Final answer:
The statement could be true if non-conservative forces do no work during an object's fall because the total mechanical energy of the system would remain constant, maintaining the sum of potential and kinetic energy.
Step-by-step explanation:
If non-conservative forces do no work during an object's fall, it implies that only conservative forces are doing work, such as gravity. According to the law of conservation of mechanical energy, the total mechanical energy in a system remains constant if only conservative forces are present.
Therefore, if an object has 20 Joules of potential energy and 30 Joules of kinetic energy at one point during its fall, it maintains a total mechanical energy of 50 Joules throughout its motion as long as there are no non-conservative forces like friction or air resistance doing work on it. So, the statement that 'If non-conservative forces do not do work during an object's fall, its potential energy at some point will be 20 J and its kinetic energy will be 30 J' could be true provided there are no energy losses to non-conservative forces.