1. VISUAL PHYSICS
School of Physics
University of SydneyAustralia

2. m = r V V = m / r gold m1 V1 gold m2 V2 rgold = m1 / V1 = m2 / V2 r V
r m
m = r V
V = m / r

3. pressure !!!

4. F A

6. Gauge and absolute pressures
Pressure gauges measure the pressure above and below atmospheric (or barometric) pressure.
Patm = P0 = 1 atm = kPa = 1013 hPa = 1013 millibars =
760 torr = 760 mmHg
Gauge pressure Pg
Absolute pressure P
P = Pg + Patm
200
100
300
400

8. 100
200
300
400

9. Impact of a molecule on the wall of the container exerts a force on the wall and the wall exerts a force on the molecule. Many impacts occur each second and the total average force per unit area is called the pressure.

11. The pressure in a fluid can be defined as the ratio of the force exerted by the fluid to the area over which it is exerted. To get the pressure at a point you need to take the limit as this area approaches zero. Because of the weak cohesive forces between the molecules of the fluid, the only force that can be applied by the fluid on a submerged object is one that tends to compress it. This means the force of the fluid acts perpendicular to the surface of the object at any point.

12. Liquid – uniform density r
p pressure acting at on surface
Weight of column
of liquid F h A
Liquid – uniform density r

13. ph ph
p0’ p0 p0
(0,0)
(0,0) h h
Linear relationship between pressure and depth.
If the pressure at the surface increases then the pressure at a depth h also increases by the same amount.

15. h
The pressure exerted by a static fluid depends only upon the depth of the fluid, the density of the fluid, and the acceleration of gravity
ph = p0 + r g h
Static pressure does not depend upon mass or surface area of liquid and the shape of container due to pressure exerted by walls.

16. Cloudy / rain
sunshine

18. ?

19. D h A B C

20. A h
patm
patm B C r

21. F2 F1 h1
oil h2 A1 A2

22. A sharp blow to the front of an eyeball will produce a higher pressure which is transmitted to the opposite side

23. Increased pressure transmitted down spinal cord
Another example is the pressure exerted by a growing tumour. This increased pressure is transmitted down the spinal column via the cerebrospinal fluid, and may be detected lower in the spinal cavity which is less invasive than trying to detect it in the brain itself.
tumor
Increased pressure transmitted down spinal cord

25. Partially submerged floating

26. Floating: partially submerged
Weight of object < weight of fluid that can be displaced by object
Volume of displaced water < volume of object
Weight of liquid displaced by partially submerged object = weight of object
Water
displaced

27. Floating: fully submerged
Weight of object = weight of fluid displaced by object
Volume of displaced water = volume of object
Water
displaced
Static equilibrium
Some fish can remain at a fixed depth without moving by storing gas in their bladder.
Submarines take on or discharge water into their ballast tanks to rise or dive

28. Sinks Weight of object > weight of fluid displaced by object
Volume of displaced water = volume of object
Water
displaced

29. Volume of water displaced
A steel ship can encompass a great deal of empty space and so have a large volume and a relatively small density.
Volume of water displaced
Weight of ship = weight of water displaced

30. Weight of ship = weight of water displaced
The buoyant force is equal to the weight of the water displaced, not the water actually present. The missing water that would have filled the volume of the ship below the waterline is the displaced fluid.
Volume of water displaced. This volume is not necessarily the volume present.
Weight of ship = weight of water displaced

31. + FLOATING: weight of object = buoyant force FB FG
Object partially submerged
Object fully submerged
top
bottom A
bottom
top ro h A h w rF ro rF

33. ?
oil
water

34. Flift + FB m
a = 0 FG
Flift + FB = FG

35. at surface is into bulk of the liquid
Cohesion: attractive forces between “like” molecules
Surface of any liquid behaves as though it is covered by a stretched membrane
Net force on molecule
at surface is into bulk of the liquid FT SF
SF = 0

36. pull up on surface
push down on surface
restoring forces

37. Which shape corresponds to a soap bubble?
Surface of a liquid acts like an elastic skin  minimum surface potential energy  minimum surface area for given volume

38. FT = 2 T L FLOATING NEEDLE FT Equilibrium FT = FG FG
Not a buoyancy phenomena FT
FT = 2 T L
Equilibrium
FT = FG FG
Length of needle, L
Coefficient of
surface tension T
Surface tension acts along length of needle on both sides

