Extremely quick question (MV calc/waves)

[Zorodius]

My physics book gives this (http://img.photobucket.com/albums/v165/Zorodius/18-13.jpg) function for the displacement of a wave element oscillating in a longitudinal wave. Later on, they take the partial derivative with respect to time, like so. (http://img.photobucket.com/albums/v165/Zorodius/18-30-2.jpg)

My question: Did my brain fall out, or is the negative sign in the second picture not supposed to be there? I'm open to either possibility right now :biggrin:

[HallsofIvy]

You are correct. You get one negative from the kx-\omega t and another from the derivative of cosine.

[Gokul43201]

My question: Did my brain fall out, or is the negative sign in the second picture not supposed to be there? I'm open to either possibility right now :biggrin:

Don't be. Understand your basics well enough that you know when something's right and when it's wrong. Your brain is doing good...just needs to feel more confident, and confidence comes from familiarity. :smile:

[Zorodius]

Thanks for your replies!