Hi, I'm trying to formulate some general ideas on how systems convert energy imparted by external forces. Could anyone please read the following and see if any of the statements and examples are correct? I really like the idea of thinking of isolated systems as "machines" through which energy flows: there is input energy in one form and output energy in other form. Here is what I have: Statement 1: Work done by external force on a system is equal to the change in total energy of the system (mechanical + thermal + any other form of energy) Statement 2: Work by done an external force on a system is always a transfer of energy in the form of kinetic energy, but the system may be instantaneously converting it to some other form of energy. Example 1: you pick up a ball When you lift the ball, your force is doing positive work to lift the ball up The ball/earth system is instantaneously converting the kinetic energy you provide to the ball by lifting it into potential energy of the system via gravity's negative work In sum, your biochemical energy that was used to impart kinetic energy to the ball has all been converted into potential energy of the earth/ball system Since originally the kinetic+potential energy of the earth/ball system was 0 and since you have increased the potential energy by lifting the ball, you have increased the mechanical energy of the system Example 2: you throw the ball upward When you throw the ball upwards, you instantaneously impart it with some kinetic energy This example is the same as example 1 except that your force is applied for only a brief moment: this means that once your force is done imparting kinetic energy, the earth/ball system starts converting all the newly added kinetic energy to potential energy while the ball is moving upwards. Once all the kinetic energy associated with upwards movement has been converted to potential energy, the system then converts the potential energy to kinetic energy associated with downwards movement as the ball falls towards the ground Statement 3: From example 1) and 2), could we generalize the isolated earth/ball system (or any other system where the conservative force is gravitational force between a large object and a small object) as a system that: continuously converts any kinetic energy associated with upwards motion to potential energy once potential energy is maximal (kinetic energy associated with any motion is depleted), begins converting all potential energy into kinetic energy associated with downwards motion Example 3: you push a block along a horizontal floor with friction: As you push the block, your force is doing work on the block/floor system to impart kinetic energy to the block Because of the kinetic frictional force between the block and the floor (which is a constant force for a given mass), the floor is continuously doing negative work on the block to convert a fixed amount the kinetic energy that your work imparts to the block. This negative work represents the kinetic energy you provide to the block being converted into thermal energy of the block/floor system In order to accelerate the block, you need to do work that exceeds the fixed amount of energy that is being converted to thermal energy by the frictional force The block/floor system is a system that converts a fixed amount of the kinetic energy imparted to the block into thermal energy of the block/floor.