Pressure P = Force per unit area = F/A SI units: = N/m 2 = Nm -2 = 1 pascal = 1 Pa

Ex. Calculate the pressure in Pa exerted on the ground by a 1.2 kg book that has length 6.0 cm, a width 8.0 cm and a height 3.0 cm when… a/ …it is lying on the side with the largest area. b/ Repeat for when it is lying on its smallest-area side.

Pascal’s Principle - The pressure applied to a fluid (like water or air) in a closed container is transmitted equally to every point of the fluid and to the walls of the container. Ex: Two cylinders with moveable pistons that contain a fluid and are connected: The pressure is the same in both cylinders. But if the areas are different, then the forces will differ.

If you call the pressure in the left-hand cylinder P left, and the pressure in the right-hand cylinder P right, then Pascal’s principle states that: P left = P right F L /A L = F R /A R Ex. If a force of 2.0 N is exerted on an area A L = 0.10 m 2, what force is needed to balance it on the other side if A R = 5.0 m 2 ?

Ex. Car breaks transmit forces using this principle.

Conversion: 1 atmosphere = 1.01x10 5 Nm -2 = 101 kPa = 760 mm Hg Ex. How much force (in N) is exerted on an inch squared at sea level by the pressure of the atmosphere (1 inch = 2.54 x m)? Use 1 N = lbs to convert to this force to pounds.

Ex. The weight of the atmosphere above one inch squared is 14.7 pounds. This force is transmitted everywhere in the air at sea level.

Ex. Bernoulli’s Principle: an increase in the speed of the fluid occurs simultaneously with a decrease in pressure. Ex. A fluid flows from a thicker tube into a smaller one. What happens to its speed as in enters the smaller tube? What happens to its KE? If it is moving with a greater v forward, will it exert the same pressure on the walls of the tube? v v

Ex. Wings.

Ex. Curve balls:

Q: Where does air pressure come from? A: When a gas molecule bounces off of a surface, it undergoes a change in v: Ex. Draw  v:  v = v f – v i = v f + (-v i )   v --> m  v -->  p --> impulse exerted by wall on molecule  = -impulse of molecule on wall --> F  t --> Pressure What determines the amount of the pressure? 1. higher T --> greater KE --> faster v --> more P 2. more frequent collisions (b/c smaller space)--> greater P vfvf vivi vfvf -v i vv wall impulse molecule impulse

Homework: Read pages: in your text. On page 288, do 14-16, 21, 22 and 27