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What was Dr.Lavelle referring to with the analogy of the boulder? The boulder is able to go down a hill without work, but for the boulder to go up a hill work is required. Can someone explain this in further detail?
He was saying that a boulder could just roll down the hill on its own, so it wouldn't require work to roll down the hill. But, the boulder will not roll up the hill on its own, so it will require work to be pushed up the hill.
This analogy was related to the concept of a reaction occurring spontaneously. I think that Dr. Lavelle just wanted to convey the idea that reactions will have a certain outcome, say the rock rolling down, if not influenced by the surrounding. The rock is not likely to go up the hill, similarly, a reaction won't proceed in a direction that requires energy unless energy is supplied to the system.
I think Lavelle was also referring to the idea of doing work on surroundings vs. on a system. If the boulder rolls down the hill, it does work on the surroundings because it releases energy (work is positive- exergonic). When the boulder is pushed up the hill, it's an endergonic process because work is being done on the system (work is negative).
Some reactions, usually exothermic reactions, occur spontaneously and are favored. There is no work forcing the reaction to occur and not heat is put into the system. Instead, the system will react on its own, releasing energy as heat. This is similar to a boulder rolling down a hill. Gravity does all the work and no energy is put into the system. On the other hand, endothermic reactions will require some work and/or heat input in order for the reaction to occur. This is then related to rolling the boulder up a hill. If the boulder is sitting there, it will not move up on its own; it will require some energy input to move up the hill.
If you literally place a boulder on the side of the mountain/hill, it will roll down on its own due to the pull of gravity. We do not expect it to go up, unless another force was acted upon it (say, a machine) that could pull it up.
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