# Homework Help: Engery of a spring mass system

1. Mar 23, 2009

### jchojnac

1. The problem statement, all variables and given/known data
Use your program for the viscously damped spring to solve this problem. Make sure you change the parameters as specified below.

A mass m=7.5 kg is attached to the end of a spring with a spring constant of k=18.3 N/m. The mass moves through a viscous damping medium with a damping constant b=1.8 kg/s giving a velocity dependent damping force Fdamp= -bv.

The motion occurs in zero gravity so set the force of gravity to ZERO in your program. Also set the equilibrium position L0=0. The mass is initially motionless and displaced from equilibrium by a distance yinitial=0.2 m.

What is the energy of the spring-mass system at the initial position of the mass? (the spring-mass system does not include the damping medium)

What is the energy of the spring-mass system when the mass first passes through the equilibrium position? (you may wish to include a logical test to help you find when this occurs)

2. Relevant equations
1/2mv^2
mgy

3. The attempt at a solution
just tried m*g for the first one so far.

2. Mar 23, 2009

### LowlyPion

Without knowing what your program is, it's a little hard to know what you are struck on.

3. Mar 24, 2009

### spidermankick

from __future__ import division
from visual.graph import *

scene.y = 400 # moves animation window down 400 pixels to make room for graph

# Constants
pi = 3.14159
L0 = .08 # equilibrium length of spring (not stretched)
g = 0 # gravitational acceleration set to zero (not on earth)
k = 14 # insert the spring constant you found for the coiled spring

# Objects
# Ceiling to hang spring from.
# block to act as mass.
# Spring is represented by a cylinder.
scene.center = vector(0,-.1,0) # you may want to adjust this to improve display

ceiling = box(pos=vector(0,0,0), size=(.3,0.005,0.005)) # make the contact pointthe origin
block=box(pos=vector(0,-0.1,0), size=(.02,0.02,0.02), color=color.yellow)

# Using the positions of the block and ceiling set the cylinder to stretch from the ceiling to the block

# Initial values
block.m = 0.45 # insert the measured mass from coiled spring experiment.
block.v = vector(0,0,0) # the vector velocity assuming the block is initially stationary
block.p= block.m * block.v

block.pos=vector(0,-L0-0.05,0) # initial position of block 0.05m from equilibrium

# Setting the timestep and zeroing the cumulative time
deltat = .0001 # you should decrease this later to test if it is small enough
t = 0
W = 0
displacement=0
Kgraph = gcurve(color=color.cyan)
Ugraph = gcurve(color=color.yellow)
KplusUgraph = gcurve(color=color.red)
Wgraph = gcurve(color=color.green)
# Loop for repetitive calculations
scene.autoscale=0
while t < 4:

Fnet= -(((block.pos-vector(0,-L0,0))*k))-((block.p/block.m)*.2) #INSERT the force of the spring on the block
displacement=(mag(block.p)/block.m)*deltat
block.p= block.p+Fnet*deltat # updates the momentum
block.pos= block.pos+block.p/block.m*deltat # updates the position
spring.axis = block.pos #updates the spring axis so it stays on the block
t=t+deltat
pmag = mag(block.p) # or whatever you've called the block's momentum

K = (pmag**2)*.5/block.m #COMPLETE this for the kinetic energy of the
block
U = ((mag(block.pos)-L0)**2)*.5*k #COMPLETE this for the potential energy of the block-spring system (note no gravity)
W = W - displacement*(mag(block.p)/block.m)*.2
Kgraph.plot(pos=(t,K))
Ugraph.plot(pos=(t,U))
KplusUgraph.plot(pos=(t,K+U))
Wgraph.plot(pos=(t,W))

4. Mar 24, 2009

### spidermankick

this is the program need to solve this problem, but i don't know what # need to be changed to solve this problem

if someone could help me that would be great

5. Mar 24, 2009

### yoyoz41

hi the energy of the spring-mass system at the initial position of the mass will be
the potential energy only which is
1/2 * k * x^2

and then plug the number in
.5 * 18.3 * (.2)^2 = .366

6. Mar 13, 2010

how would you solve their 2nd question?!

"What is the energy of the spring-mass system when the mass first passes through the equilibrium position? (you may wish to include a logical test to help you find when this occurs)"

7. Mar 19, 2010

### Sakuyan

Well I'd assume that Since Ef = Ei + Wsurr,
And they gave us how to solve the Fdamp = -bv.
So would it be something like Ef = Ei + (Fdamp * x)? Just a guess, I'm in the same mess.