1. Fluids are Affected by Forces, Pressure and Heat
Chapter 8
Fluids are Affected by Forces,
Pressure and Heat

2. 8.1 Forces 2 Groups of Forces
Force= push or pull that acts on an object
2 Groups of Forces
Contact Forces = only have an effect on objects they touch
Action-at-a-Distance Forces = can apply forces to an object without touching it

3. Contact Forces
Tension Force – force experienced by a wire or rope when pulled at both ends
Friction Force – works to slow down or stop motion due to surfaces rubbing together
Elastic Force – exerted when a spring-like object restores itself to normal shape

4. Action-at-a-Distance Forces
Gravitational Force – force of attraction between objects due to mass (depends on mass and distance of objects)
Electrostatic Force – static electricity = the excess of positive or negative charge, causes pushing and pulling forces
(Like charges repel Opposite charges attract)
Magnetic Forces – Natural magnets, electromagnets, and the Earth exert pushing or pulling forces on certain metals (iron, nickel, cobalt)

5. Measuring Mass Mass is measured in metric units (grams, kilograms)
Measured using a balance scale
Mass is the same anywhere in the universe

6. Weight Depends on Gravity
Weight is the amount of force on an object due to gravity
Bathroom scales actually measure force not mass

7. The Newton Force is measured in Newtons (N) (Sir Isaac Newton)
It is a small amount of force about enough to hold up an apple
Weight is measured in Newtons because weight is a force

8. Measuring Force Use a force meter to measure
(the most common type = spring scale)
Mass and weight are directly proportional but not the same thing! If you change one it affects the other.
To convert mass to weight multiply by
9.8 meter/second squared

9. Forces and Motion
Balanced Forces - equal in strength and opposite in direction
Sometimes keep an object stationary or in motion
Unbalanced Forces – cause a change in speed/direction of an object
Arrows are used to show direction of the force

10. Homework Textbook Questions! Page 289 Questions # 2-4, 6
Complete your Ch. 8 vocab.
Quiz next class!

11. 8.2 Pressure
Pressure = the amount of force acting over a given area on an object
Compression = decrease in volume produced by a force

12. Gases Are Compressible
Gases are easy to compress due to the space between their atoms
Less than 1% is actually the particles in a gas
Need a container and a force to create compressed gas (air compressor)

13. Pressure and Kinetic Activity in Gases
Pressure is a good indicator of kinetic energy of a gas
Heat added increases the pressure can cause explosions or implosions

14. Liquids and Solids are Very Difficult to Compress
Particles of solids and liquids are tightly packed so compression does not occur using normal means (incompressible)

15. Compression and Deformation
Deformation = change in shape without being forced into a smaller volume
Examples - Styrofoam cups, bouncing a ball
The air inside is being compressed

16. Comparing Pressure
Spreading force over a greater area means the pressure can be reduced (bed of nails)
We use the formula:
Force (measured in Newtons (N)
Area (measured in m2)

17. Calculating Pressure
A BMX rider exerts a force of 1200N on a sheet of plywood that measures 1m x 2m. How much pressure is exerted?
P= F/A
P = 1200N/(1mx2m)
P = 600 N/m2 = 600 Pa

18. Page 296 Practice Problems 1-3
Homework Questions
Textbook Questions
Page 296 Practice Problems 1-3
Page 299 Questions # 2,3,5,6

19. 8.3 Viscosity, Adhesion, and Cohesion
Viscosity = the resistance of a fluid to flow, the slower it flows the more viscose the liquid is
Due to the:
Particles in some liquids flow past each other better than others
Some particles have greater attraction to each other
Oil = complicated molecule ; water = simple molecule

20. Flow Rate of Various Liquids
You can compare liquids by measuring their flow rates = speed at which a fluid flows from one point to another

21. Effect of Temperature on Viscosity
Size and shape of liquid particles and the attraction between particles contribute to friction
Liquids Heated = viscosity decreases
Cooled = viscosity increases
Gases Heated = viscosity increases
Cooled = viscosity decreases (less friction)
Liquids and Gases are opposite!!!

22. Adhesion
Another property of fluids that can affect their flow is adhesion
Adhesion = attraction or joining of two different objects or fluids to each other
Fluids are sometimes attracted to surfaces
Example : water molecules have polarity which means one end of the molecule is positive and the other is negative – makes the molecule sticky
It is hard to get all water out of a cup

23. Cohesion
Cohesion = strength with which the particles of an object or fluid attract each other
• Water molecules attract one another at the surface creating surface tension
• Example: water strider insects

24. Homework Questions
Textbook Questions
Page 309 Questions# 1,3,4,5,6