Transcript PHYSICS CHAPTER

CHAPTER-6
6.1 Force and Motion
Contact Versus Long-Range Forces
Force =
A push or pull exerted on an object.
System =
The object
Environment =
The world around the object that exerts
forces on it.
Force has both magnitude and direction,
∴ force is a ___________ quantity.
A: vector
Contact force =
Can only act on an object if it is directly
touching the object.
Example: book/desk, book/hand
Long-Range Force =
A force that can be exerted on an object without
actually touching it.
Examples =
Magnets,
Gravity- long-range attractive force that exists on
ALL objects due to the mass of the Earth.
Agent =
The actual cause of the force, if you cannot
name it, then it does not exist
Free Body diagram =
The object is represented by a dot
and all of the forces operating on the
object are drawn in the direction of
the forces with their tails on the dot.
FBD Box on a Desk
FBD Ball on a rope/chain
FBD Ball in a Hand
FBD Box being pushed at a
constant Velocity
FBD Box Sliding on an Incline Not
Being Pushed
Hanging Traffic light
Newton’s Second Law of Motion
The acceleration produced by a net
force on an object is directlt
proportional to the magnitude of the
net force, is in the same direction as
the net force, and is inversely
proportional to the mass of the object.
Newton’s 2nd Law Continued
Basically means=
If a force is applied to an object, the
object will accelerate in the direction
of the force.
F = net force (N)
F = ma
a = acceleration (m/s2)
a = F/m
m = mass (kg)
Net Force
(Fnet)
a) The vector sum of two or more forces
b) The sum of ALL the forces acting on an
object.
c) When ALL the forces acting on an object
are “resolved” (broken down) into their
summation in each direction, their
“component” vectors are added together to
achieve some net force in a particular
direction.
Fnet EXAMPLES
Horizontal
Vertical
Horizontal & Vertical together
At some angle – then “RESOLVE”
Measuring Force: The Newton
1 Newton = 1 N
kgm
1 N = 1 s2
F = ma
N = kg x m/s2
Newton’s First Law of Motion
An object at rest will remain at rest
and an object in motion will remain in
motion in a straight line with a
constant speed unless a net force
acts on it. AKA: Law of Inertia
In other words an object tends to
keep on doing what it’s already doing.
INERTIA =
Is the tendency of an object to
resist a change in its motion.
If it is at rest it wants to remain at rest.
If it is moving it wants to continue
moving at the same speed and
direction.
Inertia depends on mass, the more
mass an object has = more inertia.
Equilibrium =
When the net force acting on an object is
equal to zero.
This does not mean that there are NO
FORCES acting on an object it simply
means that if all the forces acting on an
object are added together the sum would
be zero. There may be many forces acting
on the object.
EXAMPLES
TERMINAL VELOCITY =
The greatest speed that a “Free
Falling” object can attain.
Q: How does an object reach terminal
velocity?
Student examples.
TERMINAL VELOCITY Cont.
As an object begins to fall faster the
amount of air resistance increases
proportionally. Once the object falls fast
enough the force of gravity will become
equal to the force of air resistance. At that
point equilibrium is reached. The object
will continue to fall, but will fall at the same
speed, constant velocity, because there
will be no net force acting on the object,
forces on the object will be in equilibrium.
From page 123 Table 6-2
Ff = frictional force
FN = normal force, the force exerted by a
surface on an object perpendicular to
the surface.
Fg = force of gravity
FT = force of tension
FSP = force of spring tension
6.2 Using Newton’s Laws
Using Newton’s Second Law
Aristotle – the heavier the object, the
faster it falls (accelerates towards the
center of the Earth)
Galileo –
All objects fall at the same rate
a = -g = -9.8m/s2
All forces acting on an object must be
considered such as air resistance,
friction, …etc
Mass =
The amount of matter an object has
Weight =
The force of gravity acting on an
objects mass.
Weight = Fg = mg
Q: What does a
scale actually
measure?
A: The scale
actually measures
the FN (normal
force) or the
upward force that
opposes gravity
perpendicular to
the scale.
