# Help with displacement and velocity of waves on a string (using trig)

1. May 11, 2010

### ninjarawr

Hello all,

excited to be on the forums. I'm having trouble solving a problem from one of my past exams (prepping for final). Could you please guide me?

The displacement associated with a wave on a string has the functional form y = 0.2 cos (10x - 4t). What is the wave speed?

The answer is 0.4 m/s...but how do I get here? what is really bothering me is the "10x"...I'm used to the format y = Acos(wt), where wA = v.

thanks!

ninja

2. May 11, 2010

### zachzach

Use the wave equation since it is a wave.

3. May 11, 2010

### ninjarawr

yeah...but what do I do with the "10x"?

4. May 11, 2010

### zachzach

There is no 10x you are using the differential wave equation right?
$$\frac{\partial^2\psi}{\partial x^2} = \frac{1}{v^2}\frac{\partial^2\psi}{\partial t^2}$$