Chapter 2
Forces and Motion
Dr. Gluck
Forces and Motion
Laws of Motion
 2.1
Newton's First Law
 2.2
Acceleration and Newton's Second Law
 2.3
Gravity and Free Fall
 2.4
Graphs of Motion
Forces
PS3 (9-11) – 9b, 8a

How do Newton’s laws predict the motion of an object when
forces act on it?

What is the difference between forces applied to an object and
forces applied by an object?

According to Newton’s third law, why is motion possible?

How does the motion of a mass change due to forces acting on
it?

How can we explain the change in motion of a mass as caused by
different forces?
2A Investigation: Law of Inertia
Key Question:
Why are heavier objects harder
to start or stop moving?
*Students read text section 2.1
AFTER Investigation 2A
2A Investigation Topics

Launch cars of different mass.

Measure time intervals.

Calculate speed based on measured time intervals.

Graph speed vs. mass.

Research and define terms: inertia, weight and mass.
Chapter 2.1 Objectives and Vocabulary

Recognize that force is needed
to change an object's motion.

Explain Newton's first law.

Describe how inertia and mass
are related.





force
Newton's first law
inertia
newton
net force
What is the difference between mass and
weight?
– measure of
the amount of matter
in an object.
 Mass
 Mass
– is constant
– measure of
gravitational force
between two objects.
 Weight
– changes
depending on the
amount of gravity
 Weight
Sir Isaac Newton (1642-1727)
 English
Physicist
 Invented
 His
calculus
three laws
explain the
relationship of
forces, an objects
mass, and it's
motion.
How do we change the motion of an object?
Force
Push or pull
Any action that has the ability to
change motion.
Club hits ball. What are the possibilities?
1. Ball stays in motion at
constant speed.
2. Another force acts
on the ball.
Force is
opposite of
the motion
causing ball
to stop.
Force
applied at
an angle
and it
changes
direction.
Bottom Line: There has to be a force applied to the ball OR the
ball moves unchanged forever.
What is going on here?
Describe what is going to happen.
An object in motion stays in motion…..
How could this have been prevented??
Inertia
Property of an
object to RESIST
a change in
motion.
But what causes inertia?
MASS
If mass determines how much inertia an object
has, which of these has more inertia?
The heavier object has more
inertia.
1 pound = 4.448 N
a Newton
(N) LARGER
A Is
Newton
is SMALLER
thanor
an pound.
SMALLER than a pound?
NET FORCE is the total of ALL forces acting on an object.
Can we just add all the
forces acting on the object
mathematically in
Newton’s?
NO
Direction
matters
NET FORCE IS A VECTOR QUANTITY.
Needs magnitude and direction.
Forces in the SAME direction
Add them up:
500
200 = 700 ----
Forces in OPPOSITE directions
Add them up using direction:
500N -
200N 
Total : 300N 
2B Investigation: Newton's Second Law
Key Questions:
What is force?
What is the relationship between
force and motion?
*Students read text section 2.2
AFTER Investigation 2B
2B Investigation Topics

Measure time intervals of car moving along track.
 Calculate and compare speeds of car at different
points on track.
 Evaluate forces acting on car.
 Calculate acceleration of car.
 Use Newton's second law to calculate the force.

Graph force vs. time for the car on the track.
 Explain how speed and acceleration are different
using graphs.
Chapter 2.2 Objectives and Vocabulary
Define and calculate
acceleration.
 Explain the relationship
between force, mass, and
acceleration.
 Determine mass, acceleration
and force given two of the
quantities.

acceleration
 deceleration
 Newton's
second law

Newton’s First Law says that a
force is needed to change an
object’s motion.
What kind
of change
happens?
What is acceleration?
 Rate
at which speed changes.
f
i
What does the acceleration of an
object depend on?
FORCE
MASS
This is Newton’s Second Law
Force causes acceleration,
and mass resists acceleration.
Some things to consider when applying
Newton’s second law:
1. The NET force is what causes the acceleration.
2. If there is no acceleration, the net force must be zero.
3. If there is acceleration, there must be a net force.
4. The force unit of newtons is based on kg, m, and s.
Chapter 2.3 Objectives and Vocabulary

Describe the motion of an
object in free fall.

Calculate the speed and
distance for an object in free
fall.

Distinguish between mass and
weight.

Explain how air resistance
affects the motion of objects.
free fall
 acceleration
due to gravity
 velocity
 weight
 air resistance
 terminal speed

Free Fall
 Objects
in free fall
accelerate downward due
to gravity and no other
forces.
 Abbreviated
 Has
by “g”
a value of 9.8 m/s/s
If you know the acceleration of an
object in free fall, you can predict its
speed.
m
s
=
m
s s
s
So, is the speed (velocity) of a
falling object constant or is there
an acceleration?
A skydiver falls for 6 seconds before opening her
parachute. Calculate her actual speed at the 6-sec
mark and the distance she has fallen in this time.
Given: initial speed = 0 m/s
Time = 6 sec
Formulas: vf = gt
vf = gt
Vf = 9.8 m/s/s (6 sec)
Vf = 58.8 m/s
Looking for: final speed and
distance
vavg = v1 + v2
2
vavg =
d=vavgt
v1 + v2
2
d=vavgt
Vavg = (0+58.8m/s/s) = 29.4 m/s
2
d=(29.4 m/s)(6 sec) = 176.4 m
Calculate the final speed and distance for a skydiver who
waits only 4 seconds to open his parachute.
Given: initial speed = 0 m/s
Time = 4 sec
Formulas: vf = gt
t = 4 sec
Vf = (9.8 m/s/s)(4 sec)
Vf = 39.2 m/s
Looking for: final speed and
distance
vavg = v1 + v2
2
d=vavgt
Vavg = 0 + 39.2 m/s = 19.6 m/s
d = (19.6 m/s)(4 sec) = 78.4 m
An apple falls from the top branch of a tree and
lands 1 second later. How tall is the tree?
Given: initial speed = 0 m/s
Time = 1 sec
Formulas: vf = gt
t = 1 sec
Vf = (9.8 m/s/s) (1 sec) = 9.8 m/s
Looking for: distance
vavg = v1 + v2
2
d=vavgt
Vavg = 0 + 9.8m/s = 4.9 m/s
2
d = (4.9 m/s)(1 sec) = 4.9 m
Is there a difference between the mass of
these astronauts?
Is there a difference between the weight of
these astronauts?
Weight depends on
gravity.
The more gravity, the
more weight.
Chapter 2.4 Objectives and Vocabulary

Describe motion with position
versus time and speed versus
time graphs.

Use a position versus time
graph to calculate speed from
the slope.

Use a speed versus time graph
to calculate acceleration and
distance traveled.

slope