Chapter 6
Forces (Newton’s Laws of Motion)
6.1: Force and Motion
• A force is an interaction between one object and
another (push/pull  has both magnitude and
direction = vector quantity)
When you push on a wall, the
wall pushes back on you!
Environmental Forces
• Contact force:
– Acts on object by touching it
– Ex. Force of hand on ball, elbows on desk
• Long-range force
– Exerted without contact
Examples of contact and action-at-distance
Contact Forces
Frictional Force
Tension Force
Normal Force
Air Resistance Force
Applied Force
Spring Force
Long Range Forces
Gravitational Force
Electrical Force
Magnetic Force
Forces have “Agents”
• That are not so secret 
• Agent = specific cause of force (if you can’t
identify the cause then the force does not
exist)
– Ex. My hand causes a force upward on a baseball,
my hand = Agent
– Force of gravity: Agent = mass of Earth
Common Types of Forces
• Friction: Ff Due to surface bumps and “stickiness” of the atoms on
the surfaces of materials. ALWAYS acts opposite of direction of
motion
• Weight: Fg Force due to gravitational attraction between 2 objects,
usually earth and an object
• Normal: FN The force that supports an object against gravity
• Spring: Fsp Restoring
force
(push/pull)
If the
Normal
Force is exerted
less than by a spring
the Weight, what happens?
• Tension: FT Pull exerted by a string, rope or cable
• Thrust: Fthrust Force that moves objects such as rockets, planes, cars
and people
Defining YOUR System
• System = portion of the universe chosen for
studying the changes that take place within it
• EXAMPLES: a planet, and/or the liquid within
a glass. YOU DEFINE YOUR SYSTEM (my
definition differs a little from your text)
Examples of Systems
1. A book is at rest on a tabletop. Diagram the forces
acting on the book.
2. An egg is free-falling from a nest in a tree. Neglect
air resistance. Diagram the forces acting on the egg
as it is falling.
3. A flying squirrel is gliding (no wing flaps) from a
tree to the ground at constant velocity. Consider air
resistance. Diagram the forces acting on the
squirrel.
4. A force is applied to the right to drag a sled across
loosely packed snow with a rightward acceleration.
Diagram the forces acting upon the sled.
Draw Systems w/ bodies and w/o
Diagram of System
Free-body Diagram
How objects move: Newton’s 2nd Law
• The acceleration produced by a net force on an
object is directly proportional to the net force, is
in the same direction as the net force, and is
inversely proportional to the mass of the object.
– Units
• Mass: kg
• Acceleration: m/s2
• Force = N
• Recall: Velocity vs. time slope = acceleration
• Force vs. Acceleration slope = mass
Net Force
• Net Force: Combination of all forces on an object
• Vector Quantities: Forces shown by arrows
– Have both magnitude (how much)
and direction (which way)
Adding Vectors
• Resultant: Sum of two or more vectors
– Same direction, add
– Opposite direction, subtract
Complicated by angles (Ch. 7)
Newton’s First Law of Motion
• Every object continues in its state of rest, or a
uniform speed in a straight line, unless acted on
by a nonzero force
– Restating Galileo’s concept of Inertia
When you pull out a tablecloth, the dishes are
left in their initial state of rest.
When a driver slams on the breaks, the car
stops faster than the driver’s body, which
causes the body to lurch forward causing
whiplash.
The Equilibrium Rule
• When the net force on something is equal to zero
Tension = Stretching Force
Is this diagram in equilibrium?
Equilibrium of Moving Things
• Equilibrium = state of no change
• An object moving at a constant speed in a straight
line is in equilibrium
• Friction: Force that occurs opposite of motion
when objects are in contact
6.2: Using Newton’s Laws
Recall Newton’s 2nd Law
• The acceleration produced by a net force on an
object is directly proportional to the net force, is
in the same direction as the net force, and is
inversely proportional to the mass of the object.
– Units
• Mass: kg
• Acceleration: m/s2
• Force = N
Mass – A Measure of Inertia
• Amount of Inertia (resist in change of motion) depends on
amount of mass
– Mass: Amount of matter in an object; measured in kilograms (kg)
– DIFFERENT THAN WEIGHT!
• Weight: Force on an object due to gravity
– Weight and mass are directly proportional
• Double the Mass, Double the Weight!
Which bucket would be
harder to push? WHY?
Newton’s Second Law of Motion
If the mass increases, to produce the same
amount of acceleration, what must you do to
the force?
If the force on an object doubles and the mass
does not change, what happens to the
acceleration?
Dynamic Cart Demo
• 2 Volunteers of obviously different
mass/weight
Pg. 128-129 Example problem
Lifting a Bucket:
A 50 kg bucket is being lifted by a rope. The
rope is guaranteed not to break if the tension
is 500 N or less. The bucket started at rest,
and after being lifted 3.0 m, it is moving at 3.0
m/s. Assuming that the acceleration is
constant, is the rope in danger of breaking?
