Problem about Force, Work, and Energy

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Homework Help Overview

The problem involves a scenario where an individual, MJ, is falling from a height during a jump, with specific measurements of force and height at different points. The context includes concepts of force, work, and energy, particularly focusing on gravitational potential energy and normal force as MJ descends.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the interpretation of the scale readings and the definition of heights in relation to MJ's posture during the fall. There is an exploration of the equations used in the original attempt, with questions about their validity given the context of the problem.

Discussion Status

The discussion is ongoing, with participants raising questions about the definitions of the scale and heights involved. Some guidance has been offered regarding the interpretation of the scale as a bathroom scale measuring normal force, but there is no consensus on the setup or the equations presented.

Contextual Notes

There are uncertainties regarding the measurement points for height and the implications of MJ's posture at different heights. The original poster's calculations are questioned, particularly the assumption that the normal force acts over the entire descent from h3 to h2.

mrcat100
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Homework Statement



MJ is falling from the highest point in his jump.
h1(the highest point) = 2m, and the scale reads 0N,
h2 = 1m, and the scale reads 2500N,
and h3=?m, and the scale reads 800N.
MJ is at rest at h3

Homework Equations



E1+W of point2-3= E3

The Attempt at a Solution



mgh1+FN(h2-h3)+Fg(h2-h3)= mgh3, because there is no KE1 or KE3,
80(10)(2)+2500-2500h3-800+800h3= 800h3,
3300= 2500h3, h3 = 1.32m (obviously incorrect as h3 is higher than h2)
 
Last edited:
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What 'scale' is this - what is it measuring? Should the 800m be 800N?
 
Sorry, the question is talking about a scale as in a bathroom scale, which measures FN in Newtons
 
Then my next question is how exactly the heights are defined. At h2, he must be in contact with the scales, so the height is not being measured from scales to feet. If it's from scales to anything higher than the ankle it will depend on posture - maybe he's crouching at h2, and the heights are measured to the head. But a reasonable guess is that he's upright in each case and the heights are measured to c of g.
I don't see how you can write the equations you have in your attempt. They imply e.g. that the scales supplied an upward force of 2500N for the entire distance as he descended from h3 to h2.
 

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