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Chapter 4
Dynamics
&
Newton’s Laws of Motion
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Newton’s First Law
The Law of Inertia
 An object at rest tends to stay at
rest.
 An object in motion tends to stay in
motion.
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Inertia:
A resistance to change, a property of all
matter.
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Determining Net Force
(Using Free body Diagrams)
 The Sum of the Forces Acting on a Body is the net
force on that body.
 If an object is not moving or if it is moving at a
constant velocity the net force acting on the object
is zero. This is known as a state of equilibrium.
Try: P. 124
Practice A
Fground
Problems 1 & 2
Fresist
& p. 128
Practice B
Problems 1 & 3
Fforward
Fweight
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This is an
example of a
free body
diagram
Newton’s Second Law
The Law of Acceleration
 The acceleration of an object is in the
direction of the force applied.
 Acceleration is directly proportional to the
force applied.
 The harder you push an object the faster it
goes
 Acceleration is inversely proportional to the
mass of the object.
 The heavier the object, the less affect a
push has.
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Newton’s Law of Acceleration
 Fnet = Mass x Acceleration = ma
 Weight = Fw = mass x gravity (mg)
Try: P. 132
Practice C
Problems 1, 3 & 5
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Newton’s Third Law
Action - Reaction
 For every action there is an equal and
opposite Reaction.
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The “Normal” Force
 Also called a the
“contact force;” the
normal force is a
reaction force
 The Normal force
acts perpendicular
to the surface upon
which it acts.
Fw
FN
The force of the weight of
the statue pushes downward,
and the box exerts an equal
and opposite “normal” force
upward
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Normal Force on an Incline
Fx and Fy are
components of Fw, the
weight of the box. The
component Fy pushes
against the incline and
the incline pushes back
with an equal and
opposite force, FN
Fx= Fw Sin θ
Fy = Fw Cos θ
FN
Fy
θ
θ
Fw
Fx
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Frictional Forces
 Forces always come in pairs, hence the
Normal force, which is perpendicular to the
contact surface, has a companion force that
is parallel to the contact surface, this force
is friction
 Friction always opposes motion
 Friction depends upon two things:
 The nature of the contact between two objects
 How strong the force of contact is (The Normal
Force)
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Friction – The Mathematics
 The nature of contact is given
mathematically as µ (mu) which is
called the Coefficient of Friction
 For every type of surface contact there
are two values of μ; A static (nonmoving) value (µs) and a kinetic
(moving) value (µk)
 µs > µk which tells us that it is easier to
keep something moving than it is to start
the object moving
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Friction – The Mathematics
 As stated earlier, the second thing
which friction depend upon is the
strength of the contact is given by
the normal force, FN hence:
Ff = µFN
 Ff = force of friction
 µ = coefficient of friction
 FN = normal force
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Coefficients of Friction
Sample Problem
 A 24 kg crate initially at rest on a
horizontal floor requires a 75 N horizontal
force to set it in motion. Find the
coefficient of static friction between the
crate and the floor.
 We know that:
 Ff = µFN
 FN= mg (crate on horizontal floor)
 Ff = 75 N (force required to overcome friction)
 So: µ = Ff /FN = Ff /mg
 Substituting in the values:
 Μ = 75 N/(24 kg/9.8 m/s2) = 0.32
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Try: P. 139
Practice D
Problems 1 & 3
Overcoming Friction
Sample Problem
 A student attaches a rope to a 20 kg
box of books. He pulls with a force of
90 N at an angle of 30° with the
horizontal. The coefficient of kinetic
friction between the box and the
sidewalk is 0.5. Find the acceleration
of the box.
Try: P. 141
Practice E
Problems 1 & 4
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Wrapping Up
 Air resistance is a form of friction known as drag.
This term is also used when referring to the resistance
of water on skis, boats, etc.
 There are four fundamental forces in the Universe.
 Friction and Magnetic forces are due to interactions
between atoms, these are electromagnetic forces.
 Gravitational force is the attraction between the
masses of objects.
 The strong and weak nuclear forces are very
strong at the subatomic level but are not easily
observed. They act over very short ranges.
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