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Haveagoodday
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Homework Statement
A crane is slowly lifting a large box of mass 2 kg by means of a thick (but
massless) rope, from the ground to a height of 10.0 m.
a) How much work does the crane do on the box? How much work does
gravity do on the box?
b) The rope suddenly breaks and the box falls to the ground. What is its
speed as it reaches the ground?
c) Sofar, we have assumed that there is no friction of any kind. Now consider
the case when there is a resistive (air drag) force on the box as it drops down
from an initial height of 10,0 m. The resistive force is modeled by
with a = 0,2kg/m. What is the terminal speed vT of the box? Using Newton’s
second law, write down the differential equation for v(t).
d) The solution to this differential equation is of the form:
where tanh is the hyperbolic tangent function (look it up), and C is some
integration constant. If t = 0 is the time when the rope breaks, what should
C be? By direct integration, find x(t); the distance fallen as a function of
time. You will need that
where cosh is the hyperbolic cosine function (look it up). Find by insertion,
whether 1.67 s, 6.71 s or 7.16 s is (approximately) the time it takes do drop
to the ground.
The Attempt at a Solution
a) W=196 J
b) v=14 m/s
c) vT=96m/s and the differential equation --> v(t)= sqrt(vT(1-Ce^(-gt/(2v/vT))))
d) C=arctanh 0= 0
x(t)=sqrt(a/mg)*v*t
x(7.16)=10.13 m