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

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?

Density Density = mass/volume Mass is in kg Volume is in liters, m 3, cm 3, etc. In this class we will use m 3. 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

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

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?

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.

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.

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.

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.

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.

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.

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

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”.

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.

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.