1. Section 3: Fluids Preview Key Ideas Bellringer Pressure Buoyant Force
Comparing Weight and Buoyant Force
Pascal’s Principle
Math Skills
Fluids in Motion

2. Key Ideas How do fluids exert pressure?
What force makes a rubber duck float in a bathtub?
What happens when pressure in a fluid changes?
What affects the speed of a fluid in motion?
GLOBAL NOTE: Section 2 at top of slide needs to be changed to Section 3

3. Bellringer
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, which is always in the upward direction, opposes the weight of the object.
Examine each of the drawings shown on the next slide. Then answer the questions that follow.

4. Bellringer, continued
1. Is the buoyant force on the lump of gold greater than, less than, or equal to the gold’s weight?
2. Is the buoyant force on the balloon greater than, less than, or equal to the balloon’s weight?
3. Is the buoyant force on the boat greater than, less than, or equal to the boat’s weight?
4. Is the buoyant force on the submarine greater than, less than, or equal to the submarine’s weight?

5. Pressure How do fluids exert pressure?
Fluids exert pressure evenly in all directions.
pressure: the amount of force exerted per unit area of a surface
example: when you pump
up a bicycle tire, air particles
constantly push against each
other and against the tire walls

6. Pressure, continued
Pressure can be calculated by dividing force by the area over which the force is exerted:
The SI unit for pressure is the pascal.
pascal: the SI unit of pressure; equal to the force of 1 N exerted over an area of 1 m2 (symbol, Pa)

7. Visual Concept: Equation for Pressure

8. Buoyant Force What force makes a rubber duck float in a bathtub?
All fluids exert an upward buoyant force on matter.
buoyant force: the upward force that keeps an object immersed in or floating on a fluid

9. Buoyant Force, continued
Archimedes’ principle is used to find buoyant force.
The buoyant force on an object in a fluid is an upward force equal to the weight of the fluid that the object displaces.

10. Comparing Weight and Buoyant Force

11. Buoyant Force, continued
An object will float or sink based on its density.
If an object is less dense than the fluid in which it is placed, it will float.
If an object is more dense than the fluid in which it is placed, it will sink.

12. Density

13. Pascal’s Principle What happens when pressure in a fluid changes?
Pascal’s principle states that a change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of the fluid. In other words, if the pressure in a container is increased at any point, the pressure increases at all points by the same amount.
Mathematically, Pascal’s principle is stated as
P1 = P2.
Because P = F/A, Pascal’s principle can also be expressed as F1/A1 = F2/A2.

14. Pascal’s Principle, continued
Hydraulic devices are based on Pascal’s principle.
Because the pressure is the same on both sides of the enclosed fluid, a small force on the smaller area (left) produces a much larger force on the larger area (right).
The plunger travels through a larger distance on the side that has the smaller area.

15. Math Skills Pascal’s Principle
A hydraulic lift uses Pascal’s principle to lift a 19,000 N car. If the area of the small piston (A1) equals 10.5 cm2 and the area of the large piston (A2) equals 400 cm2, what force needs to be exerted on the small piston to lift the car?
1. List the given and unknown values.
Given: F2 = 19,000 N A1 = 10.5 cm2
A2 = 400 cm2
Unknown: F1

16. Math Skills, continued
2. Start with Pascal’s principle, and substitute the equation for pressure. Then, rearrange the equation to isolate the unknown value.
P1 = P2

17. Math Skills, continued
3. Insert the known values into the equation, and solve.
F1 = 500 N

18. Fluids in Motion What affects the speed of a fluid in motion?
Fluids move faster through small areas than through larger areas, if the overall flow rate remains constant. Fluids also vary in the rate at which they flow.

19. Fluids in Motion, continued
Viscosity depends on particle attraction.
viscosity: the resistance of a gas or liquid to flow
Fluid pressure decreases as speed increases.
This is known as Bernoulli’s principle.

20. Visual Concept: Viscosity