Download Introduction to Energy Concepts

Forces and Newton’s Laws
Physics
Classical or Newtonian
mechanics
Describes motion of macroscopic objects.
 Stays away from objects approaching the
speed of light  special relativity.
 Stays away from teeny, tiny objects 
quantum mechanics.
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What is a Force?
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A Push or a Pull!
Standard Unit is the NEWTON (N)
Types:
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Contact Forces (macroscopic)
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Normal contact stuff (support forces, pushes, pulls,..)
Friction (always in the opposite direction of motion)
Air Resistance (again always opposite in direction)
Tension (Ropes, Wires, Cables)
Field Forces
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Electric, Magnetic, Nuclear
Gravity (exists between any two objects)
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Requires lots of mass to be significant!
On earth: 1 kg is pulled on with a force of 9.8 Newtons
Practice ….
Using Gravitational Field to
Determine Weight / Mass
12 kg = __________ N
 100 grams = ________ N
 10 N = __________ kg
 I weigh 175 pounds.

How much mass is that in kg (earth)?
 What is that weight in N?
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Overview of Newton’s Laws
First Law: What happens to the motion
of an object if there is no total, net force
acting on it?
 Second Law: What happens to the
motion of an object if there are forces
acting on it?
 Third Law: What happens to the
motion of several objects when they
interact?
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Newton’s First Law of Inertia
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Every object continues in its state of rest, or
of uniform motion in a straight line, unless it is
compelled to change that state by forces
impressed upon it.
In other words; if there is no NET FORCE
acting on the object, it’s motion should remain
unchanged!
The amount of Inertia an object has depends
on its mass
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Ex’s
Inertial Balance
How does one weigh things in
space?
 You can’t
WEIGH
things in
space, but
one can
mass
object.
Inertia is WAY better than a
towel. Why?
More inertia
Egg
 Why does the blood rush to your feet
when a descending elevator stop?
 How do you get the last bit of ketchup out
of the bottle?
 Do head rests help prevent whiplash from
front-end or rear-end collisions?
 Shot put

Review…
How many Newtons does a 6 kg object
experience due to the gravitational pull of
the Earth?
 25N = _____kg
 Which of the following has the most
inertia? A submarine, a tricycle or an
SUV?
 What is inertia?

