Download newton`s first law of motion powerpoint

Honors Physics
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I can define Force.
I can explain the difference between contact and field
force.
I can draw Force (Free-body) Diagrams
I can explain the relationship between the motion of
an object and the net external force acting on it.
I can determine the net external force on an object.
I can calculate the force required to bring an object
into equilibrium.
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In its most easiest terms, force is a push or a pull on an
object.
However, in this class we will define force as “the
cause of an acceleration, or the change in an object’s
velocity.”
Therefore, force can change the direction of an object or
its speed!
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1 pound (lb) = 4.448 Newtons (N)
1 Newton (N) = 0.225 pounds (lbs)
1 kilogram (kg) = 1000 grams (g)
1 kilogram (kg) = 2.20 pounds (lbs)
Did you know?
The abbreviation for the
pound (lb) comes from
libra, the Latin word for
pound!
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Contact Forces
 Force that arises
from the
physical contact
of two objects.
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Field Forces
 Force that can exist
between objects, even in
the absence of physical
contact between the
objects.
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Field theory was developed as a tool to explain how
objects could exert force on each other without
touching.
According to this theory, the presence of an object
affects the space around it so that a force is exerted
on any other object placed within that space.
The region of influence is called a “field.”
For example, an object falls to Earth because of the
interaction between the object and the gravitational
field of the Earth.
Field forces are especially important in the study of
particle physics – but alas, this must wait until later!
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Force is a vector – it shows magnitude and direction!
A force diagram is a diagram of the objects involved in a
situation and the forces exerted on the objects.
In a force diagram, ALL of the forces acting on an object
are shown.
Another name for a force diagram is a free-body
diagram.
A free-body diagram helps to analyze a situation and
isolate an object to show the forces acting on it.
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Did you notice that the
force diagram vectors
originate from the center
of the object!!??!!??
This spot is known as the
“center of gravity!”
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The first step is to examine the motion of the object before forces
are applied.
A common misconception is that an object with no force acting
on it will always be at rest!
To test this misconception, consider sliding a bowling-ball on
different surfaces.
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Imagine a bowling-ball on a thick carpet. If you apply a force the
bowling-ball moves, however as soon as you remove the force the
bowling-ball stops immediately!
Now imagine pushing the bowling-ball on a waxed floor. If you push
with the same force it will slide much farther before coming to a rest!
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Galileo, in the 1630’s realized that a block sliding
on a perfectly smooth surface would slide forever
in the absence of an applied force.
He concluded, correctly that it is not the nature of
an object to stop once it is set in motion; RATHER
it is an object’s nature to maintain its state of
motion.
This concept was developed by Newton, in 1687
and has come to be known as Newton’s First Law
of Motion.
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An object at rest remains at rest, and an
object in motion continues in motion
with constant velocity (that is, constant
speed in a straight line) unless the object
experiences a net external force.
The tendency of an object not to
acceleration is called INERTIA.
INERTIA is the tendency of an object to
maintain its state of motion.
Large Inertia
Small Inertia
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In the absence of forces, a body will preserve its state of
motion.
When the net external force on an object is zero, its
acceleration (or change in its velocity) is zero.
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In order for the car to
accelerate – the car needs to
overcome the force of
friction, the force of air
resistance, and the force due
to the weight of the car!
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To understand how a car maintains a constant
velocity under the influence of so many forces, it
is necessary to understand the different between
external force and net external force.
External Force – A single force that acts on an object as
a result of the interaction between the object and its
environment.
 Net External Force – The vector sum of all the forces
acting on an object. Sometimes it is called the
resultant force.
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Steps to solution:
1. Define the problem and identify the variables.
2. Select a coordinate system, and apply it to the free-body
diagram.
3. Find the x and y components of all vectors.
4. Find the net external force in both the x and y directions.
5. Find the net external force.
6. Evaluate your answer.
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You leave your physics book on the top of a drafting
table that is inclined at an angle of 35o. The freebody diagram shows the forces acting on the book.
Find the net external force acting on the book and
determine whether the book will remain at rest in
this position.
Force of friction = 11 N
Force of table on book = 18 N
Force of gravity on book = 22 N