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A mass weighing 32 lb stretches a spring 6 inches. The mass is in a medium that exerts a viscous resistance of 93 lb when the mass has a velocity of 6 ft/s.

Suppose the object is displaced an additional 5 inches and released.

Find an equation for the object's displacement \( u(t) \) in feet after \( t \) seconds. Answer exactly, or round coefficients to decimal places.

\[ u(t) = \ _________________ \]

Answer :

Mass weighing 32 lb stretches a spring 6 inches. A mass is in a medium that exerts a viscous resistance of 93 lb when the mass has a velocity of 6 ft/s. The object is displaced an additional 5 inches and released. Let us determine the spring constant, k.

Using Hooke's law,

F = -kx …(1)

Where,

F = force = mass * acceleration

k = spring constant

x = extension of the spring = 6 inches = (6/12) ft = 0.5 ft

We know that Force = mass * acceleration

a = F/m = (-kx)/m

m = 32 lb = 32/32.2 slugs

Acceleration = a = -kx/m

k = -ax/m = -(32/32.2)(0.5)/(32/32.2)

k = -0.0784 lb/ft

Displacement, x = 5 inches = 5/12 ft

Time constant, τ = m/k

τ = 32/0.0784 = 408.1633

The differential equation of motion of a body with viscous resistance is given by

mdv/dt + cv = -kx, where v is the velocity of the body, and c is the viscous resistance.

Since v = dx/dt, we have,

md²x/dt² + cdx/dt + kx = 0

md²u/dt² + cd/dt(u) + ku = 0 …(2) [Let u = displacement]

As per the given data,

c = 93 lb/kv

u = Ae^(rt) …(3) [Characteristic equation of the differential equation]

Where A and r are constants, which can be found from the initial conditions.

At t = 0, u = 0

u(0) = 0, u'(0) = 0

u(0) = Ae^0 = 0, i.e., A = 0

u'(0) = Ar = 0, i.e., r = 0

Hence, u(t) = 0

[tex]u(t) = c1e^0t + c2te^0t = c1 + c2 …(4)[/tex]

Since, u(0) = 0

u(0) = c1 + c2 = 0 …(5)

At t = τ, u = 0

[tex]u(τ) = c1e^(0) + c2τe^(0) = 0 …(6)[/tex]

On solving (5) and (6), we get,

c1 = 0, c2 = 0

Therefore, u(t) = 0. Hence, the displacement of the object is zero.

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