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holezch
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Homework Statement
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.
Homework 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
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