Objectives
 Describe examples of force and identify appropriate
SI units used to measure force
 Explain how the motion of an object is affected
when balanced and unbalanced forces act on it
 Compare and contrast the four kinds of friction
 Describe how Earth’s gravity and air resistance
affect falling objects
Force
 Definition
- push or pull that acts on an object
- can accelerate or decelerate on object
ex. kicking a soccer ball at rest
wind pushing against you as you walk
 Measured
- units: Newtons (N) = 1kg m/s2
 Represented
- force vectors: arrow = direction
length = strength or magnitude
Forces Cont.
 Combining Forces
- forces acting in the same directions: add
3N + 5N = 8 N
- force in opposite directions: subtract
6N
– 2N = 4 N
 Net Force
- the overall force acting on an object after all the
forces are combined
 Two Types
- balanced
- unbalanced
Balance vs. Unbalanced Forces
 Balanced forces
- forces that combine to produce no net force
ex. tug of war with no winner
2N - 2N=0
- no net force, no movement
 Unbalanced forces
- force that results when the net force acting on an
object is not equal to 0
ex. 2 N - 4 N = 2 N
- object will move in direction of net force
Practice Problems
 You push a car with a force of 50 N, your friends
pulls with a force of 25 N. Draw force diagrams,
and calculate the net force acting upon the car. Will
the car move?
50N
+ 25N =
75 N
 You push a box towards your friends with a force of
80N while one friend pushes the box against you
with a 55 N, the other 25N. Draw force diagrams
and calculate the the net force acting upon the car.
Will the box move?
80 N
- 55 N + 25 =
0
Common Forces
 Two common forces
- gravity: force that acts between any two masses
- attractive force that can act over large distance
- Earth’s gravity acts downward toward the
center of Earth = 9.8 m/s/s = 10m/s/s
ex: Earth’s gravity holds you on the ground
Common Forces Cont.
 Friction
- force that opposes the motion of objects that touch
as they move past one another
- four types: static, sliding, rolling, fluid
- static: acts on objects that are not moving
ex.
- sliding: force that opposes the direction of
motion of an object as it slides over
a surface
ex.
Common Forces Cont.
- rolling friction: force that acts on rolling
objects
ex. soccer ball rolling across the floor
- fluid friction: force that opposes the
motion of an object through fluid
* any mixture of gases is considered a fluid
ex. airplane flying through the air
Force Diagrams
 A diagram that identifies all force acting upon on
object
1. Identify the situation of the object
ex. Static
2. Identify the force acting upon the object
ex. Gravity, static friction
3. Draw the forces acting upon the object
ex.
Fn
forces cancel out

Fg
net force = 0
Practice Problems
 An object is decelerating due to friction
 An object is static
 An object is hanging from the ceiling
Objectives
 Compare and contrast Aristotle Galileo’s and
Newton’s ideas
 Define inertia
 Explain Newton’s 1st law of motion, and apply them
to physical situations
Galileo Vs. Newton: Inertia
 Galileo
- introduced the idea of inertia
- maintained that motion of an object requires a
force
 Newton
- grasped the significance of inertia
- motion of an object requires and initial force, not
continual
- law of inertia defines natural motion and tells us
what kinds of motion are the result of applied forces
Newton’s First Law of Motion
- Newton redefined Galileo’s idea of inertia
Inertia: tendency of things to resist a change in it’s
motion
 Law 1
- Every material object continues in its state of rest
or uniform motion in a straight line, unless it is
compelled to change that state by forces impressed
upon it
- Key Word: continues
Newton’s 1st Law Cont.
ex. stamp our feet to remove snow
shake a garment to rid dust or dirt
table cloth and dishes stunt
* Holds true whether your are at rest or moving at a
constant velocity
ex. when we jump straight up, we land in our
footsteps rather than at a location equal to the
distance the earth moves during our jump?
Inertia Problems
Q: A hockey puck sliding across the ice finally comes
to rest. How would Aristotle interpret this behavior?
How would Galileo and Newton interpret it?
