Forces and Motion
Forces
What is a Force?
-A push or pull on an
object
-UNITS: NEWTONS (N)
What is a Newton?

1 Newton will cause a 1 kg object to
experience 1 m/s2 of acceleration.
VELOCITY!
Forces can cause changes in_________



ACCELERATION!
Therefore, it causes __________________
Is it a vector?
Because force deals with VELOCITY- it has
magnitude and direction

Therefore force is a vector
Force cont.
A force can cause…
 a resting object to move, or …
 A moving object to accelerate
 How?

By changing the object’s speed or
direction.
Types of Forces




Applied – contact force in the direction the
object is moving (Fa)
Tension- caused by a rope, cable, ect.,
directed away from the object (Ft)
Normal- caused by a surface (Fn)
Friction- opposes the motion of objects, must
have contact (Ff)
Force of gravity- force at a distance, caused
by attraction between two objects (Fg)
Applied Force Examples
Tension Force Examples
Normal Force Examples
Friction is a force:
Friction is a force that opposes motion between two
surfaces that are in contact
There are two main types of friction:
Static- between stationary objects (“static”=
“stationary”)
Ex- the force that is keeping this block from
sliding downhill
Kinetic- friction of movement
NOTICE: this friction depends on what two
objects are in contact! (rough vs slippery
surfaces)
Complete the concept map for the 4 types of friction! Use your
textbook (pages 332-334)
FRICTION
STATIC
SLIDING
is defined as
ROLLING
is defined as
FLUID
is defined as
is defined as
The friction between
surfaces that are
stationary
force that exists
when objects slide
past each other
force that exists
when a round object
rolls over a flat
surface (usually less
than sliding friction)
& an example is
& an example is
& an example is
a book sitting on a
table
-hockey puck on ice
-child going down a
slide
-a sled down hill
force that exists
when an object
moves through a fluid
(air, water)
& an example is
-a roller blade on a
sidewalk
-bowling ball on
bowling alley
-a car driving down
the road
-swimmer swimming
through pool
How can we decrease friction?
Watch this demo and see…
http://www.scottso.net/examples_encfri.ht
m
Can you think of a situation in which you would want to increase friction?
More on the force of Gravity:

Gravity is an attractive force  pulls objects
together
(objects that have mass are attracted to one
another)

WEIGHT: equal to Force of GRAVITY (Fg)


Not the same as MASS!!!
DO NOT CONFUSE THE TWO
“GRAVITIES!”

FORCE of Gravity= FORCE in NEWTONS


(an object’s mass x the acceleration due to gravity)
ACCELERATION due to Gravity= 9.8 m/s2
WEIGHT is a FORCE that can be
calculated…

More on that later in the unit…
Concept REWIND
So
why don’t we get
sucked into the center of
the earth?
NORMAL FORCE

Can you think of other forces that can oppose
gravity?
Tension Force and Friction Forces
Air resistance can
also be called “drag”
Forces can be COMBINED:
Net Force: Fnet : the combination of all of the forces
acting on the object
 Use Free Body Diagrams to show the net force:

Remember the Units?

NEWTONS
Free-body diagrams
Free-body diagrams
are used to show
the relative
magnitude and
direction of all
forces acting on
an object.
This diagram
shows four
forces acting
upon an
object. There
aren’t always
four forces,
For example,
there could be
one, two, or
three forces.
Problem 1
A book is at rest on a table top. Diagram the
forces acting on the book.
Problem 1
In this diagram,
there are normal
and gravitational
forces on the
book.
Problem 2
A baseball is free-falling from a spot where it
was stuck in a tree. Neglect air resistance.
Draw a free-body diagram showing the forces
involved.
Gravity is the
only force
acting on the
ball as it falls.
Problem 3
Add to the drawing of the baseball falling and
consider air resistance. A free body diagram
for this situation looks like…
Gravity pulls down
on the ball while
air resistance
opposes the
ball’s
acceleration.
Balanced Forces:

When the forces on an object are balanced, the
net force is zero (Fnet = 0) and there is no change
in the object’s motion.



