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Force
What is a Force?
Force can be defined as a push or
a pull…
or anything else that has the ability
to change motion.
Units for Force
kilogram x m/sec2
kg x
2
m/s
Newton is the
scientific unit!!!
N=kg x m/s2
To Understand Force…
You must understand the difference
between
AND
2 Kg Mass
What is Mass?
The amount of “stuff” in an object
1 Kg
Mass
1.5 Kg
Mass
2 Kg
Mass
Mass is measured in kilograms.
How is Mass Different Than Weight?
 Weight
is a measurement of force!!
 Weight is not measured in kg, it is
measured in pounds (lbs) or
Newtons.
 Gravity has an effect on the weight,
but not the mass!!
For example:
Mass is the Same:1 kg
But the WEIGHT…
7
1 Kg
Mass
2.2
Pounds
On the moon 1.0 kg would weigh 0.3 lbs.
On the sun 1.0 kg would weigh 59.5 lbs.
On Mars 1.0 kg would weigh 0.8 lbs.
Newton’s 3 laws of motion
Newton’s First Law: Balanced Forces
An object will maintain a constant state of
motion (balanced).
This means an object at rest tends to stay
at rest
and
an object in motion tends to stay in motion.
Forces that are balanced can be:
Not in motion
In Motion
Stay at rest:
a book on a
table
Stay in
motion:
body not
restrained by
a seat belt in
car accident
Table pushes up
on book
IPC BOOK
Gravity
pulls down
on book
Newton’s First Law: Unbalanced Forces
An unbalanced force is a force that changes
the motion.
The book slides and
then stops because of
In space there is no
a resistant force called
resistance to cause
friction. Friction is
friction, so a bowling
force that opposes
ball would stay in
motion.
motion
...unless another
object got in
the way…
Of course “frictional”
force happens on earth...
but, if you weren’t wearing a safety belt
you would continue your motion…
Do not let this happen to you; buckle up!
Inertia and Mass
Inertia
 an object’s resistance to any change in
motion
 The amount of laziness
 Objects with greater mass have ______ inertia
 Greater

Newton’s Second Law:
Force = Mass X Acceleration
Force and acceleration are related.
An object will have greater
acceleration if a greater force is
applied to it
Which will accelerate faster—a car
with one person pushing it or the same
car with eight people pushing it?
Newton’s Second Law:
Force = Mass X Acceleration
 The
mass of an object also affects
acceleration.
 Smaller mass=faster acceleration
 Which will accelerate faster—a car
with two people pushing it or the
same two people pushing a fully
loaded gravel truck?
How do we calculate force?
Force involves mass and movement so
therefore we calculate force by
 Mass x acceleration

F
V
m
a
Consider the problem…
“How much force is needed to accelerate a
1400-kg car 2 m/s2? ”
Step 1
Read the problem.
Draw a picture.
1400-kg
car
How much force?
2 m/s2
F=
m=
a=
Formula
Plug-in
Units, units, units!
Answer
“How much force is needed to accelerate a 1400kg car 2 m/s2? ”
Step 2
Write down what you know,
What are you trying to find?
1400-kg
car
2 m/s2
F=
m =1400 kg
a = 2 m/s2
Formula
Plug-in
Units, units, units!
Answer
“How much force is needed to accelerate a
1400-kg car 2 m/s2? ”
Step 3
Set up the formula.
Solve.
F=mxa
F=
m =1400 kg
a = 2 m/s2
Formula
Plug-in
mxa
Units, units, units!
Answer
“How much force is needed to accelerate a
1400-kg car 2 m/s2? ”
Step 4 Plug-in the numbers.
Solve.
2800kg x m/s2
F=
m =1400 kg
a = 2 m/s2
Formula
mxa
Plug-in
1400 kg x 2 m/s2
Units, units, units!
Answer
2800 kg x m/s2
Quick Review
Newton’s first law: Objects at rest stay at
rest and objects in motion stay in motion
with the same velocity unless acted on by
a net force
 Newton’s second law: F=ma
 So….objects will speed up, change
direction or stop only if acted on by a net
force

Questions:
Why does my car slow down when I take
my foot off the gas?
 Why is hard for me to move a desk with a
student sitting in it?

Friction
Friction
 Friction
is a force that opposes
motion between two surfaces that
are touching
 It also changes the energy of
motion into heat energy---try
rubbing your hands together and
feel the heat
Types of Friction
Static Friction
 The friction between two surfaces that
are in contact but not moving
 Sliding Friction
 The friction that opposes the motion of
two surfaces that are in contact and
sliding past each other
 Rolling Friction
 The friction between an object rolling
and the surface it is rolling on

Some things that affect motion!


Air resistance
 The opposition to something moving through
the air
 Depends on shape, size, and speed
 A good portion of the gas that cars burn is to
overcome air resistance
Terminal Velocity
 When an object’s weight matches the air
resistance—it is the highest velocity that a
falling object will reach
Question
What
are the three variables
that Newton’s Second Law
of motion connects?
Newton’s Third Law:
Action / Reaction
For every action there is an equal and opposite reaction.
Look at the picture below:
What is the action?
As the gases push
downward out of the rocket,
What is the reaction?
The rocket is pushed upward
by gases.
Gravity is the force that ALL objects in the
UNIVERSE exert on each other...
Newton said that gravitational force
depends on 2 things:
The DISTANCE of the objects
The MASS of the objects
LESS gravitational
force compared to...
MORE gravitational
force here.
The blue object has more
gravitational force than the
green object because the
blue object has more
MASS.