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1. Mechanics
Mechanics is a branch of physics
concerned with the behavior of physical
bodies when subjected to forces or
displacements also it deals with matter
and investigates energy.
Mechanics is divided into three branches:
1- Statics
2- Kinematics
3- Dynamics
In this slide we will discuss Dynamics.
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2. Dynamics
 The branch of mechanics that is
concerned with the effects of forces on
the motion of a body or system of
bodies, especially of forces that do not
originate within the system itself. That
is the external forces and not the
internal ones.
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3. Internal forces
 These forces generate inside the body due
to the interaction between the particles,
atoms, molecules or even inside the
nucleus.
 Forces that originate within the object
itself
 They cannot change the object’s velocity
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4. Types of forces
 In nature we have four fundamental
forces and all the other forces that
you know undergo these four which
are in order of strength:
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5. Strong force
 The Strong Force - This force binds
neutrons and protons together in the
cores of atoms and is a short range
force.
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6. Electromagnetic force
 Electromagnetic - This acts
between electrically charged
particles. Electricity, magnetism, and
light are all produced by this force
and it has an infinite range.
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7. Weak forces
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Weak Force - This causes Beta decay (the
conversion of a neutron to a proton, an
electron and an antineutrino) and various
particles (the "strange" ones) are formed by
strong interactions but decay via weak
interactions. Like the strong force, the weak
force is also short range.
IB Physics (IC NL)

8. Gravitational force
 Gravitational - This force acts
between all masses in the universe
and it has infinite range.
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9. Table of strength
Interaction Relative
strength
Range
Strong 1038 10-15
electromagnetic 1036 ∞
Weak 1025 10-18
gravitational 1 ∞
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10. Unit two: forces
In our IB, forces are to be studied from
a basic part of view and concerned with
external forces; later we might go to
forces related to the four preceding
forces.
 External force
 Any force that results from the
interaction between the object and its
environment
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11. External Forces
 Def: force is a mechanical action capable of:
i. moving a body initially at rest.
ii. Changing the motion of a body.
iii. Deforming a body.
 Usually think of a force as a push or pull
 Vector quantity
 May be a contact force or a field force
 Contact forces result from physical contact between two
objects
 Field forces act between disconnected objects
 Also called “action at a distance”
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12. Contact and Field Forces
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13. Newton’s First Law
 If a body is at rest it remains at
rest, if it is moving with uniform
motion it keeps its uniform motion
unless acted upon by an external
force.
 The net force is defined as the vector
sum of all the external forces exerted
on the object
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14. Equilibrium
 An object either at rest or moving
with a constant velocity is said to
be in equilibrium
 The net force acting on the object
is zero (since the acceleration is
zero)
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15. Condition of equilibrium
0


ext
F
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16. Inertia
 Is the tendency of an object to
resist any attempt to change its
state of motion.
 An object's inertia is directly
proportional to its mass; the
heavier an object is, the more
inertia it has. Hence, a body's mass
measures its inertia.
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17. Mass
 Is the quantity of matter found in a
body.
 Scalar quantity
 SI unit is kg
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18. Seat Belt Device
 Illustration of how one
type of seat belt
operates involving the
inertia of a block
 It protects you when
inertia keeps you
moving if the driver
suddenly applies the
brakes.
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19. Newton’s Second Law
 If a body is subjected to a force this body
accelerates. The force and the acceleration
are directly proportional and in the same
direction.
 F and a are both vectors
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20. Units of Force
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 SI unit of force is the Newton (N)
1 N = 1 kg.m.s-2

21. Sir Isaac Newton
 1642 – 1727
 Formulated basic
concepts and laws
of mechanics
 Universal
Gravitation
 Calculus
 Light and optics
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22. Horse and Barge
 The barge mass is
2.00X103 kg
 1 = 30.0o
 2 = 45.0o
 Values of the forces F1
and F2 are each 600 N
 Find the x and y
resultant forces and
associated
accelerations
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23. Gravitational Force
 Mutual force of
attraction between
any two objects
 Expressed by
Newton’s Law of
Universal Gravitation:
2
2
1
g
r
m
m
G
F
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G = 6.67 x 10-11 SI

