1. UNIT 2 MECHANICS
Chapter 5
Dynamics

2. Chapter 5A - Forces

3. Fundamental Forces Classifying forces Force
A push or pull on a system, may be a vector or scalar quantity
The magnitude is represented by F
The SI unit of force is a newton, N
Three Classes
Two broad categories
Contact and Field forces
Four fundamental

4. Facet of Science – Fundamental Forces
Read…
Facet on page 92
Four fundamental forces that are the source of all observed forces in the universe
Responsible for holding together the structure of all matter in the universe
Listed in decreasing strength
Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravitational force

5. Fundamental Forces Strong nuclear force Electromagnetic force
Strongest of all forces
Holds protons and neutrons together
Electromagnetic force
Attractions and repulsions due to electrical charges
Weak nuclear force
Holds particles inside the proton together (quarks)
Gravitational force
Exerted between all forms of matter
The weakest of all forces

6. Contact Forces Contact force Includes
Acts between systems only when one system touches another
Also called mechanical force
Includes
Compression
Pushes things together
Tension
Pulls things apart
Torsion
Twists objects
Shear
Causes portions of a material to distort
or move in opposite directions along
parallel planes
Friction
Discussed more later

7. Field Forces Field forces Principal field forces
A noncontact force exerted on a susceptible object by similar objects
Extend outward indefinitely
Diminishes rapidly with distance
Principal field forces
Gravitational field
Surrounds every material object
Everything in the universe exerts gravitational force
Magnetic field
Surrounds magnets and materials with
electricity flowing though them
Iron and other magnets
Electric field
Surround and affect electrical charges

8. Balanced and Unbalance Forces
Every object in the universe has more than one force acting on it
Why isn’t everything moving all over the place?
Balanced forces
A force that in combination with other forces acting on the same system produces a zero net force on the system
External force
A force exerted on a system by something in its surroundings
Net force
The single unbalance force acting on a system that is the sum of all forces acting on the system
When balanced it equals zero
Unbalanced force
A force that in combination with other forces
acting on the same system results in a nonzero net
force and produces a change in motion of the system

9. Chapter 5B – Newton’s Laws of Motion

10. How Newton’s Laws Came to Be…
Read….
Page 95, Imaging Inertia
Inertia
The tendency of all matter to resist change in motion
Laws of motion
Newton’s three laws of motion, which define the science of dynamics and apply to all areas of mechanics

11. The First Law The Law of Inertia Mechanical equilibrium
States that objects at rest remain at rest and objects in motion continue in a straight line at a constant velocity unless acted upon by a net external force
Inertia cannot be measured directly and has no units
Mass is a measure of inertia
Hard to see on earth because of gravity and friction
Mechanical equilibrium
The condition in which all forces acting on
a system are balance as indicated by the
system’s lack of acceleration

12. The Second Law The Law of Accelerated Motion
States that the acceleration of a system is directly proportional to the net force acting on the system and is inversely proportional to the system’s mass
Compare this to a small car and a large SUV
Formula: force=mass times acceleration
F=ma
a=acceleration
F= force
m=mass
Unit of force=Newton=(kg.m)/s2
Know this for future problems like
Example 5-1

13. The Third Law The Law of Action-Reaction Normal force
For every external force exerted on a system by its surroundings, the system exerts an equal but opposite force on its surroundings
Also called the action-reaction principle
Normal force
The force exerted by a supporting
surface on an object resting on it;
always perpendicular to the supporting
surface

14. Friction Friction Several kinds of friction
A contact force that opposes the movement of objects past each other
Vector or scalar quantity
Is everywhere and affects nearly all motion
Slows things down and wears things out
Several kinds of friction
Static-stationary objects
Kinetic-sliding objects
Rolling-rolling & stationary objects
Fluid-liquid & a gas

15. Five Factors Affecting Friction
Always occurs between systems in contact
Depends on the kinds of materials in contact
The friction vector acts on the system in the direction opposite to its motion
The magnitude of friction is proportional to the normal force acting on the system of interest
The friction between two stationary
objects is greater than the friction
between the same two objects when
they are moving past each other

16. Chapter 5C – Gravity and Free Fall

17. The Gravitational Field
Gravity
Basic property of all matter
Depends on three quantities
Masses of the two objects
Distance between the centers of the objects
Law of universal gravitation
States that the force of gravity between two objects is directly proportional to the products of their masses and inversely proportional to the square of the distance between their centers of mass

18. Free Fall Read… Air resistance Free fall Page 101
Friction on an object moving through the air
Increases with surface area of object
Free fall
The condition of an object accelerated by the force of gravity alone with no other forces acting on it
Can only take place in a vacuum

19. Gravitational Acceleration and Weight
The acceleration of an object due to gravity
9.81 m/s2 regardless of mass
Vector or scalar
Formula: derived from Newton’s second law
w=mg
w=weight in newtons
m=mass in kilograms
g=gravitational acceleration in m/s2
Example 5-2

20. Free Fall and Distance How far does an object move per second??
Formula 1 – Determine velocity
v=g t
v= free-fall speed in m/s
g=acceleration due to gravity in m/s2
t=total time of free fall in seconds
Formula 2 – plug into distance formula
d=v t
d=distance in meters
v=average speed in m/s
t=total time in seconds

21. Free Fall and Distance Uniform acceleration
A constant acceleration, such as that of an object in free fall
Formula 3 (based off other two)
Valid for any object starting from rest and dropping vertically in free fall if air resistance is not significant
d=1/2g( t)2
g=9.81 m/s2
Example 5-3

22. Free Fall and Air Resistance
Read…
5.13
Drag (5th Type of Friction)
A form of friction exerted by a fluid on an object moving in relation to the fluid
Air resistance is an example
Terminal velocity
The maximum constant speed of an object falling in the atmosphere
Occurs when drag balances with the force of gravity
Depends on the surface area compared to the mass