Problem about Force, Work, and Energy

[mrcat100]

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)

 

[haruspex]

What 'scale' is this - what is it measuring? Should the 800m be 800N?

 

[mrcat100]

Sorry, the question is talking about a scale as in a bathroom scale, which measures FN in Newtons

 

[haruspex]

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.

 

[Nickie]

I would approach this problem by first identifying the key concepts involved: force, work, and energy. I would also make sure to clarify any assumptions or missing information, such as the mass of MJ and the direction of the force.

Next, I would draw a free body diagram to visually represent the forces acting on MJ at each point in his jump. This would help me better understand the forces at play and how they are related to each other.

Then, I would use the equations for force, work, and energy to set up a system of equations that can be solved to find the unknown variables. It is important to carefully consider the direction of the forces and the signs of the values used in the equations.

Finally, I would check my solution for reasonableness and accuracy. If the answer seems incorrect, I would go back and review my calculations and assumptions to identify any potential errors.

In this specific problem, it appears that the incorrect answer may be due to a sign error in the equation used. It is also possible that there may be other forces at play that were not mentioned in the problem statement. Therefore, it would be important to double check the calculations and assumptions to ensure an accurate solution.