Elevator Problem: Find Scale Reading in Falling Elevator

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Homework Help Overview

The problem involves a scenario where a person stands on a scale in a falling elevator, specifically questioning the scale reading when the elevator is descending at a constant speed of 3.5 m/s. The subject area pertains to dynamics and forces, particularly the concepts of weight and acceleration in a non-inertial reference frame.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the implications of constant speed on acceleration, with one participant noting that if speed is constant, acceleration is zero. There is also an exploration of the net force equation and its application to the scenario.

Discussion Status

The discussion is active, with participants questioning the assumptions regarding acceleration and its effect on the scale reading. Some guidance has been provided regarding the use of net force equations, but there is no explicit consensus on the correct approach or solution yet.

Contextual Notes

There is a mention of a specific answer being incorrect according to an external source (Webassign), which raises questions about the assumptions made in the calculations. Participants are also encouraged to define a positive direction for acceleration in their equations.

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Homework Statement



A 80.0 kg person stands on a scale in an elevator.
(c) What does it read when the elevator is falling at 3.5 m/s?

Homework Equations



EF = MA

The Attempt at a Solution



I can't even attempt the solution because I don't know acceleration...
 
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Toxage said:
A 80.0 kg person stands on a scale in an elevator.
(c) What does it read when the elevator is falling at 3.5 m/s?
…
I can't even attempt the solution because I don't know acceleration...

Hi Toxage! :smile:

If the speed is constant, then the acceleration is … ? :wink:
 
tiny-tim said:
Hi Toxage! :smile:

If the speed is constant, then the acceleration is … ? :wink:

Then the acceleration is equal to 0.

EF = MA

EF = M(0)

EF = 0

Fn - Fg = 0

Fn = Fg

80(9.8) = 882 Newtons

882/9.8 = 80kg

Doesn't work... Webassign says its the wrong answer...


Did I do something wrong in my math?
 
No your maths looks fine. What is the quoted answer? If your standing on scales and not accelerating the scales will tell you your weight. When you are accelerating use this formula:

[tex]F_{net} = m(a + g)[/tex]

But MAKE SURE YOU DEFINE A POSITIVE DIRECTION AND STICK TO IT. So if you define up as positive, g = -9.81. And then when accelerating up, a is positive and vice versa.
 

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