Final answer:
Matter and energy are indistinguishable at the quantum level. Photons have momentum without mass, and the momentum and energy of objects have no theoretical upper limit but are practically restricted by the laws of physics.
Step-by-step explanation:
At the quantum level, matter and energy become indistinguishable as described by Einstein's famous equation E=mc^2. This equation shows that energy can be converted to mass and vice versa, underlying the principle that they are interchangeable, and introducing the concept of mass-energy equivalence.
Photons, which are particles of light, are massless yet they carry momentum p, given by p=E/c, where E is the energy of the photon and c is the speed of light. Therefore, photons can have momentum without mass, different from traditional matter which has both mass and momentum.
To answer the specific questions, objects with mass have an upper limit on the velocity at the speed of light c, but there's no upper limit to their momentum or energy theoretically as these can increase indefinitely with velocity but in practice, reaching the speed of light would require infinite energy due to the increase in mass, making it physically unattainable.
As for the power expended by a force, it can indeed be negative if the force opposes the direction of displacement. Comparatively, a 50-W light bulb can use more energy than a 1000-W oven if it is left on for a much longer period since energy consumption is power multiplied by time.