Acknowledgements
© 2013 Mark Lesmeister/Pearland ISD
 This work is licensed under the Creative Commons Attribution-
ShareAlike 3.0 Unported License. To view a copy of this license,
visit http://creativecommons.org/licenses/by-sa/3.0/ or send a
letter to Creative Commons, 444 Castro Street, Suite 900,
Mountain View, California, 94041, USA.
 Selected graphics and problems from OpenStax College. (2012,
June 12). College Physics. Retrieved from the Connexions Web
site: http://cnx.org/content/col11406/1.7/
 Cartoons from Looney Tunes Movie Collection, © 2005 Warner
Brothers Entertainment. Used under the fair use doctrine for
educational purposes.
 Selected questions from
Pearland High School Physics
FORCE
 Force is a push or pull exerted on some object.
 Forces cause changes in velocity such as:
 Start moving, stop moving or change direction.
 The SI unit for force is the Newton.
 1 Newton = 1 kg m/s2
Types of Forces
 Forces can act through contact or at a distance.
 Contact Force – physical contact between two
objects
 Field Force – does not involve physical contact
between two objects.
 Example include:



electrical forces
magnetic forces
the force of gravity
Part 1
Observation #1
 An object at rest remains at rest, unless something
makes it move.
Observation #2
 An object in motion continues in motion with
constant velocity, unless something makes it
change its velocity.
 Constant velocity means constant speed in the
same direction.
Combining Observations 1 & 2
 An object left alone will not change it’s velocity.
Something must cause a change in velocity.
 A force is something that causes an acceleration or
change in velocity by either:
 Changing speed
 Changing direction.
Objects and Systems
 An object is something
that has no internal
structure, or that we can
treat as having no
internal structure. Ex:
Electron
 A system is an object or
collection of objects
grouped together for
study. Ex. Atoms
External and Internal Forces
 An object cannot exert a
force on itself.
 Internal forces have no
effect on the motion of a
system as a whole.
 Only external forces are
considered in Newton’s
Laws.
Observation #3
 An object will not change its velocity unless a net
external force acts on it.
Newton’s First Law
 Objects do not change their motion without a
cause.
 Forces are what cause changes in motion.
 It is the net external force acting on an object
that determines whether it will change
motion.
Newton’s First Law
 An object at rest remains at rest, and an object in
motion continues in motion with constant velocity,
unless the object experiences a net external force.
 A net external force is required to change velocity.
Force
 SI unit of force is the Newton (N).
 1 N = 0.225 lb
 1 lb. = 4.448 N
 A force is a vector.
 It has a magnitude, measure in N or lbs.
 It acts in a particular direction.
Common Forces
 The force of gravity
(Fg)or (W) pulls straight
down.
 The force of friction (Ff )
or (f) occurs between two
objects that can slide
against each other.
 It opposes the relative
motion of the surfaces.
 Applied force (Fa ) points
in the direction of motion.
Common Forces
 The normal force (FN) or
(N) is the support force
from a surface.
 It is called “normal” because
it is always perpendicular to
the surface.
 The tension (FT) or (T) is
the force in a rope or
string.
 The tension is the same in
every part of a rope.
Free-body diagram
 Free-body diagrams consider just one object and
the forces that act on it.
 To draw a free body diagram
 Draw a dot to represent the object.
 Draw and label vector arrows representing all the
forces acting on the object.

All the vectors should be shown as acting at a single point.
Steps for Drawing Free-Body
Diagrams : add this to your notes
 Before you begin identifying the forces acting in a
situation; draw a dot, a circle or a box to represent the
isolated object under consideration
Object under consideration:
towed car
Due to the interaction between the road and the back
tires, the road also exerts a backward force of friction on
the car.
Fn
Ft
Ff
Fg
Objects on an incline- sketch this in
your notes
Part 2
Inertia
 Another way to say the First Law is to say that objects
have inertia.
 Inertia is the tendency of objects to resist changes in
motion.
 The amount of inertia an object has is determined by
its mass.
Forces
 A force is the interaction of two objects.
 There are four fundamental interactions, in other words
four fundamental forces.
P
1 kg
P
Nitrogen
e-
Carbon-14
P
P
4 kg
2 fm
Four fundamental forces
 Gravitational forces
 Strong nuclear forces
 Weak nuclear forces
 Electromagnetic forces
Weight/Mass Relationship
 Weight is the magnitude of the force of the Earth’s
gravity on an object.
 The force of gravity is shown in diagrams as FE-m
or Fg.
 Mass is a measure of the amount of matter in an
object.
 Weight and mass are proportional.
 The constant of proportionality near the surface
of the Earth is g = 9.81 N/kg.
 The weight of an object is often written as mg.

𝐹𝑔 = 𝑚𝑔
Gravitational Mass vs. Inertial Mass
 Gravitational mass is the property of an object
that determines how much weight it has at a
certain place (Earth, Moon, Mars …).
 It determines the strength of gravitational
interaction with other objects, systems or
gravitational fields
Gravitational Mass vs. Inertial Mass
 Inertial mass is the property of an object that
determines its resistance to changes in motion.
 This property shows how an objects motion
changes when it interacts with other objects or
systems.
 Experiments have confirmed that these two
properties are the same.
Equilibrium model
 Objects that are at rest or moving with constant
velocity are in equilibrium.
 According to Newton’s First Law, objects in
equilibrium have a net external force that equals 0.
 Δv = 0, ∑ F = 0
 ∑ F = 0, Δv = 0
Equilibrium Example
 A rock-climber who weighs 800 N is held in place
by two ropes. One pulls horizontally to the right,
and the other pulls up and to the left at an angle of
30o from the horizontal. What is the tension in
the ropes?
 First, draw a FBD for the rock climber and label your forces
(drawing your forces against a backdrop of a coordinate system
might help with resolving the forces)
Equilibrium Example
T1
T1
Model: Equilibrium
T1Y  T1 sin 
θ
T2
θ
F
X
FG
FG  800 N
  30
T1  ?
T2  ?
T1X  T1 cos
0
T2  T1 cos  0
T2  T1 cos 
T2  (1600 N) cos(30 )
T2  1390 N
F
Y
0
T1 sin   FG  0
T1 sin   FG
FG 800 N
T1 

sin  sin 30
T1  1600 N
Section 3
Which has more force?
 When the boxer hits the bag, which has more force,
the boxer on the bag or the bag on the boxer?
Newton’s Third Law
 If Object A exerts a force on Object B, then B exerts a
force on Object A that is equal in magnitude but
opposite in direction.
©2012 OpenSTAX College
Newton’s Third Law
 The two forces are called an action-reaction pair.
 The two forces do not balance each other, since they
act on different objects.
 For every action, there is an equal and opposite
reaction
©2012 OpenSTAX College
Newton’s Third Law
Newton’s Third Law
Action-Reaction Pairs
 Identify all the action-reaction pairs involved in a ball
sitting on a table.
Action-Reaction Pairs
 Identify all the action-reaction pairs involved in a ball
sitting on a table.