
It’s a classic mistake in physics. A force diagram is supposed to represent forces on one object only. In this example they are mixing up the forces on two objects – the bird and the branch.
In the first diagram the bird is in equilibrium. But in the second diagram, they say that the combined weight of the two birds acts on the branch. Of course they’re right that it breaks but the explanation is confusing.
The more general point is that we need to teach concepts rigorously if students are to understand them. That means making explicit all the important details. Textbooks often contain one brief example, a few recall questions, and assume that’s enough to give students the concept. Wishful thinking in our view.