Download FORCES:

Newton’s Laws
of Motion
CHHS Physics Dept.
Mr.Puckett
Unit Objectives
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After studying this unit the student will be able
to explain and work problems on Newton’s 3
Laws of motion.
Define inertia and compute problems with it.
Differentiate between mass and weight.
Explain why force and acceleration are
proportional.
Explain and diagram net force.
Solve problems with Net force.
Distinguish between force and pressure.
Draw and label free body diagrams.
Explain and ID why forces appear in pairs.
What is a Force?
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A Force is a push or a pull on an object.
Forces are vectors because they have
magnitude and direction.
Force is equitable to energy in later chapters.
Make a list of all the forces you can think of….
Think back to the elementary playground – your
first physics lab.
Units of Force: The Newton
A force is measured in terms of the acceleration it
gives a standard mass. The SI unit of force in
physics is named after the father of physics;
Newton. The Newton (N) is defined as the
force that will cause a 1.00 kg mass to
accelerate at 1.00 meter per second
squared.
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Proof:
F=ma
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=(1.00 kg)(1.00 m/s2)
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= 1 kg x m/s2
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=1N
The 4 Forces in Nature:
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There are four basic forces in
nature:
1.
2.
3.
4.
Gravity- the attractive force between all
matter in the universe.
Electromagnetic – the forces of
attraction in atoms and magnets.
The Strong nuclear force that keeps the
nucleus together.
The Weak nuclear force that causes
radioactive decay.
Formulas:
F = ma; Force = mass x accel.
 Weight = mg; Force of weight =
mass X accel of gravity.
NOTE:
1 g =1 gravity force and
2g=2
X gravity (2 x 9.8m/s2) etc.
 P= F/A; Pressure = Force/Area
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Newton’s Laws
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Newton’s First Law
•Law of Inertia-
Keeps doing what it’s
doing, unless it is forced to change.
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Newton’s Second Law
•F = ma – the acceleration is proportional
to the force applied to it.
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Newton’s Third Law
•Action/ Reaction –
forces in pairs. For
every action in nature there is an equal and
opposite force.
Newton’s First Law
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Law of Inertia
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“A body remains at
rest or moves in a
straight line at a
constant speed
unless acted upon
by a force.”
Newton's First Law of Motion
– The Law of Inertia
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The name for this tendency is INERTIA - the tendency
for an object to remain in the same state of motion (or
rest).
An object will continue in its state of rest or uniform
motion in a straight line unless an outside unbalanced
force acts upon it. When forces are balanced, there is
no acceleration.
All situations on Earth’s surface are called inertial
reference frames. A non-inertial reference frame is one
in which an object is in freefall or accelerating and may
not obey the law precisely.
Newton’s Second Law
The Sum of the Forces acting
on a body is proportional to
the acceleration that the body
experiences
F  a
 F = (mass) a
Newton’s Second Law of
Motion: Force  Acceleration
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One of the most important formulas in all of
physics is F = ma . This says that the
summation of the forces on an object is directly
proportional to the acceleration of the object. It
says if you push hard on a basketball it goes
fast and when you push easy, it goes slow…..
You already knew that !
Force is a vector quantity and is the net force of
the sum of all acting forces in all directions. The
acceleration will vary directly with the net
applied force and inversely with the mass.
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F  ma
Net Force
The Net
force is the
sum of all the
forces acting
on an object
from each
direction.
 Fx  max
 Fy  may
 Fz  maz
Newton’s Third Law
Action-Reaction
 For every action
force there is an
equal and opposite
reaction force
 When you stand on
the earth it pushes
up equal to you
pushing down.
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Newton’s Third Law of
Motion: Forces in Pairs.
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Forces in Pairs states that every balanced force
in nature is accompanied by an equal and
opposite force. Forces always are in pairs
between pairs of objects. The action and
reaction forces act on different bodies.
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Examples are a rocket blasting off, a hang
glider flying in air and you sitting in your desk.
You push down on the chair and it pushes back
up on you.
Weight vs Mass
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The weight of an object FW is the
gravitational force acting downward on
the object.
Weight = FW = mass x gravity
FW = ma = mg
Mass is the amount of matter in an
object. ( Number of atoms ), It can
also be a measure of the inertia.
Weight and Mass:
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Mass is a measure of the inertia of a body. It
depends upon the amount of matter contained within
an object. The relationship between mass and inertia
is direct. The unit of measurement for mass is the
Kilogram (kg). Formula: m = F/a. Mass is
CONSTANT.
Weight is the gravitational force exerted object.
Weight is a vector force and a negative symbol
shows force in a downward direction. A medium
apple weighs about one Newton. Formula: W
(weight) or Fg = m x g. Weight changes
according to where you are and the acceleration
of gravity changes (outer space).
Which is variable?
Mass or Weight?
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If you weigh 150 lbs on earth and you go
to the moon; how much will you weigh?
Since the moon is 1/6 as massive as the
earth you will weigh 25 lbs.
Your mass always stays the same… unless
you lose atoms or body parts.
How many kg body mass do you have to
divide your pounds by 2.2 = kg
Weight in Newton’s
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In physics, we express weight in the unit
of Newton’s.
1 Newton is defined as the force of 1 kg
of mass accelerated by gravity.
Formula: Weight = mass x acceleration.
So a person with a 100 kg body mass has
a weight of 981 Newton’s.
