I just have a feeling the teacher made a mistake... A 4-kg mass is suspended by
ID: 2025274 • Letter: I
Question
I just have a feeling the teacher made a mistake...A 4-kg mass is suspended by a cord 2 m long from the ceiling. The height of the room is 3 meters. Calculate the potential energy of the mass relative to the floor
and then...
A 4-kg mass is suspended by a cord 2 m long from the ceiling. The height of the room is 3 meters. Calculate the potential energy of the mass relative to the ceiling
he has -78 Joules for the first question and 39 Joules for the second and in my mind it makes more sense the other way around if you're using the floor as a point of reference.
Explanation / Answer
Yes you are right. First a regular secondary school answer:
Epotential = mgh
So Epotential at the height h = 1 gives Epotential1 = mg
at h = 2, Epotential2 = 2mg
so Epotential1 - Epotential2 = mg > 0
The other one:
Epotential3 = 3mg
Epotential1 - Epotential3 = -2mg
A more precise answer (although the one above should suffice)
The potential is defined by:
F=-V
In a coordinate system with the y-axis pointing up, the x-axis horizontal to the surface of the earth *(locally), the Force is pointing downwards and so F < 0 therefore V > 0 impliciting V > 0 when y goes up.. Thus your positive answer.
Turning around the coordinate system we get the opposite, a negative answer, but because the distance is twice as long, and the potential increases linearily (locally), we find also the thing you suggested for the rope-cealing problem.
Your teacher mixed the answers indeed.