FRICTION
• http://www.youtube.com/watch?v=1hhW76BI
wP4&feature=player_embedded
Friction Force
FRICTION: Force between 2 surfaces that opposes motion
• Static Friction (Fs):
Force that must be overcome to move an object at rest
• Kinetic Friction (FK):
Force of friction that opposes a moving object
Friction Equations
• Kinetic friction is weaker than static friction fk < fs
• Force of friction = coefficient of friction times the
normal force (supporting force)
• Coefficient of Friction (μ): measure of “roughness”
– Relates texture of surfaces in contact
– Calculated in lab
• Static friction: Fs = μsFN
• Kinetic friction: FK = μkFN
Example Problem: Balanced Friction
Forces (pg. 131)
• You push a 25 kg wooden box across a
wooden floor at a constant speed of 1.0 m/s.
How much force do you exert on the box?
Ex. Problem: Unbalanced Friction
Forces (Pg. 132)
• If the force you exert on the box is doubled,
what is the resulting acceleration of the box?
When acceleration is g
Free Fall
• Free fall: When the force of gravity is the only force
acting on an object (air resistance is negligible)
a = g = 9.8m/s2
• Objects of different masses fall at the same rate
The elephant has a greater gravitational
attraction than the person. BIGGER “F”.
BUT, the elephant also has a bigger mass
proportional to that BIGGER F. BIGGER “m”.
That means that the BIGGER “F” cancels the
BIGGER “m” and the objects fall at the same
acceleration.
When acceleration is less than g
Non-Free Fall
• Most of the time, air resistance is NOT negligible.
– Air resistance/ Air Drag
• Force of friction acting between object and air
• Depends on SPEED and SURFACE AREA
• When Drag Force = Force of gravity = no net force =
TERMINAL VELOCITY
Increase Speed,
increase air
resistance/ drag
force
Increase Surface Area, increase air resistance
Gliding
Increase Surface Area in Nature
“Flying” squirrels have
large flaps of skin
“Flying dragons” (lizards in
genus Draco) have long ribs
that support gliding
membranes
“Flying” frogs have very
large toes with webbing
between them
Increase Surface Area, Increase Air Drag, Slow Your Fall!
Non-Free Fall
• Air resistance decreases the acceleration due to
gravity
– Terminal velocity: Object is no longer accelerating
downward – falling at a constant rate
• Human skydiver: 150 – 200km/hr
• Parachute increases surface area, which slows terminal
speed to 15-25km/hr
Removing Air Resistance
• Air Resistance:
– Coin falls faster than feather
because the feather has less mass
and greater surface area
• Remove Air Resistance (Vacuum)
– Coin and feather fall at the same
rate
Periodic Motion
•A motion that repeats itself in equal intervals of time
is Periodic Motion.
•Simple harmonic motion (SHM): a type of periodic
motion where the restoring force is directly
proportional to the displacement
Describing SHM
• Periodic Motion/ SHM results from net forces trying
to obtain re-establish EQUILIBRIUM
• Period (T): time (in seconds) needed to repeat one
complete cycle of motion
• Amplitude: Maximum distance (in meters) the
object moves from equilibrium
The Mass on a Spring
• Period of Oscillation, T, depends on the mass
of the block and the strength of the spring but
not on the amplitude of the motion
The Pendulum
• Period depends only on The Length (L) of the
string/ pendulum and on the force of gravity,
not on the mass of the “bob” or the amplitude
of oscillation
Resonance
• Vibrations produced by one object align with
another object
• The power of resonance can be as gentle as an
adult pushing a child on a swing, or as ferocious as
the force that toppled what was once the world's
third-longest suspension bridge.
• Resonance helps explain all manner of familiar
events, from the feedback produced by an electric
guitar to the cooking of food in a microwave oven.
6.3: Interaction Forces
Newton’s Third Law of Motion
• Whenever one object exerts a force on a second
object, the second object exerts an equal an
opposite force on the first
– Action and Reaction Forces
• Equal in strength, opposite in direction
When a boxer hits a bag, he exerts a
force on the bag. The bag exerts an
equal force on the boxer opposite
the original force
Boxer = Action Force
Bag = Reaction Force
Newton’s Third Law of Motion
You swing an ax and get
it stuck in a stump…
What was the Action Force?
What was the Reaction Force?
Action and Reaction on
Different Masses
• Falling objects pull upward on Earth with as much
force as the Earth pulls downward on it!
Because the Earth has such a great
mass, we can’t see the acceleration
Action and Reaction on
Different Masses
• When you fire a cannon, the cannonball exerts
and equal an opposite force on the cannon.
Why does the cannon
move very little, but the
cannonball flies very far?
The cannon has a large mass, the
cannonball has a small mass.
Defining Your System
• Since action and reaction forces are equal and
opposite, why don’t they cancel to zero?
– When we have action and reaction systems, they are
isolated from other forces. These other forces can
cause acceleration!
The Four Fundamental Forces
Weakest
• Gravity: affects all masses (infinite range),
force carrier, Gravitons
• Weak Interaction: Small short range, forces
caused by bosons, in atomic nucleus, cause of
radioactive decay
• Electromagnetism: Large range, acts on
charged particles through photons
• Strong Interaction: works primarily on
subatomic particles but range is theoretically
infinite. Works through gluons, which hold
Strongest
protons and neutrons together.
When Newton’s Laws are not valid
• Objects moving near the
speed of light
• Objects that are very small –
on the scale of an atom
• Objects under the influence
of very strong gravitation
forces