39. k = 0.70 N.m-1 x = 3410-3 m Fspring = Fe = k x radius of ring FT + FG
R = 2010-3 m
mass of ring
m = 7.0 10-4 kg
ring

40. FT = 2 T L FLOATING NEEDLE FT Equilibrium FT = FG FG
Not a buoyancy phenomena FT
FT = 2 T L
Equilibrium
FT = FG FG
Length of needle, L
Coefficient of
surface tension, T
Surface tension acts along length of needle on both sides

41. q q Why can an insect walk on water? FT FT cosq FG FT = T L = 2 p R T
Surface tension force acts
around the surface of the leg FG q
FT = T L = 2 p R T
For one leg
FG = mg / 6

43. Flow of a viscous fluid L stationary wall plate moving with speed v
vz = v
high speed Z
linear velocity gradient L X
vz = (d / L) v
vz = (v / L) d d
low speed
stationary wall
vz = 0

44. Flow of a viscous newtonain fluid through a pipe Velocity Profile
Cohesive forces between molecules  layers of fluid slide past each other generating frictional forces  energy dissipated (like rubbing hands together)
Parabolic velocity
profile
Adhesive forces between fluid and surface  fluid stationary at surface

45. Poiseuille’s Law: laminar flow of a newtonian fluid through a pipe
Q = dV = Dp p R4
8 h L dt
p1 > p2 pressure drop along pipe  energy dissipated (thermal) by friction between streamlines moving past each other
volume flow rate Q = dV/dt
parabolic velocity profile
Dp = p1 - p2 h p1 p2 2R
Q = dV/dt L

46. Streamlines for fluid passing an obstaclev
Velocity of particle
- tangent to streamline

47. Velocity profile for the laminar flow of a non viscous liquid

48. A1 A2 r r v2 v1

49. A1 A1 A2 v2 v1 v1
Low speed
Low KE
High pressure
high speed
high KE
low pressure
Low speed
Low KE
High pressure

50. Y
Dx2 p2 A2 m v2 X
time 2 r p1
Dx1 y2 A1 m v1 y1
time 1

51. force
high speed
low pressure
force

52. high velocity flow high pressure (patm) velocity increased
low pressure
velocity increased
pressure decreased

53. 5
slow flow
(streamlines further apart)
high pressure 1
Same speed and pressure across river
faster flow
(streamlines closer together) low pressure

54. p large
p large
p small
v small
v large
v small

55. artery
Flow speeds up at constriction
Pressure is lower
Internal force acting on artery wall is reduced
External forces causes
artery to collapse

56. (1) Point on surface of liquidy1
v2 = ? m.s-1 y2
(2) Point just outside hole

57. (1)
(2)
v1 = ? rF h rm

58. C yC A yA B yB D

59. Ideal fluid
Real fluid

64. head
arm
arm
lung
lung
heart
trunk
leg
leg

66. Floating ball

67. Resultant FR
Lift FL C A B
drag FD D

68. to pressure difference low pressure region
Drag force due
to pressure difference
low pressure region
rotational KE of eddies  heating effect  increase in internal energy  temperature increases
motion of air
high pressure region
motion of object

69. NO CURVE Drag force due to pressure difference low pressure region
rotational KE of eddies  heating effect  increase in internal energy  temperature increases
NO CURVE
Drag force is opposte to the direction of motion
high pressure region

70. Tear drop shape for streamlining

71. v v vT vT t t
Object falling from rest
Object thrown down with initial speed v0 > vT

73. flow speed (high) vair + v  reduced pressure
Drag force due
to pressure difference
flow speed (high) vair + v
 reduced pressure v
vair (vball)
MAGNUS EFFECT
flow speed (low) vair - v
 increased pressure v
Boundary layer – air sticks to ball (viscosity) – air dragged around with ball
high pressure region
low pressure region

74. The trajectory of a golf ball is not parabolic
Golf ball with backspin (rotating CW) with air stream going from left to right. Note that the air stream is deflected downward with a downward force. The reaction force on the ball is upward. This gives the longer hang time and hence distance carried.

76. lift

77. lift Direction plane is moving w.r.t. the air
Direction air is moving w.r.t. plane
low
pressure
lift q
low pressure drag
attack angle
high
pressure
momentum transfer
downwash
huge vortices