Transcript Slide 1

Pressure P = Force per unit area
= F/A
SI
units:
= N/m2
= 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 Pleft, and
the pressure in the right-hand cylinder Pright, then Pascal’s principle
states that:
Pleft
= Pright
FL/AL
=
FR/AR
Ex. If a force of 2.0 N is exerted on an area AL = 0.10 m2,
what force is needed to balance it on the other side if AR = 5.0 m2?
Ex. Car breaks transmit forces using this principle.
Conversion:
1 atmosphere = 1.01x105 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 10-2 m)?
Use 1 N = 0.225 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
v
v
.
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?
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:
vi
wall
Ex. Draw Dv:
molecule
impulse
Dv = vf – vi
= vf + (-vi)
Dv
-vi


wall
impulse
vf
vf
Dv --> mDv --> Dp --> impulse exerted by wall on molecule
= -impulse of molecule on wall --> FDt --> 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
Homework:
Read pages: 280-283 in your text.
On page 288, do 14-16, 21, 22 and 27