A: Aristotle:
a constant force was not applied to the puck
therefore it would come to a stop.
Galileo & Newton:
The forces acting against the puck become greater
than that of force acting on the puck and therefore it
will come to a stop
Objectives
 Be able to understand and apply Newton’s 2nd Law
of motion
 Using Newton’s 2nd: calculate force, mass, and
acceleration
 Recognize that the free fall acceleration near Earth’s
surface is independent of the mass of the falling
object
 Explain the difference between mass and weight
Newton’s 2nd Law
 The acceleration of an object is directly proportional
to the net force acting on the object, in the direction
of the net force, and is inversely proportional to the
mass of the object
- in short: A = net force or F = ma
mass
- symbols: a = acceleration
Fnet = force
m = mass
Practice Problems
 A boy pushed forward a cart of groceries with a total
mass of 40.0 kg. What is the acceleration of the cart
if the net force on the cart is 60.0 N?
 An automobile with a mass of 1200 kg accelerates at
a rate of 3.0m/ s2 in the forward direction. What is
the net force acting on the automobile?
 A 25 N force accelerates a boy in a wheelchair at
0.5m/s2. What is the mass of the boy and the
wheelchair?
Practice Problems Cont.
 A = F/m
60.0 N = 1.50 m/s2
40.0 kg
 F = ma
1200 kg x 3.0 m/s2 = 3600 N
 M = F/a
25 N = 50 kg
0.50 m/s2
Inertia Problems
Q: Would it be easier to lift a huge truck on the Earth
or on the Moon?
A: On the moon, when you lift an object you are
dealing with weight, since weight is the gravitational
pull on the most massive body, and the moon has
less gravity than the earth
Acceleration
 Acceleration can be:
 Equal to 0: Net Forces = 0
static motion
- object is not moving or accelerating
- all forces are balanced
ex. a book at rest on the table
dynamic motion
- object is moving but not accelerating
- all forces are balanced
ex. a book sliding across the
table at a constant velocity
Acceleration Cont.
 < gravity: net forces >0
- object is moving & experiencing friction
- forces are unbalanced
ex. a feather falling to the ground
*important to remember NET FORCES
ex. air resistance is neglected: net force is the
objects weight
ex. presences of air resistance: net force is less
than the weight
objects weight – air resistance
Acceleration Cont.
- Air resistance of an object depends on 2 factors
1. The frontal area of an object
- greater the area greater the air
resistance
2. The speed of the object
- greater the speed greater the air
resistance
Acceleration Cont.
 Terminal Speed
- When acceleration of an object equals zero
- If concerned with direction; use terminal velocity
 Why?
- Velocity indicates a direction and speed
Acceleration Cont.
Ex. Skydiving
- as you fall you gain speed, air resistance
therefore builds until finally it equals your weight.
If this happens, the net force is equal to zero and
you no longer accelerate, reaching terminal speed
feather few ~ centimeters per second
sky diver ~ 200 kilometers per hour
Acceleration Cont.
 = gravity (free fall)
- object is falling at 9.8m/s2
- air resistance can be neglected
ex. Any object falling in a vacuum
Why does the object with double the mass not
accelerate greater?
Acceleration Problem
Q: A jumbo jet cruises at a constant velocity of
1000 km/hr when the thrusting force of it’s
engine is a constant 100,000 N.
What is the acceleration of the jet?
What is the force of air friction (air
resistance) on the jet?
A: The jet is not accelerating, it is at a constant
velocity and therefore the net force must be 0
meaning the thrust is canceled out by the air
resistance, therefore the air resistance must be
100,000N
Air Drag Problems
Q: Consider a man and a woman parachuting
together from the same altitude. Suppose the
man is twice as heavy as the woman and that their
same sized chutes initially open simultaneously.
Who gets to the ground first?
A: The man, he will fall a further distance before
reaching his terminal velocity
Air Drag Problems Cont.
Q: A skydiver jumps from a high flying helicopter. As
she falls faster and faster through the air, does her
acceleration increase, decrease, or remain the same?