If you are standing on the
scale, equilibrium is
reached.
Fnet= 0
Fnet= Fscale -Fg =0
 the scale is measuring
the magnitude of the
upward force needed to
offset Fg, which is your
weight. weight = Fg = mg
Weighing yourself on a floor
compared to weighing yourself in an
accelerating elevator.
Your mass = 75kg
a) floor
Weight = F = mg = (75kg)(9.8m/s2)
Weight = 735kgm/s2 = 735N
b) elevator is accelerating upward at
3m/s2 . What does the scale register?
Fscale = FN + Fnet
Fscale = mg + ma (acc of elevator)
Fscale = m(g+a)
Fscale = 75kg(9.8m/s2 + 3m/s2)
Fscale = 75kg(12.8m/s2)
Fscale = 960N
c) elevator is accelerating at 5m/s2
downward. What is the apparent
weight on the scale?
Fscale = FN + Fnet
Fscale = mg + ma
Fscale = m (g +a)
Fscale = 75kg(9.8m/s2 -5m/s2)
Fscale = 75kg(4.8m/s2)
Fscale = 360N
Apparent Weight =
the force exerted by the scale
Elevator accelerating
Fscale = mg ± ma
Fscale = m(g ± a)
The Friction Force
Friction =
The force that acts to resist the
motion of objects that are in contact
with each other.
Since friction opposes motion, the Ff
(frictional force) acts in the opposite
direction of motion.
All surfaces have some friction,
nothing is completely frictionless.
Static Friction =
The force exerted on one surface by
another when there is no relative
motion between them.
Kinetic Friction =
The force exerted on one surface by
another when the surfaces are in
relative motion.
Static Friction Force (Ffs) 0≤Ffs≤μsFN
Kinetic Friction Force (Ffk) Ffk= μkFN
μ = coefficient of friction
FN = mg
Table 6-3 Page 131
Surface
Rubber on concrete
Rubber on wet concrete
Wood on wood
Steel on steel (dry)
Steel on steel (with oil)
Teflon on steel
μs
0.80
0.60
0.50
0.78
0.15
0.04
μk
0.65
0.40
0.20
0.58
0.06
0.04
Example Problem Friction #1
A 25kg box is
pulled across a
floor at a constant
32m/s. What is the
force of the pull if
the coefficient of
static friction is
0.223 and kinetic
friction is 0.114?
SOLUTION PROCESS
1. FBD
2. Table of known/unknown
3. Equation
4. Plug in #’s
5. Steps showing ALL work
6. Solution/final answer in box/circle
FBD
Table of known/unknown
m = 25kg
vi = 32m/s
vf = 32m/s
ti = 0
tf = X
Fnet = 0
di = 0
df = X
a=0
μk = 0.114
μs = 0.224
Equation = ?
FP = Fkf
FP = FNμk
FP = mgμk
FP = (25kg)(9.8m/s2)(0.114)
FP = 27.93kgm/s2
FP = 27.93N
Example Problem Friction #2
If the force of the pull were doubled
what would the acceleration be?
Differences…?
Fnet = some value
Ff remains the same
 there will be an acceleration
Solution
Fnet = FP –Ff
ma = FP – Ff
a = (FP – Ff) / m
a = (55.86N – 27.93N) / 25kg
a = 27.93kgm/s2 / 25kg
a = 1.12m/s2
Periodic Motion
Periodic Motion =
The back and forth motion over the same
path.
Examples:
Swinging, stretched spring with a mass,
pendulum
Pendulum
The pendulum
will swing back
and forth forever
until some net
force other than
gravity acts on it.
Q: What Fnet
will eventually
cause the
pendulum to
stop moving?
A: air
resistance
Simple Harmonic Motion =
Motion that returns an object to its
equilibrium position as a result of
a restoration force acting on the
object that is directly proportional
to the object’s displacement.
Restoration Force =
A net force attempting to bring the
object back to equilibrium, it is in
the opposite direction of the
object’s displacement.
Period (T) =
The time needed to repeat one
complete cycle of simple harmonic
motion.