How much do YOU weigh in Newton’s?
Free Body Diagrams
Free Body
Diagrams are
pictures of how
forces act
upon an object
or system.
W is the weight
and Fn is the
normal force.
T is tension.
Tension
(Tensile Force)
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Tension is the force in a string, chain or
tendon that is applied tending to stretch
it. Like hanging from the monkey bars
with your arms.
Example: If you hang a box in the air
by one rope the tension force (FT ) is
the same as the weight: mass x
acceleration ( due to gravity in vertical
direction). If the box is held by 2 or
more ropes, then the amount of tension
is split between the ropes.
FT
Normal Force
The normal force on an object that
is being supported by a surface is
the component of the supporting
force that is perpendicular to the
surface. This is the force of the
earth pushing back up against
your feet.
 FN = FW
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The Normal Force is the
Earth’s response to Weight.
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Newton’s 3 law
of actionreaction forces
can be shown in
the Normal force
of the Earth
pushing up
against our
weight.
The Law of Gravity
Every mass exerts a force of
attraction on every other mass.
 The math…
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Gmm'
F 2
r
G = 6.67  10-11 N·m2/kg2
Field Forces and the
Inverse Distance Squared
In a field force like gravity, magnetic
and electrical fields; the force will
vary by the inverse of the distance.
 Fg = 1/ d2
 So if you double your distance:
Fg = 1/ 2d2 = ¼ as much force.
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Gravity Questions
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Did the Moon exert a gravitational
force on the Apollo astronauts?
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What kind of objects can exert a
gravitational force on other
objects?
Typical Problems for Forces:
Vertical Forces in an Elevator
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Note how
the motion
increases
and
decreases
the
apparent
weight on
the scale.
Elevator Vertical Analysis
The
“UP”
side.
 The
“Down”
side.
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Tension in Ropes and Life
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Tension in physics is the forces we
put through ropes – not the tests we
give.
Frictional Force
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Friction is a force that opposes the motion of two
objects that are touching each other. It does this by
creating temporary electromagnetic forces between
the contact points of the two surfaces.
Friction acts in a direction parallel to the surfaces in
contact and opposing the motion. The force exerted
by a two surfaces touching is called a contact force.
When a contact force acts perpendicular to the
common surface of contact it is called a Normal
Force (meaning perpendicular). Friction is
independent of the areas of the surfaces in contact
but is directly proportional to the mass.
What Causes Friction?
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Rough surfacesa side view of
smooth steel.
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Electrostatic
attraction
between atoms.
Friction Opposes Motion
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Note that the
vectors of force /
motion are
opposite from
friction.
Types of Frictional Force
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1.
Static friction is the force of friction that
resists the start of motion. This is always
greater than the sliding frictional force.
2. Sliding friction (also called Kinetic Friction)
is the force that resists the existing or continuing
motion.
3. Air resistance is a special case of sliding
friction. This is due to air molecules colliding with
a moving object. Ff air v2. The air friction force
is a square function of the velocity and in reality a
function of the air density altitude. We ignore it in
this course.
Friction in Life
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Kinetic Friction
is opposing
continued
motion.
Static friction
is opposing
the starting of
motion.
Relationship between Static
and Kinetic Friction.
Steps to Solve Force Problems
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a.
Read and label the problem.
b.
Draw an accurate diagram (free body
diagram). Show all forces exerting force on the
object.
c.
Choose a convenient x-y coordinate
system for resolution of vectors.
d.
Determine knowns and unknowns to
assist in choosing equations.
e.
Solve it roughly to get an idea of the
range of reasonable answers.
f.
Solve with equations.
g.
Keep track of units throughout the
equation and check the answer.
Teeter Totter Problem:
Torque
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Force1 x distance1 = Force2 x distance2
Vector Analysis of Pulling a
Sled with a 25o Angle
Force and Acceleration of an 80
kg Sled pulled at 25o with 150N
Tension analysis of a Sled
Forces on a Hanging Sign
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Note how the
two ropes have
unequal angles.
This means they
will support
unequal
amounts of the
weight of the
sign.
Analysis of a
Hanging Sign
Special Case Problem: The
Inclined Plane
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Please note the special case with the skier
going downhill (Inclined plane) . When the axis
is chosen for vector resolution it is slanted to a
normal Cartesian system. This means that the
vertical and horizontal components in the vector
switch trig functions:
Fx (parallel to surface) = mg sin  and
Fy (perpendicular to surface) = mg cos . This switch
allows us to identify the force causing the
sliding down force as the vertical sine.
The Inclined Plane
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Note that
when you
have an
incline that
the normal
force is no
longer
directly
opposite to
weight
Analysis of an Inclined Plane
Now Add Friction to an
Inclined Plane.
Analysis of Friction on an
Inclined Plane.
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Note the
change in
axis for
vertical and
horizontal in
this special
case.
Inclined Plane with Pulley
Inclined
Planes
with
Pulley
Analysis
Hooke’s Law: the Spring
Constant.
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Hooke’s Law is the
application of
Newton’s second law F
= ma to springs.
Formula: F = - kx
where k is the spring
constant and x is the
distance the
displacement. The
negative sign is the
“restoring” convention
Hooke’s Law of Springs
Examples
Hooke’s Law Problem
May the Force Be With You
Mr. Puckett flying from Clinch Mountain Tennessee 1979