Does this mean the object has NO VELOCITY?
NO!!!
 Can be at rest OR moving at a constant velocity
Ex’s? Write one down!
(Any object at rest or constant Velocity)
Unbalanced Forces
 Results when net force acting on an object is
NOT equal to zero ( Fnet is not =0 )


When an unbalanced force acts on an object,
the object accelerates in the direction of the
net force (can cause it to speed up OR slow
down)
Fig 14, p.332
http://www.darvill.clara.net/enforcemot/friction.h
tm
Decide if the following objects have balanced or
unbalanced net forces.
An airplane is flying at 150 km per hour for
30 minutes.
1.
1.
Balanced= no acceleration!
A book sitting on a table.
2.
1.
Balanced= no acceleration!
A man sprinting to the finish line,
accelerating at 2 m/s each second.
3.
1.
Unbalanced= acceleration!
Calculating Net Force and
predicting motion…
Remember: When forces are not balanced
___________________
ACCELERATION!!!
occurs!
(don’t confuse with velocity!!! It’s a CHANGE in
velocity!!!)
Example 1
Notice the upward
force of 1200
Neutons (N) is
more than gravity
(800 N). The net
force is 400 N up.
Example 2
Notice that while the normal force and gravitation
forces are balanced (each are 50 N) the force of
friction results in unbalanced force on the
horizontal axis. The net force is 20 N left.
Problem 1
A rightward force is applied to a book in order
to start it moving across a desk. Consider
frictional forces. Neglect air resistance.
Construct a free-body diagram. Then
describe the net force.
Note the applied force arrow pointing to the
right. Notice how friction force points in the
opposite direction. Finally, there is still gravity
and normal forces involved. Net force is NOT
zero because the movement is CHANGING!!
Net force is to the right!
Problem 2
A skydiver is descending with a constant
velocity. Consider air resistance. Draw a freebody diagram. Then describe the net force.
Gravity pulls down on
the skydiver, while air
resistance pushes up
as she falls.
Net force is ZERO
because there is NO
ACCELERATION!
(Velocity is constant)
Problem 3
A man drags a sled across loosely packed
snow with a rightward acceleration. Draw a
free-body diagram. Then describe the net
force.
The rightward force arrow points to the right. Friction slows
his progress and pulls in the opposite direction. Normal
forces still apply as does gravitational force since we are
on planet Earth.
Net force is NOT ZERO because it is ACCELERATING! (V
is not constant)
Another way to look at balanced and
unbalanced forces
Balanced or unbalanced?
Balanced or Unbalanced?
EVALUATION
Tell your partner the answers to these
questions:



What is force?
What is net force?
How is a free-body diagram used to
understand forces acting on an object?
Evaluation
Complete questions on the Free-body Diagram
Worksheet.
NEWTON’S LAWS
Historical Development
Aristotle (384 BC- 322
BC): Incorrectly
proposed that force is
required to keep an
object moving
Newton’s Laws of Motion
Newton (1643 – 1727):
Defined mass and
force; Introduced 3
Laws of Motion
Newton’s 1st Law of Motion
The motion of an object does not change if its
net force is zero.
 Basically -an object at rest will stay at rest
unless a force acts on it and an object in
motion will not change its motion unless a
force acts on it.
 Example: Soccer ball will remain (at rest) on
the grass unless a force acts on it
What about an object in motion?


Why does a baseball stop after you’ve thrown it?
Forces are acting on it:


Friction force (air resistance)
Force of gravity
Sometimes called the “Law of Inertia”
a) Inertia: the tendency of an object to resist
being moved or, if the object is moving, to
resist a change in speed or direction .
1) Car crash: You continue
forward because of inertia
“Science and the
Consumer”
p.348
How is inertia related to mass?
P 347

Mass is a measure of inertia.


