On the value of showing work and corrections
A quick summary is that some kids go crazy explaining every step of their work in words, and miss the point that writing the mathematics out algebraically allows you to communicate the same ideas, less ambiguously and more succinctly.
Avery has a great example from al khwarizmi’s 8th century math text. Read this, and you’ll see why books of arithmetic ran hundreds of pages in the middle ages, and algebra was topic reserved for the greatest math minds at university even in Newton’s day.
Al Khwarizmi’s text (translated)
“If some one say: “You divide ten into two parts: multiply the one by itself; it will be equal to the other taken eighty-one times.”
This has got me thinking about my students, our correction process, and why I think things might be a little different in physics.
Here’s a problem from my last test, stripped of the context
Suppose that there are 2 downward forces acting on an a basket at rest, 20N and 2N, and a third, upward force is supplied by a spring scale. What force does the spring scale exert on the basket?”
Typical student work:
Though the answer is right, I marked this wrong, and asked the students to correct it, where they have to:
- Solve the problem again from scratch, explaining each step of their work with a sentence.
- Explain the mistake in detail, looking for deep misconceptions that led to the mistake.
- Explain a deeper insight in the problem. This may be solving a different, but similar problem, but most often should be explaining a feature of the problem that isn’t always evident.
Here’s an example of a correction:
Since the basket is at rest, we know the net force is zero:
There are 3 forces acting on the basket (the magnetic force, the gravitational force of the earth, and the tension force of the spring (upward)).
Solving for the spring force, we find
Assuming down is the negative direction, and plugging in values we find
The force exerted by the spring is 22N, upward.
- My original mistake was not starting with Newton’s first law, and instead simply adding up the forces.
- My deeper understanding is that I can see if the net force known to be zero, I would not be able to solve for the tension force. So if the scale were bouncing up and down (a changing net force) it would be not be possible to determine the mass of an object in the basket.
Of course, rarely do corrections completed by students ever reach that level of explanation, but most of them do begin to see that equations aren’t ideas we simply pull out of thin air, plug values in and write down answers. They represent ideas and are only valid in certain conditions.
So in physics, I’d say showing your work is vital, and when a student is stuck, one of the best things to do is explain every step in English, rather than just searching for equations. But I do like Avery’s point that I want my students to see the value of mathematical notation for expressing ideas clearly and succinctly.
Maybe I’ll try to create some exercises where students have to explain what an equation is saying, or when it is valid…