Name
Period
Test Corrections: Ch. 3 – A or B
Copy each question you missed. You do
not have to copy the graphs, but you are
welcome to do so if you like.
 If a problem required you to do work,
you must show it on paper or reference
your scratch paper.
 Write the answer choice AND the
answer and put a box around it.
Ex. A. The speed increases.

Journal #15
10/5/11
Using some of the terms from Ch. 4,
explain why it is important to be wearing
a seatbelt in an accident.
Chapter 4
Forces and Newton’s
3 Laws of Motion
Isaac Newton (1642-1727)
Isaac Newton (1642-1727)
Isaac Newton is without a doubt one of the most influential
men in history.
Just a few of his accomplishments:
•Built the first practical reflecting telescope
•Developed a theory of color including the idea that white
light is composed of all colors of the rainbow
•Studied the speed of sound
•Developed calculus from scratch!
•Defined the 3 Laws of Motion that govern all objects
•Studied the effects of gravity (story about the apple)
Force
A
force is a push or a pull.
Force is not a thing in itself, but
rather an interaction between
two objects.
Force is a vector quantity…
direction matters in the answer!
Common Forces
Common Forces
Newton’s First Law
“The
Law of Inertia”
A
body remains at rest or moves
in a straight line at a constant
speed unless acted upon by a net
force.

Objects do not accelerate unless a net force is
applied.
Speed up, Slow down,
or change direction
Newton’s First Law
is a property of an object
most closely related to it’s mass
(not to be confused with
momentum) that explains why
objects with greater mass resist a
change in motion more than those
with a lesser mass.
Inertia
Net Force
force is the vector sum of ALL
forces acting on an object.
 Net
 If
there is zero net force, then there is zero
acceleration (constant velocity), this is a
special case called equilibrium.
 If there is a net force, there will be an
acceleration. That means that the object will
be speeding up, slowing down, or changing
direction.
Free Body Diagrams
A
Free Body Diagram is a simple drawing that
shows the magnitude and direction of all of
the force vectors acting on an object.
 The
length of the arrows in relation to each
other is VERY important.
 Each
arrow must point away from the “free
body” and be labeled appropriately.
 The
system, the object the force is applied to,
is drawn as a shaded circle.
Free Body Diagrams
 Here
is an example of a
FBD of a book at rest on
a table top.
FN
The
book is
drawn
as a ball
 Fg
is acting downward
but is “balanced” by FN
acting upward.
 Results
is no net force
and zero acceleration.
Fg
Free Body Diagrams
 Here
is an example of a
FBD of a box being
pulled by a rope at a
constant speed on a flat
surface.
and FN are still
opposite and equal.
FN
Ff
FT
 Fg
 FT
and Ff are also
opposite and equal.
Fg
Object is in motion,
but not accelerating.
Free Body Diagrams
 Here
is an example of a
FBD of a ball under free
fall conditions.
 Fg
is the only force acting
on this object.
 The
net force is down
and the object is
accelerating.
Fg
Object is in motion
and accelerating.
Journal #16
10/6/11
Draw a free body diagram for the following situations:

A car accelerates from rest on a flat road (there is both friction from the
air and the ground).

The space shuttle just after launch is accelerating upward (include friction
from the air)
HW Questions: P. 89 #1-5
Draw a FBD for the following situations:
1. A flowerpot falls freely from a windowsill. (Ignore any
forces due to air resistance.)
2. A sky diver falls downward through the air at constant
velocity. (The air exerts an upward force on the person.)
3. A cable pulls a crate at a constant speed across a
horizontal surface. The surface provides a force that
resists the crate’s motion.
4. A rope lifts a bucket at a constant speed. (Ignore air
resistance.)
5. A rope lowers a bucket at a constant speed. (Ignore air
resistance.)
Answers to HW
#1
#2
Ff
Fg
Fg
Answers to HW
#3
#4
FN
Ff
FT
FT
Fg
Fg
Answers to HW
#5
FT
Fg
Newtons’ Second Law
 The
acceleration of an object is directly
proportional to the net force acting on
the object, and inversely proportional to
the mass of the object.
Fnet
a
m
Fnet
a
m
NEWTON'S 2nd LAW
F
a
m
F
M
a
m
F
a
m
F
M
a
m
m
F
m
1
a
m
a
F
M
a
  or 

aF
F  ma
Newton’s 2nd Law Practice
Two horses are pulling a 100-kg cart in the same
direction, applying a force of 50 N each.
What is the acceleration of the cart?
A. 2 m/s2
B. 1 m/s2
C. 0.5 m/s2
D. 0 m/s2

