# Simple Harmonic Motion find Mass

1. Apr 15, 2012

### mhz

1. The problem statement, all variables and given/known data
A block is attached to a horizontal spring with k = 75 N/m undergoes SHM with an amplitude of 0.15m. If the speed of the block is 1.7 m/s when the displacement is 0.12m from equilibrium, what is the mass of the block?

2. Relevant equations
E = E'
E$_{k}$ = $\frac{1}{2}$mv$^{2}$
E$_{s}$ = $\frac{1}{2}$kx$^{2}$

3. The attempt at a solution
E = E'
E$_{k1}$ + E$_{s1}$ = E$_{s2}$
$\frac{1}{2}$mv$^{2}$ + $\frac{1}{2}$kx$^{2}$ = $\frac{1}{2}$kx$^{2}$
Where subscript 1 indicates that x = 0.12 m and subscript 2 indicates that x = 0.15 m (max energy)
Substituting in ...
m = 0.21 kg

but the answer is 0.27 kg?

2. Apr 15, 2012

### OldEngr63

With that data, the answer is m = 0.21 kg.