Amplitude =
The maximum distance the object
moves from equilibrium.
Period of a Pendulum
T = 2π√(l/g)
T= time of period in
seconds
l = length of string
in meters
g = acceleration of
gravity, 9.8m/s2
Q: what is the
only factor that
determines the
period of a
pendulum?
A: l, the length of
the string.
Resonance =
Applying a net
force to the swing
in the same
direction the swing
is moving will
increase the
amplitude of the
swing (causing the
swing to go
higher).
Mechanical Resonance =
When small forces are applied at
regular intervals to a vibrating or
oscillating object, the amplitude of the
vibration increases. The time interval
between applications of the force is
equal to the period of oscillation.
6.3 INTERACTION FORCES
Identifying
Interaction Forces
Pitcher & catcher
When the ball is
stopped by the
catcher what
forces are
present?
A: The forces
present are…
ball on the
catcher and
catcher on the
ball
Q: how do the
forces compare?
A: they are equal
and opposite
Interaction Forces Present
Fball on hand
FA on B
Fhand on ball
FB on A
Action-Reaction force pairs
One does not cause the other
The two forces exist together or
not at all
Newton’s Third Law
According to Newton, an interaction
pair is two forces that are opposite in
direction but have equal magnitudes.
Ex: The force of the catcher’s hand
on the ball is equal to the force of the
ball on the catcher’s hand.
Newton’s Third
Law =
Whenever one
object exerts a
force on a second
object, the second
object exerts an
equal and opposite
force on the first
object.
Pairs Of Forces
Transparency from text book
How does a car accelerate
Force Pair Example
Ball / Earth
If a 0.18kg tennis ball is dropped:
a) What is the force of the Earth on
the ball?
b) What is the downward acceleration
of the ball?
c) What is the upward acceleration of
the Earth?
Draw diagram & FBD
mball = 0.18kg
mEarth = 6 x 1024kg
g = 9.8m/s2
a) = (F = ma)
FEarth on ball = Fball on Earth = mg
mg = (0.18kg)(9.8m/s2)
mg = 1.764N
b) =
Downward acceleration = -g = -9.8m/s2
c) = upward acceleration of Earth (aE)
F = maE
aE = F/m
aE = 1.764N / (6 x 1024kg)
aE=.000000000000000000000000294m/s2
aE = 2.94 x 10-25m/s2
Four Fundamental Forces
1. Gravitational Interaction =
Every object in the universe exerts a
gravitational force on every other object in
the universe.
2. Electromagnetic =
The force that holds atoms and molecules
together, they are responsible for all
contact forces.
3. Strong Nuclear =
The force that holds protons and neutrons
together. This is the strongest known force
in the universe.
4. Weak Nuclear =
Responsible for some types of radioactive
decay.
Forces of Ropes and Strings
Newton’s 3rd
Law states that
the forces are
part of an
interaction pair.
FT – Fg = 0
FT = Fg
The tension (T)
in the
rope/chain is
equal to the
weight of all
the objects
below it.
Q: What is the tension in the rope?
Neither team is moving, each team pulls
with a force of 5000N.
Answer:
Since neither team is moving
Fnet = 0 = equilibrium
FT(team A on rope) - FT(team B on rope) = 0
FTA = FTB = 5000N
The two tensions are part of an
interaction pair.
Pairs are = but opposite
Each Tension (T) = 5000N
Pully Ex prob-1
W1 = 50N
W2 = 40N
Neglect friction
What is the
acceleration of
the boxes?
What is the
tension in the
cord?
Pully Ex prob-2
W1 = 100N
W2 = 45N
Neglect friction
What is the
acceleration of
the boxes?
What is the
tension in the
cord?
Pully Ex prob-3
m1 = 85kg
m2 = 110kg
μk = 0.4356
μs = 1.004
What is the
acceleration of
the boxes?
What is the
tension in the
cord?
Pully Ex prob-4
m1 = 85kg
m2 = 110kg
μk = 0.4356
μs = 1.437
What is the
acceleration of
the boxes?
What is the
tension in the
cord?