Final answer:
The best example of work being done on a system, where energy is inputted, is a car engine burning fuel to propel a vehicle. Conversely, work done by a system occurs when energy is outputted, such as a car's brakes generating heat to slow the vehicle. These principles illustrate the transfer of energy through work in physics.
Step-by-step explanation:
When work is done on a system, energy is transferred into that system. The scenario that best exemplifies work being done on a system in the options provided is (a) A car engine burning fuel to move the vehicle forward. In this scenario, the engine performs work by converting chemical energy from the fuel into kinetic energy, moving the car forward. Contrarily, work performed by a system involves energy being transferred out of the system. An example of this would be (b) A car's brakes slowing it down, where the kinetic energy of the car is transformed into heat energy in the brakes and dissipates into the environment.
In the case of the lawn mower example from Figure 7.2(a), energy from the person pushing the mower is transferred into the mower and eventually leaves the system as heat due to friction. When the briefcase is carried upstairs, the work done stores energy within the briefcase-Earth system, which can later be recovered.
In real-world scenarios, such as a car accelerating or braking, work leads to changes in the kinetic energy of the car. As the engine accelerates the car, it performs work on the vehicle. When the car brakes, energy is removed from the car, reducing its speed. Overall, the principles of work and energy transfer are evidenced in everyday phenomena such as driving a car, which provides practical insights into the concepts of physics.