Back to the Past Music is organized sound

Back to the Past Music is organized sound (one definition anyway) but WHAT THE HECK IS SOUND? ?

Sound ---

Throw a ball at the wall F Force on the “Wall” One Ball: Average Force Time

More Balls Force on the “Wall” Average Force Time

Multiple throwing “channels” Bigger the AREA the bigger the FORCE

Conclusions l l The more balls per unit time, the more the force on the wall. The faster the balls, the more the force on the wall. The heavier the balls, the more force on the wall. For an “extended” stream of balls, the larger the WALL, WALL the more the total force on the wall.

Back to the Balls

Consider a mass m with a “velocity” v m v IMPORTANT DEFINITION momentum = (mass) x (velocity) or p=mv

Remember? ?

Momentum and Impulse

DEFINITION l l IMPULSE = (Force x time it acts)=F x t IMPULSE = CHANGE IN MOMENTUM

REMEMBER? ? ? Force on the “Wall” Average Force Time

Impulse = Change in Momentum Initial Final NEWTON’S THIRD LAW Force on Wall Finial – (Initial) Directional

Consider a box of air Air Box MAGNIFIED

Magnify Some More ● ● ● MOLECULES ●

Are Molecules Something Like Tennis Balls? ? ? F F HOW BIG IS THE FORCE ON THE WALL? ?

The Force on the Wall INCREASES when …. l l l The number of molecules in the box that can strike the wall increases. The AREA of the wall increases. The speed of the molecules increases. l l This happens when the temperature of the gas increases. The “heavier” the molecules

The math Force = (constant) x (Area) x (number of molecules) x (speed) F=k n A T Define PRESSURE = Force per unit area = P P~n. T

An Example l The pressure on a 5 square foot area wall is 10 pounds per square inch (psi). What is the TOTAL FORCE on the wall? P= F/A so F=PA F=(10 lbs/ft 2)(5 ft 2)= 50 lbs l l

Air Pressure l Normal Atmospheric pressure l l 14. 7 lbs/in 2 Also called a BAR l A 2005 storm had a pressure at its center listed as 0. 920 bars or 920 millibars. § l l Milli = one thousandth Question – What is one atmosphere in pounds per square FOOT? 1 sq. ft. = 12 in x 12 in = 144 in 2