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Chapter 8

Chapter 8. Fluids are affected by forces, pressure, and heat. 8.1:Forces. “an object will not change its motion unless a force is applied” –Sir Isaac Newton. Force: A push or pull that acts on any object. Can cause an object to move (kicking a ball)

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Chapter 8

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  1. Chapter 8 Fluids are affected by forces, pressure, and heat

  2. 8.1:Forces “an object will not change its motion unless a force is applied” –Sir Isaac Newton. Force: • A push or pull that acts on any object. • Can cause an object to move (kicking a ball) • Can stop a moving object (catching a ball) • Can change the motion (ball bounces off your face) • Can change the shape of an object (crushed cans)

  3. 8.1:Forces Categories of Forces: Contact Forces • only affect objects they touch (bend, tear, move, stretch, compress or twist) • Are six types: tension, friction, elastic, applied, normal, air resistance.

  4. 8.1:Forces Contact Forces: Tension Force • The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends.

  5. 8.1:Forces Contact Forces: Applied Force • An applied force is a force that is applied to an object by a person or another object. • If a person is pushing a desk across the room, then there is an applied force acting upon the object. • The applied force is the force exerted on the desk by the person.

  6. 8.1:Forces • Contact Forces: Normal Force • The normal force is the support force exerted upon an object that is in contact with another stable object. • For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.

  7. 8.1:Forces Contact Forces: Friction Force • The friction force is the force exerted by a surface as an object moves across it. • There are at least two types of friction force - sliding and static friction. • Ex. a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of the books motion. • Ex. Brakes

  8. 8.1:Forces Contact Forces: Air Resistance Force • The air resistance is a type of frictional force that acts upon objects as they travel through the air. • The force of air resistance is often observed to oppose the motion of an object.

  9. 8.1:Forces Contact Forces: Elastic Force (Spring) • The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. • An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position.

  10. 8.1:Forces Categories of Forces: Action-at-a-Distance Forces • are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation. • Are three types: gravitational, electrostatic, magnetic

  11. 8.1:Forces Action-at-a-Distance Forces : Gravity Force • The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. • By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth.

  12. 8.1:Forces Action-at-a-Distance Forces: Electrostatic Force (Static Electricity) • Is the excess of positive (+) or negative charges (-) • Similar charges repel (push away) • Unlike charges attract (pull together)

  13. 8.1:Forces Action-at-a-Distance Forces: Magnetic Force • Certain metals have an electromagnetic field that can either attract (pull) or repel (push) other metals.

  14. 8.1:Forces Reading Check Answers, p. 278 1. (a) Action-at-a-distance: gravity 1. (b) Contact: elastic 1. (c) Contact: friction 1. (d) Action-at-a-distance: electrostatic

  15. Watch Brainpop:“Force” http://www.brainpop.com/science/motionsforcesandtime/force/preview.weml

  16. 8.1:Forces Measuring Mass • Mass: the amount of matter in an object (more particles) • Is measured in g, kg • Best to use a balance scale to measure • Gravity pulls equally on objects of equal mass • Mass stays the same wherever it is measured

  17. 8.1: Forces Weight Depends on Gravity • Weight is the amount (measurement) of force on an object due to gravity • Weight changes depending on where it is measured • Weight is technically a force The Newton • Force is measured in Newton's (N) • 1 Newton = the amount of force needed to hold up an apple against the force of gravity (approximately) • Force = Mass  Acceleration

  18. 8.1:Forces Reading Check Answers, p. 281 1. Newtons 2. Weight is the amount of force due to gravity on an object. Mass is the amount of matter in an object. 3. A balance scale 4. Gravity pulls equally on objects of equal mass, so a balance scale will work in any gravitational environment. 5. Weight depends on how much gravity there is. Different gravity means different weight.

  19. 8.1: Forces Measuring Force • A device called a Force Meter is used. • it uses springs or elastics to that stretch or compress in response to being pushed or pulled • A device called a Force Sensor is also used • It senses pushing or pulling • Mass and Weight are directly proportional: 1 kg = 9.8 N • Ex. 50 kg ( 9.8 m/s²) = 490 N

  20. 8.1: Forces Forces and Motion • Balanced Forces: are equal forces in opposite directions, Net force = 0 • If balanced forces act on an object that is not moving, it will not move • If balanced forces act on an object that is moving the object will keep moving in the same (constant) direction and speed

  21. 8.1: Forces Forces and Motion • Unbalanced Forces: are forces that are not equal • they can cause a change in speed or direction of an object • Using diagrams of arrows to represent different forces is a good idea

  22. 8.1:Forces Reading Check Answers, p. 285 1. Force meters are equipped with a spring or similar elastic device that stretches or compresses in response to being pulled or pushed. 2. Friction works against the motion of objects, tending to slow motion. 3. Balanced forces are equal in strength and oppose each other in direction. Unbalanced forces are not equal and cause a change in the speed or direction of an object. 4. Unbalanced forces can cause an object to begin moving, increase its speed, slow down, or change direction.

