As a raindrop falls, it evaporates while retaining its spherical shape. If we make the further assumptions that the rate at which the raindrop evaporates is proportional to its surface area and that air resistance is negligible, then a model for the velocity v(t) of the raindrop is 3(k/p) (k/p)t + ro Here p is the density of water, ro is the radius of the raindrop at t= 0, k < 0 is the constant of proportionality, and the downward direction is taken to be the positive direction. (a) Solve for v(t) if the raindrop falls from rest. v(t) = dv dt -v=g. (b) This model assumes that the rate at which the raindrop evaporates-that is, the rate at which it loses mass-is proportional to its surface area, with constant of proportionality k<0. This assumption implies that the rate at which the radius r of the raindrop decreases is a constant. Show that the radius of the raindrop at time t is r(t) = (k/p)t + 50₁ Letting A denote the surface area of the raindrop and m the mass, the model assumes that and r. dmk A. dt Since mass equals density times Select and a sphere of radius r has volume m = A- and surface area we obtain the following formulas form and A in terms of p

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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As a raindrop falls, it evaporates while retaining its spherical shape. If we make the further assumptions that the rate at which the raindrop evaporates is proportional to its surface area and that air resistance is
negligible, then a model for the velocity v(t) of the raindrop is
3(k/p) -v = g.
dv
dt (k/p)t + ro
Here p is the density of water, ro is the radius of the raindrop at t = 0, k < 0 is the constant of proportionality, and the downward direction is taken to be the positive direction.
(a) Solve for v(t) if the raindrop falls from rest.
v(t) =
(b) This model assumes that the rate at which the raindrop evaporates-that is, the rate at which it loses mass-is proportional to its surface area, with constant of proportionality k < 0. This assumption.
implies that the rate at which the radius r of the raindrop decreases is a constant. Show that the radius of the raindrop at time t is r(t) = (k/p)t + ro.
Letting A denote the surface area of the raindrop and m the mass, the model assumes that
dm
dt
and r.
Since mass equals density times ---Select-- and a sphere of radius r has volume
= k. A.
m =
A =
and surface area
, we obtain the following formulas for m and A in terms of p
Transcribed Image Text:As a raindrop falls, it evaporates while retaining its spherical shape. If we make the further assumptions that the rate at which the raindrop evaporates is proportional to its surface area and that air resistance is negligible, then a model for the velocity v(t) of the raindrop is 3(k/p) -v = g. dv dt (k/p)t + ro Here p is the density of water, ro is the radius of the raindrop at t = 0, k < 0 is the constant of proportionality, and the downward direction is taken to be the positive direction. (a) Solve for v(t) if the raindrop falls from rest. v(t) = (b) This model assumes that the rate at which the raindrop evaporates-that is, the rate at which it loses mass-is proportional to its surface area, with constant of proportionality k < 0. This assumption. implies that the rate at which the radius r of the raindrop decreases is a constant. Show that the radius of the raindrop at time t is r(t) = (k/p)t + ro. Letting A denote the surface area of the raindrop and m the mass, the model assumes that dm dt and r. Since mass equals density times ---Select-- and a sphere of radius r has volume = k. A. m = A = and surface area , we obtain the following formulas for m and A in terms of p
Plugging these formulas into the differential equation gives:
dt
).
.
55 55 55
r(t) =
Plugging in r(0) = ro gives C =
dr
dt
dr
dt
dr
= k· 4πr²
= k· 4πr²
= k
=
dt
Integrating the above differential equation gives
r(t) =
Je + C.
(assuming r = 0)
, so that
(c) If ro = 0.05 ft and r = 0.008 ft 10 seconds after the raindrop falls from a cloud, determine the time at which the raindrop has evaporated completely. (Round your answer to one decimal place.)
sec
Transcribed Image Text:Plugging these formulas into the differential equation gives: dt ). . 55 55 55 r(t) = Plugging in r(0) = ro gives C = dr dt dr dt dr = k· 4πr² = k· 4πr² = k = dt Integrating the above differential equation gives r(t) = Je + C. (assuming r = 0) , so that (c) If ro = 0.05 ft and r = 0.008 ft 10 seconds after the raindrop falls from a cloud, determine the time at which the raindrop has evaporated completely. (Round your answer to one decimal place.) sec
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