VISUAL PHYSICS School of Physics University of Sydney Australia

1. VISUAL PHYSICS School of Physics University of SydneyAustralia

2. gold m1V1 gold m2V2 rgold = m1 / V1 = m2 / V2 r V r m m = rV 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 = 101.3 kPa = 1013 hPa = 1013 millibars = 760 torr = 760 mmHg Gauge pressure Pg Absolute pressure P P = Pg + Patm 200 100 300 0 400

8. 200 100 300 0 400

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. p0 pressure acting at on surface Weight of column of liquidF h A Liquid – uniform densityr

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 + rg 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. sunshine Cloudy / rain

18. ?

19. D h A C B

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. 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. 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. 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 bottom top A ro A h h w rF ro rF

33. ? oil water

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

35. 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. FLOATING NEEDLE Not a buoyancy phenomena FT FT = 2 TL 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 FT + FG ring radius of ring R = 2010-3 m mass of ring m = 7.0 10-4 kg

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

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

43. Flow of a viscous fluid 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. Q = dV = DppR4 8 hL dt Poiseuille’s Law: laminar flow of a newtonian fluid through a pipe 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 Streamlines for fluid passing an obstacle v 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. Dx2 p2 Y A2 m v2 X time 2 r p1 Dx1 y2 A1 m v1 y1 time 1