# A SHM problem

1. Feb 28, 2014

### paulzhen

A block of 5kg connected to the free end of a spring is hanging from the ceiling and is in SHM. At t=0, the block is travelling upwards and its displacement is 15cm from the equilibrium. The block reaches its maximum displacement 0.3s later. What is the general equation of the displacement of the oscillating block_

y=A cos (ωt+ø)

I use t=0, y=15cm and t=0.3, y=A to get 2 equations:

15=A cosø
0.3ω+ø=0

However there are 3 unknowns in these 2 equations, and I just do not know how to get the 3rd equation from the given quantities. Is this question miss something? Should it tell me the value of force constant k or sth. else? Please help to solve, thanks a lot!

2. Feb 28, 2014

### lawmaker

They should have given the value of spring constant k.

3. Feb 28, 2014

### paulzhen

really? is there no way to solve it with all known conditions?

4. Mar 15, 2014

### lawmaker

Kinetic energy is 0 at the extremes in simple harmonic motion.So, Spring constant$\times$
amplitude$\div$2=0. This gives you the value of k(spring constant). Now you can solve the problem easily.

Last edited: Mar 15, 2014
5. Mar 15, 2014

### lawmaker

I am so sorry. It is spring constant times the square of the amplitude the whole divided by 2=0