Physics 101 Chapter 12-­‐14 Proper4es of ma7er Different states of ma7er Solid Gas Plasma Physics 101, Prof. M. Nikolic Liquid 2 Different states of ma7er Physics 101, Prof. M. Nikolic 3 Different states of ma7er Physics 101, Prof. M. Nikolic 4 Pressure Pressure – force divided by area F
P=
A
F – force A – area SI units: N/m2 = 1 Pascal (Pa) Nota4on for pressure: P (again)! Blaise Pascal 1623 – 1662 Physics 101, Prof. M. Nikolic 5 Exercise: Pressure What is the pressure from a 60-­‐kg person on the ground whose two feet cover an area of 20 cm x 25 cm. First we convert from units of cm to SI units of m: A = 20cm × 25cm = 0.2m × 0.25m = 0.05m 2
Then we find the force of gravity è weight of the person Fg = mg = 60kg ⋅10m / s 2 = 600N
F mg
600N
2
p= =
=
=
12000N
/
m
= 12000Pa
2
A A 0.050m
Physics 101, Prof. M. Nikolic 6 Conceptual ques4on: Pressure vs. force Who exerts more force on the ground, a 50 kg (110 lb) woman wearing spike heels or a 100 kg (220 lb) man wearing combat boots? Assume most of their weight is on their heels. The woman exerts more force. Both exerts the same force. The man exerts more force. ✓ Need more informa4on Physics 101, Prof. M. Nikolic 7 Conceptual ques4on: Pressure vs. force Who exerts more pressure on the ground, a 50 kg (110 lb) woman wearing spike heels or a 100 kg (220 lb) man wearing combat boots? Assume most of their weight is on their heels. The woman exerts more pressure. ✓ Both exerts the same pressure. The man exerts more pressure. Need more informa4on Physics 101, Prof. M. Nikolic 8 Density Density = mass divided by volume m = d ⋅V
m
d V
m
=V
d
m
d=
V
m – mass of the object V – its volume SI units: kg/m3. Density of water: d = 1 g/cm3
d = 1 kg/L
d = 1 ton/m3 = 103 kg/m3
“A pint’s a pound the world around” d = 1 lb/pint
Physics 101, Prof. M. Nikolic 9 Exercise: Density What is the mass of a waterbed with dimensions 2 m by 1.5 m by 0.20 m? Density of water – 1000 kg/m3 Waterbed is a cube. Volume of the cube is V = h ⋅ l ⋅ w = 2m ⋅1.5m ⋅ 0.2m = 0.6m 3
m = d ⋅V = (1000kg / m 3 )⋅ 0.6m 3 = 600kg
Physics 101, Prof. M. Nikolic 10 Conceptual ques4on: Density Three blocks each have the same mass. Which has the greatest volume? 1 kg of lead (d = 12
tons/m3). 1 kg of iron (d = 8
tons/m3). m = d ⋅V
Smaller density è Bigger volume 1 kg of aluminum (d = 3 tons/m3). ✓ All have the same volume. Physics 101, Prof. M. Nikolic 11 Gases Gases and liquids both flow and exert pressure Gases –  Have molecules much farther apart –  Expand to fill the available space –  Are compressible The atmosphere is held in place by gravity –  Otherwise the energe4c molecules would fly away –  It is compressed by the weight of Mass of air in lecture in lecture hall: the air above it •  It gets thinner as you go up m = dV
= (1 kg/m 3 )(5 m × 20 m × 10 m)
–  Density (at sea level): d ≈ 1 kg/m3 = (1 kg/m 3 )(10 3 m 3 ) = 10 3 kg = 1 ton
Physics 101, Prof. M. Nikolic 12 Atmospheric pressure •  The air above us has weight and we are being forced to bear that weight. •  The pressure of the Earth’s atmosphere varies with al4tude. •  Atmospheric pressure at see level is about 1.013 x 105 N/m2 = 10 340 kg per every square meter Physics 140, Prof. M. Nikolic The compensa4on for atmospheric pressure comes from, among other things, our blood pressure. 13