Mechanical energy of a spring system

1. The problem statement, all variables and given/known data
A damped mass-spring system oscillates at
285 Hz. The time constant of the system is
8.8 s. At t = 0 the amplitude of oscillation
is 1.3 cm and the energy of the oscillating
system is 36 J.
Part 1: What is the amplitude of oscillation at t =
8.7 s?
Answer in units of cm
Part 2: How much energy is dissipated in the first
period (8.7 s interval)?
Answer in units of J

Part 3: How much energy is dissipated in the second
period (8.7 s interval)?
Answer in units of J

2. Relevant equations
A = A(initial) * e-(t/time constant)
E = E(initial) * e-(t/time constant)

I followed the method of the attached picture and couldn’t get the correct answer.

3. The attempt at a solution
Part 1: Answered correctly:
A(8.7 s) = (1.3 cm) * e-(8.7/8.8) = 0.4837088518 cm

Part 2:
Change in mechanical energy between 0 and 7.8 seconds:
ΔE = -E(initial) * (e-(8.7/8.8) – e-(0/8.8))
ΔE = -(36 J) * (e-(8.7/8.8) – 1)
ΔE = 22.604985 J
Which was incorrect

Part 3:
ΔE = -(36 J) * (e-(17.4/8.8) – e-(8.7/8.8))
ΔE = 8.4109474 J
Which was also incorrect.

Attached Images
File Type: jpg phy123.jpg (29.3 KB)

http://ift.tt/1ewH9ai

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