Transcript 4.2 Newton’s 2nd & 3rd Laws

Forces
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A force is a push or a pull
any influence that can change the velocity of
an object
There can be no acceleration without a force
is a vector having both magnitude and
direction. Like any other vectors, force
vectors are represented by arrows.
Newton’s Laws of Motion
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An object at rest stays at rest until an outside
force causes it to move.
An object in motion continues to move in the
same direction at the same speed until a force
stops it or changes its direction.
So, an object at rest will stay at rest, and an
object in motion will remain in motion unless
acted by an outside force.
Newton’s First Law of Motion is also known as
the Law of Inertia
Inertia
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Inertia is an objects resistance to a change
in motion.
The inertias between two objects can be
compared when a spring is used to
accelerate different masses
The acceleration of an object depends on
how much force is applied to an object's
mass.
Inertia Example
An astronaut in
outer space will
continue drifting
in the same
direction at the
same speed
indefinitely, until
acted upon by an
outside force.
Mass
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The quantitative measure of the inertia of
an object is an object's mass
Mass refers to the quantity of matter in an
object.
?
m
Balance
If it balances, the masses are
equal. Einstein asserted that
these two kinds of masses are
equivalent.
Weight
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Weight is the pull of gravity exerted on an
object (with mass).
weight = mass x acceleration of gravity
Near the surface of the Earth,
g = 10 m/s2.
Mass is recorded in Kilograms, while weight
is recorded in Newtons.
Newton = Kg x m/s2
Force Diagrams
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By drawing all the force vectors acting around
an object you can create a force diagram.
Free-body diagrams are a very important part
of determining how forces are acting on an
object.
N
f = 22 N
38 N
8 kg
mg
Net Force
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F1
F2
F3
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Fnet
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The Net Force is the
combination of all forces
acting on an object and can be
determined through vector
addition.
Unbalanced forces make
objects accelerate.
Balanced forces are at
equilibrium (no acceleration)
and have a Net Force of Zero
Like a tug of war
Balanced Forces
Unbalanced Forces
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The forces don’t cancel out
Cause a change in motion
Act as one force
Friction
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Friction always opposes the motion of an object.
The friction forces are parallel to the contact surface
and occur when…
One body slides over the other, or…
They cling together despite and external force.
The forces shown are an action-reaction pair.
f
Acme Hand
Grenades
v
Two Kinds of Friction
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Static friction
fs
Must be overcome in order to
budge an object
 Present only when there is no
relative motion between the
bodies, e.g., the box & table
top
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Kinetic friction
Weaker than static friction
 Present only when objects are
moving with respect to each
other (skidding)
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FA
objects still or
moving together
fk
FA
a to the right
v left or right
Tension Force
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T
m
mg
The force of Tension exerted
on a cable is equal in
magnitude to the force of
Weight in the object being
held by the cable, but in the
opposite direction, if the
object is in Equilibrium.
Normal force
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When an object lies on a table or on the
ground, the table or ground must exert an
upward force on it, otherwise gravity would
accelerate it down.
This force is called the normal force.
N
In this particular case,
N = mg.
m
mg
So, Fnet = 0; hence a = 0.
Normal forces aren’t always up
“Normal” means perpendicular. A normal force is
always perpendicular to the contact surface.
N
mg
For example, if a
flower pot is setting
on an incline, N is
not vertical; it’s at a
right angle to the
incline. Also, in this
case, mg > N.
Air Resistance
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The force of the air against a moving object
Increases as the velocity of the motion increases
The size and shape of the object also effect the
air resistance
Larger surface area more resistance
Car designers try to minimize it
Overcoming air resistance uses more fuel
Terminal velocity
 Force
of gravity is constant
air resistance increases as
you speed up
 until the force is equal
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Equal forces, no
acceleration
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constant velocity
terminal velocity