  23. 8.1: Forces Some Videos: • http://www.bing.com/videos/search?q=unbalanced+force&view=detail&mid=5A230DEC7A1E811BB9B45A230DEC7A1E811BB9B4&first=21 • http://www.bing.com/videos/search?q=force+of+motion&view=detail&mid=FF4B199810C72857D7E5FF4B199810C72857D7E5&first=21

  24. 8.1:Forces Check Your Understanding: # 1 - 7

  25. 8.1:Forces Check Your Understanding: 1. Yes. The goalie caused a change in the motion of the soccer ball, so there was a force was applied. 2. Basaam’s mass would be the same. The force of gravity does not change one’s mass. 3. The force of gravity changes from planet to planet. Bathroom scales rely on gravitational force to pull the user onto the scale; the greater the pull, the greater the weight reading. The scale would give an accurate measure of weight for the planet the person was standing on, but most likely the gravitational force would be different from Earth’s so the weight would be different as well. 4. The force is not a contact force, it is an electrostatic force, which is an action-at-a-distance force. The balloons do not have to be in direct contact with the hair in order to exert a force.

  26. 8.1:Forces Check Your Understanding: 5. (a) Different jumpers have different masses, so the amount of gravitational force will vary from person to person. A more massive person will stretch the bungee cord more when he/she jumps, so the bungee cord must be carefully adjusted. 5. (b) Yes, it could. The more force applied to the bungee cord, the more it stretches. A scale next to the bungee cord that showed the amount of stretch would be an indirect indicator of force.

  27. 8.1:Forces Check Your Understanding: 6. There are many different forces in the diagram: • Tension force in the ropes: arrows can be forward or backward or both • Friction force on bottom of box: arrows backward • Gravitational force on all items: arrows downward • Elastic force on wagon springs: arrows upward, downward, or both • Electrostatic force between cat hair and comb: arrows upward, downward, or both Note: There are several other minor forces present in this diagram: such as the frictional force between the girl’s shoes and the ground, the tires and axles, tire treads and ground, and perhaps even a small amount of wind resistance.

  28. 8.1:Forces Check Your Understanding: 7. (a) The forces are balanced. The diagram should indicate equal and opposite facing arrows against the car. 7. (b) The sketch should indicate both arrows facing in the same direction, reflecting the driver and passenger cooperating to push the car in the same direction. Forces are unbalanced as the car begins its motion forward. When the car is moving at steady speed while being pushed, the force applied by the driver and passenger would be in balance with the friction force acting against the car. Accept either correctly explained response.

  29. 8.2:Pressure • Pressure: is the amount of force that acts on a given area of an object • Pressure is measured with this formula: P = F/A or (P)ressure = (F)orce / (A)rea • Since force is measured in Newton's (N) and area is measured in square meters (m²) then pressure is measured in Newtons per square meter (N/m²) • 1 N/m² = 1 pascal (Pa) – this is very small unit of pressure • 1000 pascal = 1 kilopascal (kPa) - this is commonly used

  30. 8.2:Pressure - Questions Oh No…Math! • A football player is tackled by another player and lands with the combined weight of both players on his knee. If the combined weight of the players is 2400 N and the player’s knee measures 0.1 m by 0.1 m, how much pressure is exerted on the turf when the player lands on his knee? • A skateboarder lands on all four wheels after riding a railing. If the skateboarder has a weight of 900 N and the area on the bottom of a single wheel is 0.0001 m2, what pressure does the skateboard put on the ground? • A charity fundraiser fits 12 students into a small car. If the combined weight of the car and students is 1600 kg and the combined area of the wheels touching the ground is 0.08 m2, what is the pressure placed on the ground by the car and students?

