# Ratio for Kindetic and Potential energy

1. Feb 5, 2010

### Awer1

If the phase angle for a block-spring system in SHM is φ and the block's position is given by x = xm cos(ωt+φ), what is the ratio of the kinetic energy to the potential energy at time t = 0? State your answers in terms of the given variables.

x =xm Cos(wt + phi)
v = -w(xm)sin(wt +phi)
K = (1/2 mv2)
P = (1/2 m w2 x2)

dividing K/P = [v/wx]2
substitute v and x we get: [(-w(xm)sin(wt +phi)/(w)(xm Cos(wt + phi))]2
simplifying we get: [-wtan(wt + phi)/w]2 = [tan(wt + phi)]2
at t = 0 we should get the following: [v/wx]2 = [tan(phi)]2

The answer is marked wrong, am I interpreting the question incorrectly? I know what ratio is.. and ratio for Kinetic energy and Potential should be K/P right?

2. Feb 5, 2010

### thebigstar25

you are absolutely right !! its odd that it is marked wrong .. maybe you should change the form of your answer? try different form maybe that will work ..

3. Feb 5, 2010

### Awer1

oh good I thought I was going crazy. I was only allowed three attempts. so I used three different formats that were all the same thing.. still marked wrong.

4. Feb 5, 2010

### thebigstar25

it happens, sometimes you have the right answer but i.e you have to express it in a certain form or in a certain sigificance .. :) anyway, dont worry you had the right answer ..