Question

The motion of a simple spring hanging from the ceiling can be modeled with a cosine function. The bottom of the spring has a maximum height of 7 feet 4 inches and a minimum height of 6 feet 2 inches from the floor. It takes 2 seconds for the spring to expand from its minimum length to its maximum length. What is a cosine function that models the spring’s length in inches above and below its average, resting position? Express the model as a function of time in seconds

Answer 1

To get the average resting position of the spring, we must subtract the minimum from the maximum height of the spring with respect to the floor.
So,
7 + (4/12) - [ 6 + (2/12) ] = 1.5 ft

Half of this is where the resting position of the spring is located with respect to the minimum height. If the resting position of the spring is set at 0, then the amplitude of the wave is:
A = 1.5 / 2 = 0.75

Since it takes 2 seconds for the spring to expand from minimum to maximum,
Period = 2 = 2π / n
n = π

The cosine function is:
 y = A cos nt

where y is the distance from the resting position and t is the time in seconds.
y = 0.75 cos πt