The discussion focuses on the mechanics of elevator lift systems, specifically analyzing the power dynamics between the elevator, its counterweight, and the motor. The key equation used is P=Fv, where the power is derived from the force exerted by the weights of the elevator (m1) and counterweight (m2) and their velocities. The participants clarify that the energy difference in the system during a finite time Δt is represented as (m1-m2)gΔh, and they emphasize the importance of understanding the signs in work done by the motor and gravity. The conversation concludes with a clear understanding that power can have directional implications despite being a scalar quantity.
PREREQUISITESMechanical engineers, physics students, and professionals involved in elevator design and maintenance will benefit from this discussion, as it provides insights into the fundamental mechanics of lift systems and power dynamics.
Yes, the motor is working at that rate in raising the elevator, but what is happening on the other side? How much power is being transferred, and which way, between the counterweight and the motor?Janiceleong26 said:
Energy difference would be (m1-m2)gΔh ?stockzahn said:
m2gv, and anti-clockwise? But I thought power is scalar? Does the minus sign comes from the velocity?haruspex said:
Correct. And instead of distance and energy you need velocity and power - so what to do?Janiceleong26 said:
Divide it by Δt and you will get velocity and (m1-m2)g is the force, so force times velocity gives power. I got it! Thanks!stockzahn said:
Even scalar values can have sign. If A does positive work on B then B does negative work on A.Janiceleong26 said:
Oh I see, thanks. But why is there a negative sign before the mass?haruspex said:
Because m(-g) is the force from gravity. The motor has to supply an equal and opposite force, -m(-g).Janiceleong26 said: