Fluids and

Buoyant Force

Matter

solid liquid gasDefinite:

massvolumeshape

Definite:mass

volume

Definite:mass

Fluid – state of matter in which molecules and atoms are free to move past each other.

gasliquid

will take the shape of the

container within its

own definite volume

will take the shape and fill

the entire volume of the container (no

definite volume)

volume – [m3] - Vmass – [kg] - m

Density – the concentration of matter of an object. density [kg/m3] – ρ

SG = Specific Gravity (no units!)

What’s gold’s SG and what is its density in g/cm3 ?

19300 kg/m3 = density of gold is SI

Buoyant ForceThe upward force exerted on a body immersed or floating in the fluid (liquid or gas)

f oV VThe volume of the displaced fluid = the volume of the object (part of the object immersed in it)

Critical thinking

Two solid spheres of the same radius are placed in two identical beakers with water. One sphere is made of gold (ρ = 19600 kg/m3) and the other of iron (7860 kg/m3). Which sphere will displace more water? Explain and illustrate your answer.

Critical thinking

Two solid spheres of the same mass are placed in two identical beakers with water. One sphere is made of gold (ρ = 19600 kg/m3) and the other of iron (7860 kg/m3). Which sphere will displace more water? Explain and illustrate your answer.

Archimedes’s Principle

Any object completely or partially submerged in a fluid experiences an upward buoyant force equal in magnitude to the weight of the fluid displaced by the object.

Sink or Swim?If the object will stay afloat or totally immersed in the fluid without sinking all the way.

If the object will sink

b oF m g

b oF m g

Sink of Swim?

How do submarines ascend and descend?Draw free body diagrams for each case and

compare density of the sub to that of a surrounding water.

Critical thinkingWhy do objects (people) appear to weigh less in

water?If a 5.0 kg object appears to weigh 30.0 N in

water, what is:A) buoyant force exerted on it?B) volume of the displaced fluid?C) volume of the object?D) Density of the object?If the same object is submerged into another

fluid and weighs only 15 N in it, what is the density of the fluid?

Lab – determine the density of washers and density of a fluid

You have: a cup of water, a set of nuts and washers, a spring scale, a cup of unknown fluid.

Design an experiment and determine the required densities.

A floating raftThe dimensions of the

raft are: 4.0m x 4.0m. It is made of wood with SG = 0.55. How much height of the raft is below the water? How many people can get on it without getting their feet wet if the average mass of a person is 55 kg?