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Demo

Demo. 1 cm aluminum cube (it ’ s weight is small) Measure its mass w balance. Kid ’ s baseball (hollow sphere of plastic) it ’ s weight is more than the cube. The heavy one floats!. Buoyancy. Why do boats float on water?. Demonstration.

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Demo

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  1. Demo • 1 cm aluminum cube (it’s weight is small) Measure its mass w balance. • Kid’s baseball (hollow sphere of plastic) it’s weight is more than the cube. • The heavy one floats!

  2. Buoyancy

  3. Why do boats float on water?

  4. Demonstration • Ping pong ball on the table. Why doesn’t the ball fall down? • Ping pong ball in the aquarium. Why doesn’t the ball fall down (“sink”) ? • In both cases, there is an upward force on the ball! • At its core, it is as simple as that, the water pushes up; we call it the “buoyant” force.

  5. “Hey, are you saying that the water pushes up on the ball?” How can that be? Why would water do that to a ball? Answer is coming up…

  6. Have you ever jumped off of a diving board and gone down about 10 feet below the surface of the water? What does that feel like?

  7. The deeper you go, the greater the pressure.What is wrong about this drawing of the water pressure on a box that is underwater?

  8. Is this better or worse?

  9. OK, why is the pressure greater the lower you go? To see why, let’s build the marshmallow tower!

  10. One marshmallow

  11. Add one on top.

  12. Keep Stacking!

  13. Something is happening …

  14. What’s happening to the one on the bottom?

  15. Did you ever build a human pyramid?

  16. What is the worst job in the human pyramid (and why is that job so bad)?

  17. Why is the pressure greater at greater depth? • What keeps the people at the top of the pyramid from falling? • What keeps the water at the top of the ocean from falling? • What keeps the air, high in the atmosphere, from falling to earth?

  18. Why is pressure in a fluid greater,at greater depths? • Deep in the ocean, the water molecules are pressed together by the weight of the molecules above. At the bottom the molecules are densely packed. It takes a large force to squeeze water, and in return, the molecules are pushing back with a large force. This is why the pressure down there is so crushingly huge. • The same thing is happening low in the atmosphere: The air molecules down here are being pressed together by the molecules up there. The density of oxygen at the earth’s surface is much greater than the density where airplanes fly (that’s why planes carry oxygen masks). The air pressure up there is much less than the air pressure down here.

  19. So, since the pressure increases the lower you go, every object is “buoyed” up.There is a _ _ _ _ _ _ _ force on every object in a fluid.

  20. So, since the pressure increases the lower you go, every object is “buoyed” up.There is a buoyant force onevery object in a fluid.

  21. Every object in air or in water has an upward force on it.

  22. Review …{Fill in your blank spaces}

  23. Why do objects float?

  24. Why do objects float? The fluid pushes up.

  25. Why do objects float? Why does the fluid push up? The fluid pushes up.

  26. Why do objects float? Why does the fluid push up? The fluid pushes up. The pressure on the bottom of the object is greater than the pressure on the top.

  27. Why do objects float? Why does the fluid push up? The fluid pushes up. Why is the pressure greater on the bottom? The pressure on the bottom of the object is greater than the presure on the top.

  28. Why do objects float? Why does the fluid push up? The fluid pushes up. Why is the pressure greater on the bottom? The pressure on the bottom of the object is greater than the presure on the top. The molecules that are lower are squished by the molecules that are higher.

  29. Why do objects float? Why does the fluid push up? The fluid pushes up. Why is the pressure greater on the bottom? The pressure on the bottom of the object is greater than the presure on the top. The molecules that are lower are squished by the molecules that are higher. This produces the buoyant force that holds up ships and balloons.

  30. What about a rock? If there is always a buoyant force, then why do some things sink?

  31. Why doesn’t every object float?

  32. Why doesn’t every object float?(Remember that just because there is a force on an object, that does not mean the object will go in the direction of the force. For example, gravity is putting a downward for on you right now, but you are not moving down.)

  33. OK then, why, exactly, do some objects sink, and some float?

  34. Consider a brick that someone has just dropped into some water.Draw the forces thatact on the brick…

  35. Please label these forces:

  36. Ladies and Gentlemen … A sinker! Buoyant Force Weight

  37. Consider a basketball that someone has managed to get underwater. They release the ball.Please draw the forces…

  38. Please label these forces:

  39. A floater! B W = Mg So, why do some objects sink and some float?

  40. Fill in the blankswith a ‘<’ or a ‘>’. If it floats, then B W.If it sinks, then B W. …

  41. Fill in the blankswith a ‘<’ or a ‘>’. If it floats, then B>W.If it sinks, then B<W.

  42. Checkpoint page: • The lower you are, the greater the P_ _ _ _ _ _ _.

  43. Checkpoint page: • The lower you are, the greater the Pressure. • B exists because the bottom and top of an object are at d _ _ _ _ _ _ _ _ depths.

  44. Checkpoint page: • The lower you are, the greater the Pressure. • B exists because the bottom and top of an object are at different depths. • The B on an object depends on its s _ _ _ (not its weight).

  45. Checkpoint page: • The lower you are, the greater the Pressure. • B exists because the bottom and top of an object are at different depths. • The B on an object depends on its size (not its weight). • An ice cube rises and a steel cube sinks, but the B is the s _ _ _.

  46. Checkpoint page: • The lower you are, the greater the Pressure. • B exists because the bottom and top of an object are at different depths. • The B on an object depends on its size (not its weight). • An ice cube rises and a steel cube sinks, but the B is the same. • What is different about an ice cube and a steel cube?

  47. Checkpoint page: • The lower you are, the greater the Pressure. • B exists because the bottom and top of an object are at different depths. • The B on an object depends on its size (not its weight). • An ice cube rises and a steel cube sinks, but the B is the same. • What is different about an ice cube and a steel cube? The weight of the cubes is different! • At the very bottom of the ocean is a rock. Is there a buoyant force on that rock?

  48. Checkpoint page: • The lower you are, the greater the Pressure. • B exists because the bottom and top of an object are at different depths. • The B on an object depends on its size (not its weight). • An ice cube rises and a steel cube sinks, but the B is the same. • What is different about an ice cube and a steel cube? The weight of the cubes is different! • At the very bottom of the ocean is a rock. Is there a buoyant force on that rock? YES, but its weight is greater than the buoyant force that acts on the rock.

  49. Is there a Buoyant force on you right now? • Give a reason for or against. • Why do helium balloons float? • Why do air-filled balloons sink? • Why do you sink in air but float in water?

  50. Beach Ball.Why does the ball stop rising? 6 7 8 5 4 3 2 1

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