(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

An object with an initial velocity V0 of 14 m/2 falls from a height of 240m and buries itself in 0.20 m of sand. The mass of the body is 1.0 kg. Find the average resistive force exerted by the sand on the body. Neglect air resistance.

2. Relevant equations

change in K = W (of resultant force)

change in K + change in U (potential energy) = 0 [conservative forces]

change in K + change in U + work done by friction = 0 [non conservative friction]\

...et c

3. The attempt at a solution

Solution from textbook:

the kinetic energy of the body as it is about to hit the sand:

K = (mV0^2)/2 + mgh

where m is mass g is gravity h is the height from the sky to the sand

Also, from the work-energy principle: (then it says "approximately")

K = Fs, where F is the average resistive force of the sand onto the body and s is the distance into the sand.

This part is weird to me.. shouldn't change in K equal to the work done as the object falls from the sky? How come K = Fs?

Then they just plug things in and solve.. I just don't get how they could just ignore the non-conservative force from friction and how they could just say that K = Fs..

Thank you.. please help

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# Energy, non-conservative and conservative forces question

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