Why does the earth

1
Why does the earth have more gravity, i.e. g is
larger on earth, than does the moon?
Physics 1710Warm-up Quiz

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1. The earth as an atmosphere.
2. The earth has a larger diameter.
3. The earth has more mass.
4. The earth rotates faster.
5. None of the above.

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Physics 1710Chapter 14 Fluid Dynamics
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• Analysis
• The earth has an atmosphere because of its
  larger g, not vis versa.
• The larger diameter means smaller g1/R2
• The larger mass of the earth m 4/3p?R3 makes a
  larger gGM/R2
• Gravity has nothing to do with the rate of
  rotation.

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Physics 1710Chapter 14 Fluid Dynamics
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• Summary
• Pressure is the force per unit area. P F/A
• Unit of pressue Pacal N/m2
• The hydrostatic pressure is P Po ?gh
• Archimedes Principle Fbouyant ?fluid g V
• Equation of Continuity A1v1 A2v2
• Bernoullis Equation P ½ ?v2 ?gy
  constant.

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Physics 1710Chapter 14 Fluid Dynamics
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• 80/20Pressure Force per unit Area
• P F /A

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Physics 1710Chapter 14 Fluid Dynamics
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80/20The unit of pressure in SI units is the
Pascal Pa 1 Newton/ m2

1 Newton force
1 Pascal pressure
1 square meter area
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Physics 1710Chapter 14 Fluid Dynamics
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80/20Standard atmosphere 1.0 atm 101
kPa 14.7 psi

Tire pressure A few 100 kPa
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Physics 1710Chapter 14 Fluid Dynamics
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Which applies the greatest pressure on their foot?
8
Which applies the greatest pressure on their foot?
Physics 1710Chapter 14 Fluid Dynamics

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1. Man.
2. Elephant.
3. Woman.

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Physics 1710Chapter 14 Fluid Dynamics
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Force 200 lb 891 N
Foot Area 36.2 in2 0.023 m2
p F/A 200/(2 x 36.2) 2.8 psip 891/(2 x
0.023) 19 kPa
Force 4,000 lb 17,800 N
Foot Area 43.5 in2 0.028 m2
p F/A 4000/(4 x 43.5) 23. psip 17800/(4 x
0.028) 159 kPa
Force 100 lb 445 N
Foot Area 12.7 in2 .0082 m2
p F/A 100/(2 x 12.7) 3.9 psip 445/(2 x
0.0082) 27 kPa
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Physics 1710Chapter 14 Fluid Dynamics
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Force 100 lb 445 N
Heel Area 0.16 in2 .0001 m2
p F/A 100/(2 x 0.16) 313. psip 445/(2 x
0.0001) 2200. kPa
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Physics 1710Chapter 14 Fluid Dynamics
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• The Mysterious Swami Seat

12
Physics 1710Chapter 14 Fluid Dynamics
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• 80/20Pressure Force per unit Area
• p F /A

Same Force Different Area ?Different Pressure
13
Physics 1710Chapter 14 Fluid Dynamics
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• Pressure is the hydraulic stress
• Hydraulic Stress Bulk Modulus B
• Stress modulus x strain
• s F/A p B e B ?V/V

?V
P
V
14
Physics 1710Chapter 14 Fluid Dynamics
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• Pressure is isotropic

V
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Physics 1710Chapter 14 Fluid Dynamics
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• Pressure is isotropic

F1 A1P F2 A2P
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Physics 1710Chapter 14 Fluid Dynamics
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• Hydrostatic Pressure is due to weight of fluid
  above

F m g (Ah)?fluid g P F/A ?fluid g h
A
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Physics 1710Chapter 14 Fluid Dynamics
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• Archimedes Pressure

F A?P - mg A ?fluid g ?h mg F Fbouyant -
mg Fbouyant ?fluid g V
A
V
- mg
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Physics 1710Chapter 14 Fluid Dynamics
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• Archimedes Pressure

Fbouyant ?fluid g V
V
- ?fluid V g
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Physics 1710Chapter 14 Fluid Dynamics
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• Continuity

V constant if fluid is incompressible. A1v1 A2v2
20
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Physics 1710Chapter 14 Fluid Dynamics

• Work done on fluid

?W ½ m v2 mg y dW/dV ½ ? v2 ?g y d
(Fy)/dV dF/dA ?P ½ ? v2 ?g y ?P
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Physics 1710Chapter 14 Fluid Dynamics

• Summary
• Pressure is the force per unit area. P F/A
• Unit of pressue Pacal N/m2
• The hydrostatic pressure is P Po gh
• Archimedes Principle Fbouyant ?fluid g V
• Equation of Continuity A1v1 A2v2
• Bernoullis Equation P ½ ?v2 ?gy
  constant.

22
Physics 1710Chapter 11 App E E

• Why does the platform spin faster when he brings
  his arms in?

Peer Instruction Time
23
Why does the platform spin faster when he brings
his arms in?
Physics 1710Chapter 11 App E E

1. He increases his angular momentum.
2. He increase his moment of inertia.
3. He decrease his moment of inertia.
4. He pushes against the inertia of the weights.
5. None of the above

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24
Where should the fulcrum be place to balance the
teeter-totter?
Physics 1710Chapter 14 Fluid Dynamics

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25
Which way will the torque ladder move?
Physics 1710Chapter 14 Fluid Dynamics

1. Clockwise
2. Counterclockwise
3. Will stay balanced

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