Final answer:
The statement that a non-conservative force doing work on an object will result in a gain or loss of mechanical energy is true. The work-energy theorem supports this by stating that the work done by non-conservative forces equals the change in mechanical energy.
Step-by-step explanation:
When a non-conservative force is doing work on an object, and it is the only force doing work, the object will either gain or lose mechanical energy. This statement is True. Non-conservative forces, such as friction, cause a change in the mechanical energy of a system by converting it into other forms of energy, like thermal energy. This is in contrast to conservative forces, which do not cause such changes in mechanical energy.
The work-energy theorem tells us that the change in mechanical energy (which includes both kinetic and potential energy) is directly related to the work done by non-conservative forces. So, if non-conservative forces are doing work on an object, the mechanical energy of that object will change.
As a practical example, consider the act of stopping a vehicle. You must exert more work to stop a truck compared to a mosquito, because the truck has greater mechanical energy that needs to be dissipated, usually as thermal energy due to friction.