Although you may not be familiar with the specific details, you have seen buoyant forces at work. You know from experience that certain objects float in air or in water. This is because of the force that pushes, or buoys the object up. This force opposes the weight of the object, which is always in the downward direction. Examine each of the drawings shown below. Then answer the items that follow.

1. Is the buoyant force on the lump of gold greater than, less than, or equal to the gold’s weight? Explain your answer.

2. Is the buoyant force on the balloon greater than, less than, or equal to the balloon's weight? Explain your answer.

3. Is the buoyant force on the boat greater than, less than, or equal to the boat’s weight? Explain your answer.

4. Is the buoyant force on the submarine greater than, less than, or equal to the submarine’s weight? Explain your answer.

Bellringer/ 3.2 Notes

I. Buoyant Force

The upward force that fluids exert on matterA. Buoyancy –

1. Objects that float are less dense than water.

2. Objects that sink are more dense than water.

Write these questions down:

1. What is “pushing up” on the anchor?2. The force of ___ pulling the anchor down is greater than the force of the ___

pushing the anchor up.3. Since you are less ___ than water, the force of ___ pulling you down is smaller

than the ___ of water pushing you up.4. Each cube displaces its ___ ___ of water.5. The metal cube weighs ___ Newtons and will sink/float.6. The cork cube weighs ___ Newtons and will sink/float.7. Will the plastic cube sink or float?

B. All fluids exert pressure

Why does ice float?

1. This pressure increases with depth

C. Also, ice is less dense than water

D. Archimedes’ Principle – Buoyant force in a fluid equal to the weight of the fluid that the object displaces

1. The weight of the displaced water = the buoyant force.

How do giant cargo ships like this that weigh over 3 million

pounds not sink?

E. The hollow shape of the ship decreases the ship’s density.

II. Fluids & Pressure

A. Pressure – The force exerted on a given area of an object

areaforcepressure

1. Units: N/m2

PascalsAtmospheresPSI (Pounds per square inch)

III. Pascal’s Principle

A. A change of pressure at any point in an enclosed fluid will be transmitted equally to all parts of the fluid.

2

2

1

1AF

AF

1. Hydraulics apply Pascal’s Principle

2. The braking system in your car uses Pascal’s Principle

IV. Viscosity

A. Viscosity – Resistance to flow

1. A characteristic of liquids

2. A thick liquid such as honey will flow slower than water

3. This is because there is a stronger attraction between the molecules that make up honey

V. Bernoulli’s Principle

A. Bernoulli’s Principle – The faster a fluid flows, the less pressure it will apply to its surroundings

Slower air, more pressure = down force

Faster air, less pressure

The spoiler on a car help slow air flow, creating a pressure difference between the air flowing over the car and the air flowing under the car.

The opposite is true with an airplane wing. The faster flowing air above the wing helps create lift so that the

airplane can fly.

Low Pressure

Low Pressure

High Pressure

High Pressure

High Pressure

High Pressure