Section 2

How can a huge ship float?• Because the force pushing up on the ship

opposes the weight of the ship pushing down

Supporting force is called : Buoyancy

Buoyancy:• Ability of a fluid (liquid or a gas) to exert an

upward force on an object immersed in it.• If the buoyant force is less than an object’s

weight it sinks.

Buoyant force on an object is = to the weight of the fluid displaced by the object.

Example: • Place a block of wood in water. At first it

starts to sink. • While it sinks it pushes water out of the way. • Once the weight of the water displaced = the

weight of the block

Example continued:

• Once the weight of the water displaced = the weight of the block it floats!

If the weight of the water displaced is less than the weight of the block it sinks.

Figure 13

Would a steel block the same size as a wood block float in water?

They both displace the same volume and weight of water.

Therefore the buoyant force on the blocks is equal.

What is different?

The volume of the blocks and the volume of water of water displaced have different masses.

If they have different masses they have different densities. Remember D = m/v

The density of the steel block is greater than the density of water.

While the density of the wood block is less than the density of the water.

An object will float if its density is less than the density of the fluid it is placed in.

Reshape the steel block into the shape of a hull filled with air

Now the same mass has a larger volume

Since the volume has increased the density will decrease and air is less than the density of water

Pressure:• Pressure = Force / Area• Earth’s atmosphere exerts pressure all

around us

According to Pascal’s principle, pressure applied to a fluid is transmitted throughout the fluid.

• Ex. Squeezing toothpaste

Hydraulic machines that move heavy loads

The pressure applied to the small cylinder is transferred to the large cylinder

Since pressure remains constant throughout the fluid there is more force available to life the heavy load.

Calculate Forces Page 487

Bernoulli studied the properties of moving fluids such as water and air

Bernoulli’s principle: • As the velocity of a fluid increases, the

pressure exerted by the fluid decreases.

Bernoulli’s Principle Example:

• Take a piece of paper and blow across the top of it

• The paper will rise

• This is because the velocity of the air over the paper is greater than the air below the paper

• The air pressure pushing down on the paper is lower than the pressure pushing upward

Bernoulli’s Principle We use this principle today when

designing aircraft wings

We also use this principle when spraying fertilizers, herbicides, and insecticides to our yards.• A hose-end spray works by Bernoulli

principle

Fluid Flow Fluids have the ability to flow

Viscosity:• The resistance to flow by a fluid

Fluids vary in their tendency to flow• Water has a low viscosity because it flows

easily• Cold syrup has a high viscosity because it

flows slowly

Fluid Flow When we tip a container to begin flow

the flowing particles transfer energy to the stationary particles

Flowing particles pull the other particles

If the flowing particles do not effectively pull the other particles it has a high viscosity or high resistance to flow

Fluid Flow If the flowing particles can easily pull

the other particles into motion then the liquid has a low viscosity or a low resistance to flow!