Who would you rather be tackled by…a toddler or
a defensive lineman?
What is easier to move? An empty garbage can
or a garbage can full of lead? Why?

The empty garbage can has less mass= less inertia=
less resistance to being accelerated.
Newton’s 2nd Law of Motion
“the acceleration of an object is equal to the net
force acting on it divided by the object’s mass.”
Acceleration = Net Force
Mass
Units:
F= N
M= kg
A= m/s2 = N/kg
a= F
m
F=ma
Weight is a result of F=ma
Mass: the amount of matter in an object
Weight: measure of the gravitational force
Weight = Mass x Acceleration due to gravity
W = mg


g = 9.8 m/s2
Units for weight = N
Basically Newton’s 2nd Law
W is Force
W=mg
a=F/m
g is Acceleration (due to gravity)
Rewind


Do you remember Inertia? How is this
related to mass?
How is that different than weight?
Mass is a measure of the inertia of an object;
weight is a measure of the force of gravity
acting on an object.
Your weight on You would weigh You would weigh
Earth is affected
less on the
almost nothing
by Earth’s
Moon b/c gravity
in interstellar
gravity
on Moon is less
space
PRACTICE





After lab…
See Math Skills p.350
Read p 350, do practice problems #1-3 on
351
If done with lab and above…
Complete handout (scenarios A-D)
Net force= 0

Net force= 5N to the left

Net force = 0

Net force = 15 N upward
Newton’s Third Law of Motion
Newton’s Third Law
“when one object exerts a force on a 2nd
object, the 2nd object exerts an equal and
opposite force on the first object.”
Two forces are called action and reaction
forces
(Also called “interacting forces”)
Action and Reaction Forces:
Example: Pushing your hand against a wall
Equal &
opposite
1) Hand pressing on wall = ACTION
2) Wall exerting a force on hand =
REACTION
Action-Reaction forces do not cancel each
other out
Example: Swimming  action-reaction forces
do not act on the same object.
 Action= pushing backward on the water,
 Reaction = water pushing forward on the
swimmer
 causes the swimmer to move because that
force is overcoming the friction on the
swimmer.
Action-reaction forces can produce motion, but
not all do
it depends on what other forces are acting
on the objects.
Pushing a wall = no motion
(because the wall has other forces acting on
it…gravity, etc)
Swimming = motion
(because force of water is greater than
friction on the swimmer)
Rewind

State in simple terms Newton’s Three Laws.
(in a way you can remember which is which)



1st Law of Inertia (An object at rest will stay at rest…)
2nd- F=MA
3rd Action-Reaction (explains conservation of momentum)
Extension- do
not teach in
2014
Newton’s Laws explain Projectile Motion:

Projectile motion= the curved path that an object
follows when thrown, launched, or otherwise
projected near the surface of the Earth [Fig 13,
p.358]
What FORCES are affecting the
acceleration? (A=F/M)



Force of the “throw”,
Air resistance and
gravity
Water Balloon Toss http://library.thinkquest.org/2779/
How does this affect the acceleration?

The combination of an initial forward velocity and the
downward force of gravity causes the ball to follow a
curved path.

Animation:
http://www.physicsclassroom.com/mmedia/vectors/hlp.cfm
Another Law

Law of conservation of Momentum
Momentum: the product of the mass &
velocity of an object
An object has a large momentum if the
product of its mass and velocity is large.
Momentum=mass x velocity
SI Units: kg x m/s
p=mv
Law of Conservation of Momentum:

momentum cannot be created or destroyed,
so the total momentum is conserved [Fig 20,
p.364]

In a closed system, the loss of momentum of
one object equals the gain in momentum of
another object—momentum is conserved.
Usually that’s not the whole story

There’s one more concept to explain the
acceleration of an object
Impulse
Impulse is related to momentum and force
Definition: Change in Momentum over time
Impulse=Force x Time
SI Units: N(s)
I=Ft
The fact that impulse depends on both force
and time means that there is more than one
way to apply a large impulse to an object