Liquids

1. Liquids “water, water every where, nor any drop to drink…” - Coleridge

2. What is a liquid? • Liquid is one of the four states of matter • What are the other three? • A liquid is incompressible • That means that you won’t change its volume no matter how much you squeeze it • A liquid does not hold its shape • It will conform to the shape of its container • How does a liquid differ from a gas? How does a liquid differ from a solid?

3. Density • Density = mass/volume • Mass is in kg • Volume is in liters, m3, cm3, etc. • In this class we will use m3. • The book refers to “weight density”. This means weight/volume, as opposed to mass/volume • IMPORTANT NOTE: the density of a fluid does NOT change at different depths

4. Fluid Pressure • Recall: pressure = force/area • The force of a fluid on the bottom of a container = its weight (a force) • area of the bottom of the can • Recall that the density of a fluid is the same everywhere in the container • So pressure = weight density X depth

5. Remember pressure? • There is a big difference between force and pressure. • The same force can have 2 different pressures depending on the area • Would you rather a woman step on your toes with the heel of a running shoe, or with high heels?

6. More about pressure • Look at figure 19.2 on page 274 • Imagine you have two identical blocks. If you stack one on top of the other, you have twice the pressure underneath. • Likewise, fluid pressure increases if you have twice the liquid • Important note: FLUID PRESSURE DOES NOT DEPEND ON AMOUNT OF LIQUID. ONLY ON THE DEPTH • The water pressure 1m below the surface of a small pool is the same as 1m below the surface of a huge lake.

7. Buoyancy • Buoyancy is when things seem to weigh less underwater. • Submerged objects experience a buoyant force. • When an object is placed underwater, some of the water is displaced, or pushed aside. The volume of water displaced = the volume of the object submerged.

8. Archimedes’ Principle • An immersed object is buoyed up by a force equal to the weight of the fluid it displaces. • An immersed object DOES NOT get buoyed up by its own weight.

9. Buoyancy demonstration • Let’s do a demonstration: • Weigh an empty container • Submerge an object into a different full container of water. • Catch the overspill in the first container • Weigh the amount of water displaced. • This will be the upward force on the submerged object.

10. Another way of looking at buoyancy • Remember that fluid pressure is different at different depths? • Well, a block submerged in water will have a greater pressure on the bottom part of the block than it will on the top part of the block • This is because the bottom part is deeper in the water. • This difference in pressure is the same no matter how deep the block is underwater. • Therefore, the buoyant force will be the same no matter how deep it is.

11. The weight of water • 1 liter = 1,000 cubic centimeters (1,000 cc) • 1 liter of water has a mass of 1 kg. • The weight of 1 kg is 9.8 Newtons • So, 1 liter of water weighs 9.8 Newtons.

12. Demonstration: Floaters and Sinkers • I need some volunteers to guess if certain objects (mostly foods) will float.

13. Why does something float? • The only thing floating depends on is DENSITY. • An object that is denser than water will sink and an object less dense than water will float. • This does NOT mean that “heavy objects will sink and light objects will float”.

14. More on Floatation • Iron doesn’t ordinarily float • But ships made of iron do. Why? • Remember, floating depends on density. If the density of the object is lower than the density of water, the object will float. • But if it works out that the AVERAGE density of the object is less than the density of water, that is sufficient.

15. Pascal’s Principle • “Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions.” • This is because fluids are incompressible • See page 284 in the book for pretty pretty pictures.