The QI effect

Have you ever seen the programme QI? It's one of my favourites, in fact is just about the only programme I watch on real TV, everything else, The Wire, Breaking Bad etc. is streamed from the net. QI stands for quire interesting, its a sort of quiz programme hosted by the impressively knowledgeable Steven Fry. The scoring is mostly negative, contestants loosing points for saying the obvious wrong answer. It's just like a physics class; teacher asks question knowing that a wrong answer will be given so he/she can explain why the correct answer is right. Same thing with test questions, every year I give the same question knowing the wrong answers that some students will produce. Here is a classic example:

A car accelerates from rest for 30 seconds, travels at constant velocity for a further 30 seconds then slows down to stop in 10 seconds. Draw a displacement time graph for the motion. The graph shown here is a common incorrect answer but you can see that it isn't far off. The first part has got an increasing gradient the last part a decreasing gradient and the middle constant gradient, it's just that they haven't been fitted together correctly. Shows the student has an understanding of the relationship between gradient and acceleration but hasn't got the bit about the sign of the velocity being related to the direction of travel and didn't notice that the final displacement represented by the graph is zero. Interesting how many times this solution appears, almost as many as the one I blogged about last year (Jerking around)