1. Forces in Fluids
Chapter 3

2. Buoyancy in a Gas
An object surrounded by air is buoyed up by a force equal to the weight of the air displaced.
Exactly the same concept as buoyancy in water. Just substitute air for water in the statement
If the buoyant force is greater than the weight of the object, it will rise in the air!

3. Can a Balloon Rise Forever?
Since air gets less dense as altitude increases, the buoyant force decreases as you go up. So helium balloons don’t rise forever!!!

4. Bernoulli’s Principle
When the speed of a fluid increases, internal pressure in the fluid decreases.

5. Bernoulli’s Principle
Flow is faster when the pipe is narrower
This is why water flows faster when you put your thumb over the end of a garden hose
So pressure is lower in a gas that is moving faster!

6. Four Forces on an Airplane

7. A Plane Needs Lift
Faster moving air above the wing exerts less pressure than slower moving air below the wing
The increased pressure below exerts an upward force called lift.
Lift is the force that is opposite gravity

8. How Do You Get a Big Lift?
Lift depends on the size of the wing and the speed that the air is moving around it
Speed is determined by thrust
Thrust is a force created by a power source and gives a plane a forward motion.
More thrust = moving faster
Moving faster = faster moving air
Faster moving air = more lift

9. What about Wings Big engines create a lot of thrust and speed
You can get enough lift with a small wing
A glider has no engine and less thrust
A glider must have large wings to have enough lift

10. What about Birds?
A bird with a big wing size is a glider. An example is a hawk or an eagle
A bird with smaller wings has to work harder by flapping its wings to stay in the air

11. Overcoming Drag
Drag is the force or fluid friction that slows forward movement
The movement can be of an airplane, boat, or person.
It is in the opposite direction of motion (remember Newton’s #3)

12. Reducing Turbulence
Drag is caused by an irregular flow of fluid called turbulence
Turbulence reduces lift
Airplane flaps change the shape of the wing to reduce drag
Birds adjust their wing feathers in the same way!

13. Bernoulli’s Principle and Curveballs
Slower air (where the air and direction of the ball are opposite) has higher pressure and pushes the ball.

14. Bernoulli’s Principle and an Atomizer

15. Boyle’s Law

16. Boyle’s Law Density is mass divided by volume.
Halve the volume and you double the density and thus the pressure.