# Homework Help: 2 SHM problems

1. Oct 18, 2008

### subwaybusker

I have two problems:

1. The problem statement, all variables and given/known data

I am given the positions and velocities of a mass on a spring at two times, which is four equations. I need to find the position and velocity of the mass at t=1s.

2. Relevant equations

0.35=-A$$\omega$$sin$$\phi$$
0.1=Acos$$\phi$$

-0.2=-A$$\omega$$sin($$\omega$$t$$\phi$$)
0.17=Acos($$\omega$$t$$\phi$$)

3. The attempt at a solution

I tried to divide the top two and the bottom two equations such that i got tan phi and tan omega t + phi, but after that i don't know how to manipulate the equations. i tried the tan identity but i couldn't do anything.

then i tried using 1/2*kx^2=1/2*mv^2 to get $$\omega$$ but the $$\omega$$ for the two times were different..

Second question:

wall-spring-mass-spring-wall

the two springs are of equal length and have equal k constant. I need to prove that when the mass is displaced VERTICALLY it does not have simple harmonic motion, assuming the vertical displacement is very small compared to the length of the spring.

Attempted Solution:

I drew a picture of the mass being displaced downwards and i got y=Lsintheta, but i know i am supposed to prove that the differential equation is not linear, so y (vertical displacement)ends up on the right of the Diff Eqn with a power or something. The Lsintheta isn't helped me, cause from what I have I can't see why y isn't linear.

2. Oct 19, 2008

### Andrew Mason

Check the question. The general solution to the differential equation:

d^2x/dt^2 = kx/m

is

x = Asin (wt + phi ) where w^2 = km

AM

PS for some reason Latex does not appear to be working

Last edited: Oct 19, 2008