NET FORCE
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The sum of all forces acting on an object is the
object’s NET FORCE.
When the NET FORCE is zero, the object is said
to be at equilibrium.
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Sometimes Equilibrium occurs and the object is at
rest
Sometimes Equilibrium occurs and the object is
moving AT CONSTANT VELOCITY!
Ex’s – Support forces and scales
Practice problems
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80 kg = ___________ Newtons
15 kg = ___________ Newtons
1000 N = __________ Kg
50 grams = ________ N
20 pounds = _______ Newtons
Will a scale reading increase or decrease as you
accelerate up in an elevator?
Will a scale reading increase or decrease as you
move downward at constant velocity in an
elevator?
Diagrams and Vectors
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Force Diagrams
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Should consist of a small picture of the object
Also consist of scaled arrows that represent force acting
on the object. Longer the arrow, greater the force.
Each “Force Arrow” should be labeled as “Object B on
Object A”, where Object A is the object your concerned
with.
Ex’s…. Practice …. Practice …
Vectors: Force is a VECTOR – therefore we
should follow the rules for adding vectors.
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If in the same direction they just add up.
If in the opposite direction they just subtract.
If at an angle, line the tails up together and create a
parallelogram. The diagonal of the parallelogram
(starting from the “tails”) is the Resultant.
Ex’s … Practice … Practice
HONORS
Check for Understanding…
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Examine the diagram below of 3 books of
different masses atop a scale.
What is the mass of the middle book?
With what force does Book C exert on Book B?
Draw a complete force diagram for each book !!!
Book A
Mass = 1 kg
Book B
Mass = ?
Book C
Mass = 2.5 kg
Scale = 65 Newtons
Check for Understanding… Again..
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Examine the diagram below of 3 planters of
different masses hanging from each other.
Plant B exerts 120 Newtons of Force on Plant A.
What is the mass of Plant B?
Draw a complete force diagram for each plant !!!
Ceiling
Plant A
Mass = 4 kg
Plant B
Mass = ?
Plant C
Mass = 7 kg
Warm-up
1.
2.
3.
A student drags a sled across loosely
packed snow with a rightward
acceleration. Draw a force diagram of the
student and another diagram of the sled.
A car is traveling at constant velocity up a
a hill to the right. Draw a force diagram
for the car.
An airplane, traveling to the right, is
accelerating up and to the right. Draw a
force diagram for the plane.
Jar of Flies
•
A bunch of flies are in a capped jar. You place the jar on a
scale. The scale will register the most weight when the flies
are
a) Sitting on the bottom of the jar.
b) Flying around inside the jar.
c) …. Weight of the jar is the
same in both cases.
Pulley Practice
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Draw the force diagram for the block on
the table. The mass of the little block is 10
kg. and the mass of the larger block is 5
kg. It is in static equilibrium.
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The mass of the block is 4
kg. Find all five of the
tensions labeled in the
diagram. (hint: draw force
diagrams for the pulleys)
Draw force diagrams for A and B.
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Block A in the figure is
heavier than block B
and is sliding down the
incline at constant
velocity. All surfaces
have friction. The rope
is massless, and the
massless pulley turns
on frictionless bearings.
Kink
Water is shooting out the end of a pipe. The
end of the pipe is bent into a figure 6.
Ignoring the effects of gravity, which of the
following is true?
a) The water shoots out
in a curved arc.
b) The water shoots out
in a straight line.
Friction
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Two types, static and kinetic
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Static Friction describes the resistive forces between
two objects at rest.
Kinetic Friction describes the resistive forces between
two objects that slide past each other.
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Friction depends only on two things; Normal Force
(support force) and the coefficient of friction
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Kinetic Friction is always less than static.
Ex’s Tires and road
Coefficient of friction describes how “sticky” the two objects
are. These are listed in tables somewhere….
Ffriction = μ Fnormal
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Ex…
Newton’s Second Law
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If there is a net force on an object – it will
accelerate. (Understand Motion first!)
The acceleration will be proportional to the net
force on the object
 But the acceleration will be inversely
proportional to the mass (inertia) of the object
 Acceleration = Net Force / Mass
 Ex’s
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Free Fall (a = 9.8 m/s2)
In the absence of air resistance and
friction, all objects should fall at the same
rate regardless of mass/weight.
 Known as the “law of falling bodies”
 While heavier objects have more force
acting on them, they also have more
inertia, which keeps the ratio of F/M the
same.
 Feather and Hammer Drop on the moon.
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Non Free-Fall (a < 9.8 m/s2)
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Objects falling through the air will experience a
drag force (air resistance)
The amount of Air drag depends on …
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The cross-sectional area of the object (size)
The speed of the object
The density of the air
The “texture” of the surface of the object (coefficient
of drag, CD)
Ex
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At one moment a skydiver (50 kg) is falling with an
acceleration of 5 m/s/s, what is the force of air drag?
Friction
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Frictional Force only depends on the surface
features (roughness) and the support force on
the object.
Contrary to what many people think, Friction has
very little to do with Area of contact.
Two types:
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Static Friction: Contacting surfaces lock together like
puzzle pieces when at rest.
Kinetic Friction: Surfaces ride along the edge of each
other when in motion.
Static Friction is always grater than Kinetic.
Ex’s: Phone book friction, simple experiment
Newtons Third Law
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For every action force there exists a reaction
force that is the same in magnitude but opposite
in direction.
Ex’s
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Action: My feet push on the floor with 650 N of force
Reaction: The floor pushes on my feet with 650 N of
force
Action: My cars’ tires push backwards on the
pavement with 10,000 N of force.
Reaction:
Vectors (Honors)
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To resolve multiple vectors at various
angles:
Setup a x-y coordinate system
 Break down each vector into x and y
components
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component = hyp x Cos θ
 Y component = hyp x Sin θ
X
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Add up the x’s and y’s and follow
Pythagorean to get resultant. X2 + y2 = R2