(I'm assuming "total energy" is the horizontal line on top)
Well, gravitational potential energy varies linearly with height. Therefore, C can be ruled out immediately.
Now, we also know it can't be B, because if it were falling at constant velocity, then kinetic energy would not be increasing (nor would total energy remain constant).
Same with A, in its own sense: If there's friction slowing it down, it would be losing energy.
So, we know that all gravitational potential energy is converted to kinetic energy (not lost to any form of friction), implying that the object
is falling, and falling freely, therefore D is correct.
Nah, then v would have constant acceleration, and m*v^2 wouldn't have a constant incline.
The horizontal axis isn't measuring time, though, it's measuring distance.
Gravitation potential energy = mass * height * gravitational constant. In other words, for every inch lost in height, you're gaining the same amount of kinetic energy and losing the same amount of gravitational potential energy. In other words, it
is linear, not with respect to time, but with respect to distance dropped.
It's trivial to show that there is no friction involved at all, as friction implies energy is leaving the system, yet "total energy" (also visualized as gravitational potential energy + kinetic energy) remains constant.