Problem about Force, Work, and Energy

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SUMMARY

The discussion centers on a physics problem involving MJ's descent and the forces acting on him at different heights. The key heights are h1 at 2m with a scale reading of 0N, h2 at 1m with a reading of 2500N, and an unknown height h3 where the scale reads 800N. The equations used to analyze the situation include energy conservation principles and the work-energy theorem, leading to an incorrect calculation of h3 as 1.32m. The participants clarify the nature of the scale and the definition of heights in relation to MJ's posture during the measurements.

PREREQUISITES
  • Understanding of gravitational potential energy (mgh)
  • Familiarity with the work-energy theorem
  • Knowledge of forces and Newton's laws of motion
  • Basic concepts of static equilibrium and normal force (FN)
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  • Study gravitational potential energy calculations
  • Learn about normal force and its implications in static scenarios
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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)
 
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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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