# Damped harmonic motion problem

Homework Statement:
Determine the deviation at a time when the amplitude has
dropped to 1/5 of the original value.
Relevant Equations:
A=A0 e^-bt
Hi !
Problem :
y = 5 e^-0.25t sin (0.5.t) (m, s). Determine the deviation at a time when the amplitude has
dropped to 1/5 of the original value.
I tried with A=A0 e^-bt=5 e^-0.25t
- Do i need to determine the time here or recreate the deviation equation when A decreased ? I don't understand the meaning of the question .

Delta2
Homework Helper
Gold Member
Since I know french is your mother language, I think you translated wrong the term deviation. I think the problem wants the moment in time where the amplitude has drop from 5 to 1.

what is the original value of the amplitude?
What is the value of amplitude after time t?

Delta2
Homework Helper
Gold Member
Ah on second thought by deviation you might mean the value of y. Then you first have to determine the time ##t_0## that the amplitude drops from 5 to 1, and then find ##y(t_0)##.

Last edited:
Steve4Physics
hehe i'm mixing french with english what gives a masterpiece.
Sorry for the late response, i'm tired cause of no sleep, i'll be back in 8hours from now to work on it.

Delta2
Ah on second thought by deviation you might mean the value of y. Then you first have to determine the time ##t_0## that the amplitude drops from 5 to 1, and then find ##y(t_0)##.
Hi !
You mean that i need to determine the value of t0 from the equation
A=A0 e^-bt0
and then calculate y(t0) ?

berkeman
Mentor
Hi !
You mean that i need to determine the value of t0 from the equation
A=A0 e^-bt0
and then calculate y(t0) ?
It would help a lot if you could start to learn how to post mathematical equations using LaTeX instead of using just the keyboard to try to write equations. There is a helpful "LaTeX Guide" link just below the Edit box that will get you started.

For example, what you wrote above would look like this (I think, assuming that I'm interpreting what you wrote correctly):
$$A(t) = A_0 e^{b_0 t}$$
and then you calculate ##y(t_0)##?

Also, I'm confused by the title of the thread referring to "damped harmonic motion", but I don't see any oscillating term in your equations. Can you clarify that? Thanks.

meher4real
The problem is from damped harmonic motion course.
I'll learn how to do it, thank you
this is my results A(t)=A0e^{-0.25t} => 1=5e^{-0.25t} then t=6.43s
yf=5e^{-0.25t}sin(0.5\pi t)
yf=0.17ms-1

Steve4Physics
Homework Helper
Gold Member
this is my results A(t)=A0e^{-0.25t} => 1=5e^{-0.25t} then t=6.43s
yf=5e^{-0.25t}sin(0.5\pi t)
yf=0.17ms-1
I think the English word you need is 'displacement' (not 'deviation').

For t, you have a rounding error. What is 6.43775 rounded to 3 significant figures?

##y(6.43775) = 1 sin(0.5 \pi \times 6.43775) = [##not ##0.17## and not ##ms^{-1}##].

I think you have made two mistakes:
b) incorrectly rounded the incorrect answer! (0.17558 rounded to 2 significant figures is 0.18, not 0.17).

Delta2, berkeman and meher4real
Thank you for your detailed response.
so t=6.438s.

Steve4Physics
Homework Helper
Gold Member
Thank you for your detailed response.
so t=6.438s.
No.

From Post #1, y is a displacement measured in metres. For example y could be the height of a mass bouncing up and down on a spring, with y=0 the equilibrium height. Positive values of y are above the equilibrium position; negative values of y are below the equilibrium position. So y is a value in metres, not radians.

Note that ##0.5 \times \pi \times 6.43775 = 10.112##. This is a value in radians.

When you work out sin(10.112) you must make sure your calculator is set to radians. You are not working out sin(10.112º).

The value of sine itself has no units. When you multiply the amplitude (1m) by sin(10.112), the answer must be in metres.

meher4real
You the man, thank you.

haruspex