Download UNIT 2 MECHANICS

UNIT 2
MECHANICS
Chapter 5
Dynamics
Chapter 5A - Forces
• Objectives:
– Describe the properties of a force
– Categorize forces as contact or field
forces
– List and briefly describe various contact
forces
– List and describe the three main field
forces
– Explain how more than one force can
act on an object at the same time,
producing either balanced or
unbalanced forces
– Calculate the weight of an object given
its mass, using a simple direct
proportion
• Assignment: Section Review, page 109
Fundamental Forces
• Let’s Read…
– Page 104 Introduction
• Classifying forces
– Force
• A push or pull on a system, may be a vector or scalar
quantity
• The magnitude is represented by F
• The unit of force is a newton, N
• Two broad categories
– Contact and Field forces
• Four fundamental
Facet of Science – Fundamental Forces
• Let’s Read…
– Facet on page 105
• 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
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Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravitational force
Fundamental Forces
• Strong nuclear 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
Contact Forces
• Contact force
– 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
• Field forces
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
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
– Page 107
Weight
• Remember that weight is the gravitational
attraction exerted on an object’s mass, this is the
gravitational force
– Has the same units as force, newtons
– Directly proportional to the mass
• Formula is weight = proportionality constant
times the mass
– w=km
– w=weight
– k=proportionality constant
=9.81N/kg
– m=mass
– Example 5-1
Chapter 5B – Newton’s
Laws of Motion
• Objectives:
– Summarize the historical development of
the concept of inertia
– State Newton’s first law of motion in your
own words and note its significance
– State Newton’s second law in your own
words and express it as an equation
– Use Newton’s second law in calculations
– State Newton’s third law in your own
words, and discuss the importance of
identifying the system of interest
– Describe friction and the factors that
generally affect its magnitude
– Classify examples of friction into one of
the four types of friction
• Assignment: Section Review, page 114
How Newton’s Laws Came to Be…
• Let’s Read….
– Page 109, Introduction
• 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
The First Law
• The Law of Inertia
– 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
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
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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 5-2
The Third Law
• The Law of Action-Reaction
– 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
– Page 113
– The force exerted by a supporting
surface on an object resting on it;
always perpendicular to the supporting
surface
Friction
• 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
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Static-stationary objects
Kinetic-sliding objects
Rolling-rolling & stationary objects
Fluid-liquid & a gas
Five Factors Affecting Friction
1. Always occurs between systems in contact
2. Depends on the kinds of materials in contact
3. The friction vector acts on the system in the
direction opposite to its motion
4. The magnitude of friction is proportional to the
normal force acting on the system of interest
5. The friction between two stationary
objects is greater than the friction
between the same two objects when
they are moving past each other
Chapter 5C – Gravity and
Free Fall
• Objectives:
– Summarize the principle of universal gravitation
in your own words
– Write the formula for the law of universal
gravitation and identify each variable and
constant in the formula
– Define free fall and describe the conditions
under which it exists
– Show how weight is related to Newton’s second
law
– Calculate velocity and distance of free-falling
bodies, given the necessary data
– Explain how drag affects falling objects and how
it relates to terminal velocity
– Give examples of how air resistance can be a
help or a hindrance
• Assignment: Section Review, page 121
– Start Worksheet in Class tomorrow
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
The Gravitational Field
• Fg= G Mm
Formula
r2
• Fg is force of gravity in newtons
• G is a proportionality constant
– Universal gravitation constant
– One of the four fundamental constants of physics
– 6.67 x 10-11 N.m2/kg2 (in order to cancel out units)
• M is the mass of one object in kilograms
• m is the mass of the other object in kilograms
• r is the radius or distance between the objects’
centers of mass in meters
Free Fall
• Let’s Read…
– Page 116, 5.1
• Air resistance
– Friction on an object moving through the air
• 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
Gravitational Acceleration and Weight
• Gravitational acceleration
– 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 5-3
Free Fall and Distance
• How far does an object move per second??
• Formula 1 – Determine velocity
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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
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d=v t
d=distance in meters
v=average speed in m/s
t=total time in seconds
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 5-4
Free Fall and Air Resistance
• Let’s Read…
– 5.14
• Drag
– 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
Chapter 5D - Momentum
• Objectives:
– Define momentum and describe the
factors that affect linear momentum
– Show how angular momentum is
different from linear momentum
– Calculate linear momentum
– Discuss the factors that change
momentum
– Define impulse and identify each of the
terms in the impulse formula
– Explain how linear momentum can be
conserved
• Assignment: Section Review, page
125
Linear Motion and Momentum
• Let’s Read…
– Page 121, Introduction
• Momentum
– A property of a moving object directly proportional to its mass and
speed
• Linear momentum
– The momentum of a system moving in a straight line
• Angular momentum
– Momentum of a rotating system
• Formula
p=mv
– p=vector for momentum
– m is mass of the system in kilograms
– v is velocity of the system in m/s
• No derived unit, just kg.m/s
• Example 5-5
Impulse
• Changing Momentum
– Page 122, brown box
• Impulse
– The change of a system’s momentum, directly
proportional to the force exerted and the time
interval over which the force is applied
– Vector or scalar
Collisions, Explosions, and the
Conservation of Momentum
• A fundamental conservation law of mechanics
stating that in a system of colliding objects,
the sum of their momentums before the
collision is equal to the sum of their
momentums afterward if no external forces
act on the objects
• Read page 123
– Section 5.17
Facets of Science
• Johannes Kepler
– Page 124
• What are the three laws of planetary motion?
1. Planets move in ellipses with the sun at one
focus
2. An imaginary line from the sun to a planet
sweeps over an equal area in equal time
3. The square of a planet’s orbital period is directly
proportional to the cube of the planet’s mean
distance from the sun
Using Newton’s Laws to Solve Problems
• Read page 125
– Section 5.18
TOMORROW!!
• Vocabulary Quiz
– Includes all vocabulary throughout
the entire chapter, PowerPoints, and
board; not just the box at the end.
• Complete Chapter Review in Class
• Study for Chapter 5 Test