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Welcome back to Physics 211

Welcome back to Physics 211. Today’s agenda: Problems on pressure Atmospheric pressure Buoyancy. Reminder. Exam 3 next Thursday in class Material: conservation of momentum, equilibrium of extended bodies, torque, angular velocity, pressure in liquids Practice exam at course website

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Welcome back to Physics 211

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  1. Welcome back to Physics 211 Today’s agenda: Problems on pressure Atmospheric pressure Buoyancy

  2. Reminder • Exam 3 next Thursday in class • Material: conservation of momentum, equilibrium of extended bodies, torque, angular velocity, pressure in liquids • Practice exam at course website • Closed book

  3. Pressure problem water g g h mercury x y z z x y Px=Py=Pz all increase Pg<Ph<Px Px=Py=Pz Pg<Px Pg increased

  4. Moral … • Relate pressures at different points by imagining a path entirely through liquid to open surface. • Pressure determined by net change in vertical displacement relative to free surface I.e rgDh

  5. Atmospheric pressure • How big is atmospheric pressure? • How does one measure it?

  6. Two disks that make a tight seal contain a small space that is initially filled with air. The space is then evacuated, so that atmospheric pressure holds the two disks together. 1. A one-kilogram mass 2. Two physics text books 3. A three-gallon bucket of water 4. Four (physics) students Atmospheric pressure is about 15 psi (pounds per square inch), or 100 000 N/m2. The radius of the disks is about 6 inches, or 15 cm. If the evacuated disks are hung from the ceiling, what objects can they support?

  7. Demo: Crushing barrel or soda can

  8. Mercury barometer

  9. Another barometer is placed next to the one discussed previously. The second barometer uses a wider glass tube but is otherwise the same. The top of the column in the second barometer is the height in the narrow barometer. 1. higher than 2. lower than 3. at the same height as 4. “Don’t know.”

  10. Mercury has a density that is about 14 times greater than that of water. If you were to build a barometer that uses water instead of mercury, how would the height of the column of water needed compare to that of the mercury? 1. higher than 2. lower than 3. equal to 4. Can’t tell.

  11. Why did we not have to consider theforces exerted by air in other situations (before starting hydrostatics)? What would we find if we did?

  12. A glass beaker is filled with water and placed on a balance. A person holds a finger into the water. The reading on the balance will 1. go up 2. go down 3. stay the same 4. Can’t tell.

  13. Demo – weigh your finger in water …

  14. Archimedes’ principle The magnitude of the buoyant force is equal to the weight of the fluid displaced by the object. The buoyant force is the total upward force exerted on an object by the surrounding fluid.

  15. Demo: Weighing displaced water

  16. Derivation … rgh1 h1 h2 rgh2 Net force=rg(h2-h1)A=weight of displaced water!

  17. Two identical bricks are held under water. Brick A is just beneath the surface of the water, while brick B is at a greater depth. The force (exerted by the hand) required to hold brick B in place is 1. larger 2. the same as 3. smaller than the force required to hold brick A in place. 4. Can’t tell.

  18. A plastic container with a tightly closed lid contains 30 metal balls. When placed in a tank of water, the container sinks to the bottom of the tank. If the experiment is repeated with only 25 balls, the container floats but is almost completely submerged. The 5 balls that were removed are now added one by one, and the experiment is repeated each time. What will happen? 1. The container will continue to float as a few more balls are added and then sink at once to the bottom. 2. As more and more balls are added, the container will sink to deeper and deeper positions in the tank. 3. I am not sure which of the two statements is correct.

  19. A cubical block made of a certain type of plastic has a density of 0.75 g/cm3. The density of water is 1.0 g/cm3. If the block is allowed to float in water, what fraction of the volume of the block would be below the water level? 1. one quarter 2. one half 3. three quarters 4. some other fraction

  20. Consider an object that floats in water but sinks in oil. When the object floats in water, half of it is submerged. If we slowly pour oil on top of the water so it completely covers the object, 1. the object moves up 2. the object stays in the same place 3. the object moves down 4. Can’t tell.

  21. The density of aluminum is about 2.7 times greater than that of water, so a block of aluminum will sink when placed in water. How is it possible to build a boat using only aluminum?

  22. How do boats float? • If the density of an object is less than that of water, an object floats in water. • If the density of an object is greater than that of water, an object sinks in water. • …but boats can be made of metal…

  23. explanation If object is hollow it may displace a weight of water which is greater than the objects weight

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