A: Acceleration decreases because the net force acting
on her decreases. Net force is equal to her weight
minus her air drag, and being that air drag, increases
with speed, net force and hence acceleration
decrease, Newton’s 2nd Law
Objectives
 Explain Newton’s 3rd law of motion and relate it to
everyday events
 Explain how action and reaction forces are related
according to Newton’s 3rd law
 Be able to define a system of interactions
Newton’s 3rd Law
 Action Reaction Forces
- whenever on object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object.
ex. produces motion:
car tires push backwards against the
road the road pushes forward against the
car tires
ex. no motion:
hand pushes against the wall wall pushes
against your hand
Newton’s 3rd Law Cont.
- if we drop a sheet of tissue paper in front on the
heavy weight champion of the world and challenge
him to hit it in midair with 50 lbs of force, his best
punch couldn’t even come close
Why?
- a force is not a thing in itself but makes up an
interaction between itself and another (system)
- the action force and the reaction force
- net force only equals 0 when force are
equal and opposite
ex.
System of Interactions
 Define the system
- Jack and Jill: Nf =0, hence no
acceleration,
force is internal to the system
ex.
- Jack: Nf > 0, acceleration, force is external
to the system
ex.
- Jill: = Nf > 0, acceleration, force is external
to the system
ex.
Systems of Interactions Problems
Q: One cold and rainy day, you car battery is dead and
you must push the car to get it started. Why can’t
you push the car by remaining comfortably inside
pushing against the dash?
A: Pushing again the dash creates a internal force to
the system, in order to accelerate the car you must
have an external force to the system
Objectives
 Define momentum
 Understand how momentum and Newton’s laws
related
 Explain how impulses affect momentum
Momentum
 Definition
- a quantity defined as the product of an object’s
mass and its velocity
 Formula
-M=mxv p=mxv
 Units
- Kg x m/s
Momentum Problems
 Which object has more momentum: a car traveling at a
speed of 10 km/hr or a baseball pitched at 150 km/hr.
Explain your answer
 What is the momentum of an 0.30 kg blue jay flying at
17 m/s?
 What is the mass of the train car moving at 14 m/s with a
momentum of 140 kg-m/s? of the car moving at 10 m/s with
a momentum of 100kg-m/s? What is the total momentum of
the system?
Momentum Answers
 The car has more momentum because its mass is so
much greater than the baseball. It compensates for
the difference in velocity
 p = m x v 0.30 kg x 17 m/s = 5.1 kg-m/s
 a.) 10 kg b.) 10 kg c.) total = 240 kg/m/s
Momentum Cont.
 Why is a heavy truck harder to stop than a small
moving car at the same speed?
- truck has a greater mass, therefore greater
momentum
 Can you change an objects momentum?
- Yes, using forces, but most importantly “how
long” that force is applied
ex. force applied briefly to a stalled car,
small change in it’s momentum
ex. force applied over an extended time
interval, greater change in momentum
Impulses
 Definition
- product of force and time interval
 General
- the relationship between impulse and momentum
can be seen by rearranging Newton’s 2nd law
(a = F/m)
- time interval part of impulse is “buried” in the term
for acceleration (change in v/t interval)
- F x t interval = change in (mass x velocity)
-shorthand: Ft = mv
Impulses Cont.
- rearrangement of Newton’s 2 law explains, why
“follow through” is important in increasing the
momentum of things
Q: Would there be a difference in the momentum of a
long barrel cannon or a short barrel cannon, and if so
which would be greater?
A: Long Barrel: time interval is increased
Impulses Cont.
- decrease momentum over a long time, a smaller force
results
ex. A truck out of control is better off hitting a
haystack than a brick wall. When the truck hits
the haystack, the time of impact may be extended
100 times, force is reduced by a 100 times
*If you wish the force of impact to be smaller, extend to
time of impact
Boxer:
Momentum Problems
Q: Explain how a karate expert can sever a stack of
bricks with the blow of his bare hands.
A: He imparts a large impulse to the bricks in a short
time, hence producing considerable force.
***Remember small “t” large “F”