  31. 8.2:Pressure - Answers Answers: P = 2400/0.01 (0.1 x 0.1) = 240,000 Pa or 240 kPa P = 900/0.0004 (0.0001 x 4) = 2,250,000 Pa or 2250 kPa P = 1600/0.08 = 20,000 Pa or 20 kPa

  32. 8.2:Pressure Review: • Volume: is the amount of space taken up by a substance • in a gas particles have lots of space between them • gas has an indefinite volume (determined by surroundings) • gas is a fluid • Pressure is the amount of force that acts on a given area of an object

  33. 8.2:Pressure • Compression: is a decrease in volume produced by a force • Gas can be compressed - because of all the space between particles • This can cause a shape change - because as pressure increases particles are pushed together • As a result – the gas takes up less volume or has been compressed • Liquids and solids are incompressible

  34. 8.2:Pressure • Compressed gas can be used for doing work – example: compressed air; works by taking a large amount of air and compressing it into a smaller volume • Compressed gas can cause an explosion or an implosion Implosion: heat cause particles in an open and heated container to move faster and farther apart. If the container is closed the particles slow and move closer together. This causes the pressure inside to be less than pressure outside Explosion: heat cause particles in container to move faster and farther apart. This causes pressure on the inside of the container until…

  35. 8.2:Pressure http://www.bing.com/videos/search?q=compressed+gas+explosion&view=detail&mid=15403AC7CC8ADBCD3A0415403AC7CC8ADBCD3A04&first=21 Check This Out: http://www.youtube.com/watch?v=OJeMnZuOOJU

  36. 8.2:Pressure Compression and Deformation: • Liquids and solids are incompressible – the particles are already too close together • Force is just transmitted from particle to particle • If there are any air pockets within a solid, it can be deformed • Deformation is a change of shape of a solid without a change of volume (smaller volume) • As the air pockets are compressed the object is deformed

  37. 8.2:Pressure - CYU Check Your Understanding p.299 #1 - 6

  38. 8.2:Pressure Reading Check p.296 1. Pressure 2. Compression 3. A gas can easily be compressed because there is a large amount of space between its particles. 4. A gas that is trapped in a container and heated will increase in pressure as the fast-moving particles bounce against the sides of the container. When the pressure exceeds the ability of the can to hold it, an explosion will occur. 5. If the pressure outside of a container is greater than the pressure on the inside, the walls of the container can be forced inward. 6. Liquids and solids have very little space between particles, so they cannot be forced together by ordinary means.

  39. 8.2:Pressure – CYU Answers 1. As a force is applied, it briefly pushes the particles of air inside the volleyball closer together. 2. A gas can easily be compressed because there is a large amount of space between its particles, but solids and liquids have almost no space between particles and therefore are very difficult to compress. 3. As the helium balloon rises, the particles inside the balloon move farther apart because there is less external pressure on the balloon forcing the particles together. The expansion of the balloon increases until the balloon pops.

  40. 8.2:Pressure – CYU Answers 4. (a) The pressure when deep diving is greater due to the weight of the water. This pressure compresses the air in your ear, which then applies a force to your eardrum. 4. (b) When you come back to the surface, the pressure and force on the parts inside your ear come back to normal. 5. The particles in the can are moving fast and far apart when heated, forcing particles out of the can. When cooled, the particles rapidly move closer together. The particles take up less space and so create an area of low pressure inside the can. As a result, the relatively high pressure surrounding the can applies a force to it and crushes it.

  41. 8.2:Pressure – CYU Answers 6. (a) Football player: P = F÷A = 950 N ÷ (1 m × 0.5 m) = 1900 Pa Horse: P = F÷A = 6000 N ÷ (2.5 m × 2 m) = 1200 Pa Bricks: P = F÷A = 8500 N ÷ (2 m × 2 m) = 2125 Pa 6. (b) Bricks, football player, horse 6. (c) Pressure also depends on the area over which the force is applied. In this case, the heaviest object had its weight spread over a rather large area.

  42. 8.2:Pressure Watch Bill Nye the Science Guy: “Pressure”

  43. 8.3: Viscosity, Adhesion, and Cohesion Viscosity: the resistance of a fluid to flow • The slower the flow the greater the viscosity • Flow depends on type of particles: • Size of molecules - larger molecules have a harder time to flow past each other and increase viscosity • Shape of molecules – more complicated molecules have a harder time to flow past each other and increase viscosity • Intermolecular forces - attraction between particles of different charges will cause viscosity to increase

  44. 8.3: Viscosity, Adhesion, and Cohesion • Thick liquid – greater viscosity, slower flow rate • Thin liquid – less viscosity, faster flow rate Viscosity Tests • http://www.youtube.com/watch?v=3KU_skfdZVQ • http://www.youtube.com/watch?v=OKENGIhaf1o&feature=related

  45. 8.3: Viscosity, Adhesion, and Cohesion Flow Rate: speed at which a fluid flows from one point to another. • Viscosity is affected by temperature • Viscosity affects liquids more than gases • Remember viscosity = resistance to flow • Therefore viscosity increases = less flow

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