Urgent m3 question please helpWatch
does this link not work? https://www.thestudentroom.co.uk/att...hmentid=624950
PE is negative because they start from P(where they've assigned the PE to zero) and the object is projected downwards, hence the PE is
Potential energy is measured relative to a reference point. In this case they've taken the position halfway up the circle as the zero point, hence any point below that has negative potential energy.
Potential energy is a measure of the work done in going from the reference point to your desired point. If that point is above the reference, then you have to do work to the mass and it has a +ve potential energy. If it's below the reference point, then the mass is doing work, losing energy (converted to KE if in free fall), and we have -ve potential energy.
The important value is the loss or gain in potential energy going from one point to another.
With P as the reference point:
PE before =0, PE after = -mgrsin(t)
(t for theta). So loss in energy is 0 - (-mgrsin(t))= mgrsin(t)
If we take the bottom of the circle as the reference point:
PE before (at P) is r, PE after = r-mgrsin(t)
Loss in PE = r - (r-mgrsin(t)) = mgrsin(t) as before.