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Forces and
Newton’s Laws
of Motion
Tension
• Tension (T)- force that acts through a
solid object such as a rope or chain;
directed along the rope and pulls equally
on the objects on either end.
.
Normal Force
• Normal Force (Fn)- contact force that
always acts perpendicular to the surfaces
that are touching
Free-Body Diagram (FBD)
• Free-body
diagrams are
drawings used to
show the relative
magnitude and
direction of all
forces acting upon
an object in a
given situation.
Basic Force Diagrams
A Car on a Level Surface
All forces on the car are vertical, so no horizontal
force can be generated.
A Car on a Banked Turn
The normal force on the car due to the road is no longer
vertical, so a component of the normal force acts in the
horizontal direction.
Normal force (N) - contact pressure that pushes surfaces together.
It is easier to drag a light chair across a room than a heavy table.
This is because the weight of the table exerts a greater normal force.
FBD: A book is at rest
on a table
A girl is suspended motionless from
a bar which hangs from the ceiling
by two ropes.
An egg is free-falling towards the
ground. Neglect air resistance.
A flying squirrel is gliding (no wing
flaps) from a tree to the ground at
constant velocity. Consider air
resistance.
A rightward force is applied to a
book in order to move it across a
desk with a rightward acceleration.
Consider frictional forces. Neglect
air resistance.
Real Life Application
More real life…
http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html
Another Force Diagram…
The chandelier
is separated
from the chain
for
interpretation
purposes only!
http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html
Bring in the anglesa bit more complex 
http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html
Box on an Incline
http://img.sparknotes.com/figures/1/174f9bfb5307940e1bb1c2ddac88fd8d/problem1_5.gif
Net Force
Balanced = No Net
Force = Fnet = 0 N
Unbalanced = Net
Force = Fnet ≠ 0 N
• Determined by
combining ALL
forces acting on
an object.
– Zero net force =
zero
acceleration
– If there is a net
force, there will
be an
acceleration.
http://4.bp.blogspot.com/_fDHmeCjBbk/SS4hRNbLwaI/AAAAAAAAAHw/gx0p9HIDS1o/s1600-h/12.gif
Balanced Forces (zero net)
www.physicsclassroom.com
Unbalanced Forces (non-zero net)
Imagine pushing your text book
so that it would slide across the
desk…
www.physicsclassroom.com
Equilibrium
• Balance
– In regards to motion
an object has
constant velocity if it
is in equilibrium.
– NO acceleration.
• Types:
– Static Equilibrium
– Dynamic Equilibrium
Static Equilibrium
Velocity is zero
Scales pushing up
Examples:
Weighing yourself on a set of scales
Car parked on an incline
Friction
Normal
Weight down
Weight down
Dynamic Equilibrium
Velocity is nonzero and constant
Examples:
Driving at constant velocity
Normal up
Air resistance
Force from road
Friction
Weight down
Terminal velocity when parachuting
Weight down
Friction
• The force that opposes the motion between two
surfaces that are in contact.
• Friction is the "evil monster" of all motion. Regardless of
which direction something moves in, friction pulls it the
other way.
– Move something left, friction pulls right. Move something up,
friction pulls down.
• It appears as if nature has given us friction to stop us from moving
anything. 
• Friction is a force that “appears” when there is relative
motion between two objects.
– Although two objects might look smooth, microscopically, they
are very rough and jagged.
Friction
• A force that opposes motion. Friction acts
in a direction opposite to the object's
direction in motion. Without friction, the
object would continue to move at a
constant speed forever
• Static Friction: when object is at rest
• Kinetic Friction: when object is moving
– Sliding Friction:
– Rolling Friction:
Friction is not
always a bad
thing!
• Walking! Your foot pushes backwards on
the ground and friction opposes the push
of your foot by pushing your foot forward
Coefficient of Friction
• A constant that depends on the
two surfaces in contact
• Ff = FN
• (mu)….coefficient of friction
Coefficient of Friction - Friction on an Incline
Newton’s First Law (1642-1727)
• “The Law of Inertia”
• A body remains at
rest or moves in a
straight line at a
constant speed
unless acted upon by
a net force.
– Objects do not
accelerate unless a
net force is applied.
Newton’s 1st Law of
Motion
Objects at rest remain
at rest and objects in
motion remain in
motion, unless acted
upon by an outside
force.
http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html
Newton’s 2nd law of Motion
…mathematically
Net Force = (mass)(accel)
Fnet = ma
NEWTON'S 2nd LAW OF MOTION
F
a
m
F
M
a
m
F
F
a
m
M
a
m
m
F
m
1
a
m
F
a
 
aF
or
M
a


F  ma
When the acceleration is g
we have Free Fall
m
2m
F
2F
F
g
m
2F
g
2m
Terminal Velocity
Net Force
Acceleration = g
Velocity = 0
but motion is about to begin
mg
F
Acceleration < g
v increasing downward
Acceleration << g
v still increasing downward
mg
F
just not as rapidly as before
mg
F
Acceleration = 0
mg
Terminal velocity
Falling with Air Resistance & Terminal
Velocity
• When falling the force of air resistance becomes
large enough to balance the force of gravity.
• At this instant in time, there is no net force — the
object stops accelerating (see D below); terminal
velocity has been reached.
Newton’s Third Law
• Action-Reaction
• When one object exerts a
force on another object,
the second object exerts
a force of equal strength
in the opposite direction
on the first object.
Example of Newton’s 3rd Law:
http://bp3.blogger.com/_vQA3QRUnk3M/RrgyfrRniPI/AAAAAAAAAMs/DYl
hM7pDeI8/s1600-h/newton
Newton’s Third Law of
Motion
For every action, there is always
a reaction of equal (magnitude)
and opposite (direction) reaction.
“action” or “reaction”
refers to force.
Action/Reaction forces do
NOT act on the SAME object!
Reaction: road pushes on tire
Action: tire pushes on road
Reaction: gases push on rocket
Action: rocket pushes on gases
Action- Reaction Forces
• Do Action-Reaction forces cancel
each other?
• No, they are acting on different
objects. Forces can only be added
together when they are acting on
the same object.
http://quest.nasa.gov/space/teachers/rockets/principles.html
Newton’s 3rd Law of
Motion
Reaction
Force: the
ball is
pushing the
guy
rightwards
Action
Force: A
guy is
pushing a
ball
leftwards
Tug-a-war
• If Fido and Rover
play tug-a war, how
do the “pulls” of the
dogs compare?
• If each dog pulls
with 50 N of force,
what is the tension
force in the middle
of the rope
(between the
dogs)?
• While driving, Mrs. Ingle observed a bug
striking the windshield of her car. The bug
hits the windshield and the windshield hits
the bug. Which of the two forces is greater:
the force on the bug or the force on the
windshield? EXPLAIN!
Putting Newton's Laws of Motion Together
http://www.s
pacetoday.org
/images/Rock
ets/ArianeRo
ckets/Ariane5
LaunchAriane
space.jpg
• An unbalanced force must be exerted for
a rocket to lift off from a launch pad or
for a craft in space to change speed or
direction (First Law).
• The amount of thrust (force) produced by
a rocket engine will be determined by the
rate at which the mass of the rocket fuel
burns and the speed of the gas escaping
the rocket (Second Law).
• The reaction, or motion, of the rocket is
equal to and in the opposite direction of
the action, or thrust, from the engine
(Third Law).
http://quest.nasa.gov/space/teachers/rockets/principles.html