A brick of mass 8 kg hangs from the end of a spring. When the brick is at rest, the spring is stretched by 3 cm. The spring is then stretched an additional 4 cm and released. Assume there is no air resistance. Note that the acceleration due to gravity, g, is g=980 cm/s2. Set up a differential equation with initial conditions describing the motion and solve it for the displacement s (t) of the mass from its equilibrium position (with the spring stretched 3 cm). s (t) = 4 cos (sqrt (980/3) t) cm

s (t) = 0.04cos (sqrt (980/3))

Step-by-step explanation:

Find spring constant. Use balance force between weight of brick and spring elastic force when brick at rest:

Weight of brick = force of spring

mg = kx

Mass brick, m = 8kg

Gravity constant, g = 9.8 m/s2

Spring elongation, x = 0.03m

Hence, spring constant, k = mg/x

= 8*9.8/0.03

= 7840/3

Since there is no other external forces, spring acts in simple harmonic motion

-kx = ma

a = - kx/m

note that a is the acceleration which is the double derivative of distance over time. Hence

d2y/dx2 = - kx/m

d2y/dx2 + kx/m = 0

Note that this equation is similar to simple harmonic motion:

d2y/dx2 + (w2) x = 0

Comparing these two equations we found:

w2 = k/m

Using the values obtained earlier:

w2 = (7840/3) / 8 = 980/3

w = sqrt (980/3) = 18.07 rad/s

Since the movement of the spring will be sinusoidal, similar to the movement of pendulum, we use the general equation for oscillating motion:

s (t) = A sin (wt + c)

Note that in initial condition when t=0,

displacement of spring s = A = 0.04m

Hence 0.04 = 0.04 sin (0+c)

sin c = 1

c = π/2 - hence indicating that the motion is π/2 further than the equation.

Using trigonometry identity, we know that cos (theta) = sin (π/2 + theta)

So, we can change sin (wt+π/2) to cos (wt)

Updating the equation, we'll get

s (t) = 0.04 cos (wt).

w = sqrt (980/3)

Finally,

s (t) = 0.04cos (sqrt (980/3)) (in m)

Or

s (t) = 4cos (sqrt (980/3)) (in cm)