24. Weight
 The magnitude of the gravitational
force acting on an object of mass m
near the Earth’s surface is called the
weight w of the object
 w = m g is a special case of Newton’s
Second Law
 g is the acceleration due to gravity or
gravitational field strength.
 g can also be found from the Law of
Universal Gravitation
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25. More about weight
 Weight is not an inherent property of
an object
 mass is an inherent property or mass is
invariant.
 Weight depends upon location.
 Weight changes with g
 g= 9.81ms-2 in Paris, 9.83ms-2 at the
pole and 9.78ms-2 at the equator.
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26. Newton’s Third Law
 If object 1 and object 2 interact, the
force exerted by object 1 on object 2 is
equal in magnitude but opposite in
direction to the force exerted by object
2 on object 1.
 Another version: for every action there is
an equal and opposite reaction.
 Equivalent to saying a single isolated
force cannot exist
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27. Newton’s Third Law cont.
 F12 may be called the
action force and F21
the reaction force
 Actually, either force
can be the action or
the reaction force
 The action and
reaction forces act
on different objects
 Point of application
is always the center
of mass.
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28. Some Action-Reaction
Pairs

 is the normal force,
the force the table
exerts on the TV
 is always
perpendicular to the
surface
 is the reaction – the
TV on the table

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29. More Action-Reaction pairs

 is the force the
Earth exerts on
the object
 is the force the
object exerts on
the earth

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30. Forces Acting on an Object
 Newton’s Law
uses the forces
acting on an
object
 are
acting on the
object
 are
acting on other
objects
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31. Applications of Newton’s
Laws
 Assumptions
 Objects behave as particles
 can ignore rotational motion (for now)
 Masses of strings or ropes are
negligible
 Interested only in the forces acting
on the object
 can neglect reaction forces
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32. Free Body Diagram
 Must identify all the forces acting
on the object of interest
 Choose an appropriate coordinate
system
 If the free body diagram is
incorrect, the solution will likely be
incorrect
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33. Free Body Diagram,
Example
 The force is the
tension acting on
the box
 The tension is the
same at all points
along the rope
 are the
forces exerted by
the earth and the
ground
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34. Free Body Diagram, final
 Only forces acting directly on the object are
included in the free body diagram
 Reaction forces act on other objects and so
are not included
 The reaction forces do not directly influence
the object’s motion
 In free body diagrams you can select one
point to be the point of application of all the
forces acting on the object.
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35. Solving Newton’s Second
Law Problems
 Read the problem at least once
 Draw a picture of the system
 Identify the object of primary interest
 Indicate forces with arrows
 Label each force
 Use labels that bring to mind the
physical quantity involved
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36. Solving Newton’s Second
Law Problems
 Draw a free body diagram
 If additional objects are involved, draw
separate free body diagrams for each object
 Choose a convenient coordinate system for
each object
 Apply Newton’s Second Law
 The x- and y-components should be taken
from the vector equation and written
separately
 Solve for the unknown(s)
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37. Equilibrium revisited.
 Easier to work with the equation in
terms of its components:
 This could be extended to three
dimensions
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38. Equilibrium Example –
Free Body Diagrams
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39. Inclined Planes
 Choose the
coordinate
system with x
along the incline
and y
perpendicular to
the incline
 Replace the force
of gravity with its
components
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40. Multiple Objects –
Example
 When you have more than one
object, the problem-solving
strategy is applied to each object
 Draw free body diagrams for each
object
 Apply Newton’s Laws to each
object
 Solve the equations
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41. Multiple Objects –
Example, cont.
 A fish weights 40.0 N
when at rest.
 Determine the weight
when a=2.00 m.s-2 up
 When a=2.00 m.s-2
down
 What is the weight if
the cable were to
break?
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42. Multiple Objects –
Example, cont.
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43. Forces of Friction(not in IB)
 When an object is in motion on a
surface or through a viscous
medium, there will be a resistance
to the motion
 This is due to the interactions
between the object and its
environment
 This is resistance is called friction
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44. More About Friction
 Friction is proportional to the normal
force
 The force of static friction is generally
greater than the force of kinetic friction
 The coefficient of friction (µ) depends
on the surfaces in contact
 The direction of the frictional force is
opposite to the direction of motion
 The coefficients of friction are nearly
independent of the area of contact
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45.  Static friction acts
to keep the object
from moving
 If F increases, so
does ƒs
 If F decreases, so
does ƒs
 ƒs µ n
Static Friction, ƒs
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46. Kinetic Friction, ƒk
 The force of
kinetic friction
acts when the
object is in
motion
 ƒk = µ n
 Variations of the
coefficient with
speed will be
ignored
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47. Block on a Ramp, Example
 Axes are rotated as
usual on an incline
 The direction of
impending motion
would be down the
plane
 Friction acts up the
plane
 Opposes the motion
 Apply Newton’s Laws
and solve equations
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48. Connected
Objects
 Apply Newton’s Laws
separately to each
object
 The magnitude of the
acceleration of both
objects will be the
same
 The tension is the
same in each diagram
 Solve the simultaneous
equations
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49. More About Connected
Objects
 Treating the system as one object
allows an alternative method or a
check
 Use only external forces
 If treating the system as one object then
tension is no more considered and it will
be an internal force. The mass is the
mass of the system
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