1. Physics: Principles with Applications, 6th edition
Lecture PowerPoints
Chapter 4
Physics: Principles with Applications, 6th edition
Giancoli
© 2005 Pearson Prentice Hall
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2. Inertia and Free Particle Model
Force – Push or pull on an object
Unit is newton (N)
Force is a derived SI unit – dimensions are kg m/s2
Types of Forces
Fundamental forces (all field forces)
Field forces
Contact forces

3. Inertia and Free Particle Model
Fundamental forces (weakest to strongest)
Gravity*
Acts between any two objects with mass
Acts at infinite distances
Weak interactive
Involved in some types of nuclear phenomena
Acts at distances up to m
Electromagnetic
Operates between electrically charged particles
Strong interactive
Holds nucleus of an atom together
Acts at distances up to m
*Einstein’s General Theory of Relativity states that gravity bends space around the object rather than attracting it

4. Inertia and Free Particle Model
Field force
a force acting on a particle at various positions in space
Field forces can affect an object without touching it
These are the fundamental forces

5. Inertia and Free Particle Model
Field force

6. Inertia and Free Particle Model
Contact force
Types of contact forces (continued)
Normal force – contact force that acts perpendicular to the common surface of contact
Example: Gravity acts on a book on a table, but the book does not fall through the table because of the normal force of the table pushing upward on the book

7. Inertia and Free Particle Model

8. Inertia and Free Particle Model
Contact force
Types of contact forces (continued)
Buoyancy – an upward force exerted by a fluid that opposes the weight of an immersed object.
Example: Boat floating in water

9. Inertia and Free Particle Model
Contact force
Buoyancy
Source:

10. Inertia and Free Particle Model
Contact force
Types of contact forces (continued)
Elastic Restoring –

11. Constant Force Tension Exists in a flexible cord connecting two masses
Force of tension the same at all points in cord
Tension is a pull-only force (can’t push a cord)
The net force of tension is 0N because each mass pulls on the other with equal tension

12. Inertia and Free Particle Model
Contact force
Tension example –
Source:

13. Constant Force Example
Two boxes connected by cord
FP acts on Box A, FT acts on Box B
Can treat as one system - both boxes accelerate at same rate [FP / (mA + mB)]

14. Inertia and Free Particle Model
Contact force
Types of contact forces (continued)
Tension – Present in objects that have the property of elasticity (when you stretch them and let go, they return to their original size (example: rubber band)
Tension is high when object stretched
Tension is low when object not stretched
Tension is the same throughout the object

15. Inertia and Free Particle Model
Contact force
Acts between objects that are touching
Types of contact forces
Friction – Exists between two solid surfaces because even the smoothest surface is quite rough on a microscopic scale.
Dry friction arises from a combination of
inter-surface adhesion
surface roughness
surface deformation
surface contamination

16. Inertia and Free Particle Model
Contact force
Types of contact forces (continued)
Examples of friction
Dry friction – includes static friction and kinetic (sliding) friction
Static friction - on between two or more solid objects that are not moving relative to each other. 
Kinetic friction (sliding friction) - two objects are moving relative to each other and rub together 
Rolling resistance – force that resists the rolling of a wheel or other circular object along a surface caused by deformations in the object and/or surface
Air resistance - Opposes the motion of objects moving in air (examples: open parachute, sail on sailboat)

17. Constant Force Friction
Exists between two solid surfaces because even the smoothest surface is quite rough on a microscopic scale.
The vector friction is always opposite the direction of the applied force or motion

18. Constant Force Friction Types of friction:
If no motion occurs, referred to as static friction
Example: You push on a refrigerator, and it finally starts to move.
Static friction kept it from moving

19. Constant Force Friction Types of friction:
If motion occurs, referred to as kinetic (sliding) friction
Example: You let go of the moving refrigerator, and it starts to slow down then stops due to sliding friction

20. Constant Force Friction Types of friction:
When object rolls on surface, referred to as rolling resistance
Example: One reason your bicycle slows if you stop pedaling

21. Constant Force Friction Types of friction:
Air resistance opposes the motion of objects moving in air (examples: open parachute, sail on sailboat)
Terminal velocity* is highest velocity attainable by an object as it falls through a fluid (gas or liquid). At terminal velocity, resistance = weight of object.
Terminal velocity in air for a skydiver in freefall is 195 km/h (122 mph or 54 m/s).
*Source:

22. Constant Force Friction
What causes friction?
Surfaces are not smooth even if they appear to be (microscope)
Dry friction (static friction and sliding friction) arises from a combination of
Inter-surface adhesion
Surface roughness
Surface deformation
Surface contamination

23. Constant Force Friction
Coefficient of static friction (μs)
If you know Fs, you know only one of the infinitely many possible static frictional forces that could be exerted between the two bodies.
Which force do you know? The maximum static frictional force.

24. Constant Force Friction
Coefficient of kinetic (sliding) friction (μk)
The amount of kinetic friction Fk a body experiences is proportional to the size of the normal force N exerted on the body by the structure it slides against.

25. Constant Force Friction
Coefficient of kinetic (sliding) friction (μk)
Note: The statement “kinetic friction is a function of normal force only-- surface area has nothing to do with it” is true ONLY as long as you are dealing with two rigid bodies that are sliding relative to one another.

26. Constant Force Friction

27. Constant Force Friction

28. Inertia and Free Particle Model

29. Inertia and Free Particle Model
Contact force
Coefficients of friction (μk and μs)