![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjqVSpm9PjBPHx24klrt859EuZ2ZeDpmfFYjJ1JWoNrevK9I0QpOnGEZkz51-4yzJ2GmbgacdIB8R1-uvTnFRK1INGACfskRt-TOG2O7eP8a2D6sUL6twHpwNpFGY3FAsaLsjnw1TQpwE8/s400/Picture+2.png)
I like this picture. And one reason I like it is because it reminds me that there is no correct way to a model a situation using a free-body diagram. Here are four free-body diagrams.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhb_V_wXKjrhTD8sOSoD66OxUbqzJXd7rAfgyrP9wGPLigq44vjW5bLOOGLZ7M4FCpbupPcWXRgOLZC8NuCIi6ZfqcN9PZMi3UAkGy5KudYlkMxkw9lzBlwlIhNMT28s_C8vIabmSoBQjg/s400/Picture+3.png)
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiKq78v_ujZPgMNCHgCMeg6eW7JJH7DS4WzpJF49t5biuVklAt8xFfrvcUTfVPrVyQEZm5MYy-JMm11fWGa5qlSVSNFYv1b4rEEcv7xKaYJU22hsQmnozAthzLhWaaaKFw9KYyCQlVGTEM/s400/Picture+4.png)
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiRKK1h5Twq-opVNshG-DpfsO-KAUthUpNYP81DzmnEDn2M3uVlc7joR1ahwDQnorNuURq7r9_dJeJBXdq71xANpiLjNbZIBhr_Lql62DjhRmHu8PmhpzpN3YrM9kknwCpKT6v_U9Lpi-8/s400/Picture+6.png)
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgD-zw-CNbxxs9LKN4FI7D_MrJVQn4VE2T1dJY4anN5ArhAGD7ztDEv_DbDkjawBdM9-4kJjdi596FZWiJJGHYGS_ANsxi29l7d3TFp54RCFhgGiyZEYvK831R7OwxcyZZD-Rd6N8ROiiw/s400/Picture+7.png)
While the models becomes more sophisticated; I don't think any of these models is correct or incorrect. Depending on its purpose, sometimes the simplest model is the best one. All of this makes me wonder what dis-service we do to students when we over-emphasize the correctness of a free-body diagram and under-emphasize the fact that we are building and refining models. As we come to notice and become concerned about more nuanced features of a situation, we may (or may not) choose to attend to those features.
Brian, this is absolutely awesome! I wonder what students would actually do with (or to?) this problem, especially after we have kept hammering them with the "simple" free body diagrams in intro physics...
ReplyDeleteYeah, I think it could be interesting to see what students do, especially if you were to explicitly ask them to draw more than one free-body diagram to model the situation. They would probably need some practice, and might go better in groups, but I think it could go well if done right.
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