in my homework, there's a diagram of a horse pulling a boat along the canal. The force acts at an angle to the actual direction of the movement. One of the questions is: is it better to use a long or short rope?
The best way to pull the boat would be for the horse to be directly in front of the boat. This way all of the horse's energy would be put into forward motion of the boat, none into sideways motion.
Obv this can't be the case because horsie would drown :-(, so we want to minimise the angle between the horse and the 'straight ahead' direction for the boat, to maximise the energy that goes into pulling the boat forward (as opposed to sideways).
A longer rope does this.
Interestingly, a longer rope would also weigh more, and the horse would expend some extra energy just hauling the rope around! But this would not outweigh the benefit of the longer rope unless the longer rope was really really long. Your teacher probably did not think of this.