Sound and Wavelengths

I never realized that playing an instrument could have so much to do with physics before.  While playing the piano, fingers hit different notes producing different sounds.  When my sister presses a key on the piano, a pedal connected to the key pounds a string in the back, creating sound waves.  The sound waves are produced by compression and decompression in the air.  The region of compressed air is called a condensation and the region of decompressed air is a rarefaction.  The distance between two maximum condensations or two minimum rarefactions is called a wavelength.  If she strikes a low note, the sound waves produced have longer wavelengths.  If she hits a high note, the sound waves produced have shorter wavelengths.  When a sound wave reaches our ear, the wave causes the eardrum to vibrate.  We technically don't "hear" anything, but we pick up vibrations that our brain interprets into sounds.  

Negative Velocity

Velocity can be either positive or negative, but just because something has a negative velocity doesn't mean that the object is moving slower than something with a positive velocity.  Velocity deals with speed and well as direction.  There is always a positive direction as well as a negative direction, which pairs up respectively with a positive and negative velocity.

During softball, the pitcher releases the ball in the positive direction at a certain speed.  This will have a positive velocity because it is traveling forward.  After the ball makes contact with the bat, it travels in the negative direction with a certain speed.  This speed may be faster or slower than the original speed the pitcher pitched the ball at, but it will always have a negative velocity because it travels backwards.  If, for example, the ball came towards the batter with a velocity of +85 mph and left the bat with a velocity of -90 mph, the ball has a greater speed with the negative velocity, just in the opposite direction.