D$1$: An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, burn a fuel to create heat which is then used to create motion. Electric motors convert electrical energy into mechanical motion, pneumatic motors use compressed air and others, such as clockwork motors in wind$-$up toys use elastic energy. In biological systems, molecular motors like myosins in muscles use chemical energy to create motion.

D$2$: The internal combustion engine is an engine in which the combustion of a fuel $($normally a fossil fuel$)$ occurs with an oxidizer $($usually air$)$ in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high$-$temperature and high$-$pressure gases produced by combustion apply direct force to some component of the engine. The term internal combustion engine usually refers to an engine in which combustion is intermittent.

a$.$ Which terms do you think have the highest log tf weighting in the above two documents? First make a guess, without any complicated computation. Then, check your guesses using the formulae from the slides and the book.

b$.$ Which index terms would you choose for D$1$ vs D$2$ so that they are differentiated from each other? The index terms you choose should be able to describe the two documents with respect to each other, so you can say D$1$ is about X while D$2$ is about Y$.$

Consider the query term: Engine.

c$.$ Which of the two documents is more relevant to the query? Does tf support your decision?