# Conservation of energy in a spring mass system help

1. Jul 1, 2009

### eddierolling

hi i hope some one can help me with this i'm going out of my mind with it lol. i'm new to this forum so please forgive any errors in where this thread is or if it contains something it shouldn't.

a force of 160N is applied to a sliding mass of 13kg fixed on top of a spring. the spring force is 190N at any given instant and a frictional force of 15N exists. the spring stiffness is 195 N

A calculate the acceleration of the mass at the instant
B Determine the Displacement
C Calcutale the elastic energy of the spring
D compare and contrast the two principles applied. (i have to apply the conservation of energy and D'Alembert's principle)

i've used so far the equations (as i have wrote them down)

F+Fs+Fi+Fr+W=0 (W being weight not work)

Fi=-Ma

Dsplacement x=Fs/-k

and PE=0.5kx^2

Fi=82.53 N (up)
a=-6.348 ms^-2 (down)
x=-0.974 m (down)
PE= 92.56 J

its just the last part with the conservation of energy and how to compare the two principles, its baffled me. plus i hope i've got the rest wright up to now!

any help any one can give will be brillient

thanks

2. Jul 1, 2009

### eddierolling

P.S.
i'm sorry for any spelling mistakes, i know its terrible spelling lol.

3. Jul 2, 2009

### eddierolling

thanks to any one who looked and had a go.

i am excited to inform that i solved the problem in a blaze of excitment when i realised!

thanks again

4. Jul 2, 2009

### Redbelly98

Staff Emeritus
Way to go!

And, welcome to Physics Forums