Answer B
Reason: If we consider positive direction to be the direction of pull
then, according to Newton’s second law,
Fnet
a=
,Since Fnet  50 N  50 N  100 N,
m
a=
100 N
= 1m/s2
100 kg
Newton’s Third Law
Action-Reaction
Law
Two forces that make up an
interaction pair of forces are equal
in magnitude, but opposite in
direction and act on different
objects.
Newton’s Third Law
For every action, there is
always an equal (magnitude)
and opposite (direction)
reaction.
By “action” or “reaction”, we
mean a force.
Action/reaction forces do not
act on the same object.
Reaction: gases push on rocket
Action: rocket pushes on gases
Reaction: road pushes on tire
Action: tire pushes on road
Identify at least six pairs of action-reaction
force pairs in the following diagram:
Location
Mass
Weight
Earth
18.4 kg
180 N
Moon
18.4 kg
30 N
Orbiting
Earth
18.4 kg
0N
1/6 of
Earth’s
Mass
 Mass
is the amount of matter in an
object (not to be confused with weight)
 Also
considered a measure of the
inertia of an object
 measured
in SI unit of kilograms
(kg)… if mass is given in grams you
must convert!
Weight
Weight
is the downward force
upon an object due to
acceleration caused by gravity

weight = mass (kg)  accel. due to gravity (m/s2)
Fg = mg
measured
in Newtons (N)
The
weight of a 10 kg brick is...
A) 98 N
B) 10 kg
C) 9.8 kg
D) 10 N
E) 98 kg
Mass and Weight
 On
the Moon, the force of gravity is only
1/6 as strong as on the Earth. (approx.
1.63m/s2)
 While
orbiting, you are practically
weightless but your mass remains
unchanged.
 Your
mass does not depend on where
your are.

e.g. Earth, Moon, or space
Falling with Air Resistance
Air
resistance (drag
force) increases with
speed and increased
cross-sectional area
and can be effected
by the size and shape
of an object.
Terminal Velocity
Net Force
Acceleration = g
Velocity = 0
but motion is about to begin
mg
F
Acceleration < g
v increasing downward
Acceleration << g
v still increasing downward
mg
F
just not as rapidly as before
mg
F
Acceleration = 0
mg
Terminal velocity
Terminal Velocity

Terminal velocity occurs when the drag force of air
resistance becomes large enough to balance the force of
gravity.

At this instant in time, there is no net force — the object
stops accelerating (see D below); terminal velocity has been
reached.
Journal #17
10/11/11
Two friends Mary and Maria are trying to pull a 10kg chair in opposite directions. If Maria applied a
force of 60 N and Mary applied a force of 40 N, in
which direction will the chair move and with what
acceleration?
A.
The chair will move towards Mary with an acceleration of 2 m/s2.
B.
The chair will move towards Mary with an acceleration of 10 m/s2.
C.
The chair will move towards Maria with an acceleration of 2 m/s2.
D.
The chair will move towards Maria with an acceleration of 10
m/s2.
Answer: C
Reason: Since the force is applied in opposite direction, if we consider
Maria’s direction of pull to be positive direction then, net force = 60 N
– 40 N = 20 N . Thus, the chair will move towards Maria with an
acceleration.
Fnet 20 N
2

 2 m/s
m 10 kg
Question 1
If a stone is hung from a mass-less rope, at which place on the rope
will there be more tension?
A. The top of the rope, near the hook.
B. The bottom of the rope, near the stone.
C. The middle of the rope.
D. The tension will be same throughout the rope.
Answer 1
Answer: D
Reason: Because the rope is assumed to be without mass,
the tension everywhere in the rope is equal to the
stone’s weight .
Question 2
In a tug-of-war event, both teams A and
B exert an equal tension of 200 N on
the rope. What is the tension in the
rope? In which direction will the rope
move? Explain with the help of
Newton’s third law.
Answer 2
Team A exerts a tension of 200 N on the rope.
Thus, FA on rope = 200 N. Similarly, FB on rope =
200 N. But the two tensions are an interaction
pair, so they are equal and opposite. Thus, the
tension in the rope equals the force with which
each team pulls (i.e. 200 N). According to
Newton’s third law, FA on rope = FB on rope. The
net force is zero, so the rope will stay at rest as
long as the net force is zero.
Journal#
 You
place a watermelon on a spring
scale at the supermarket. If the mass
of the watermelon is 4.0kg, what is
the reading on the scale in Newtons?
Journal#

A train engine is accelerating while pulling two box
cars of equal mass of 1.0x104-kg. If the acceleration
of the cars is 2.0m/s2, with what force must the engine
be pulling. (ignore friction from the rails and the air)
Journal#

A train engine is accelerating while pulling two box
cars of equal mass of 1.0x104-kg. There is an
opposing frictional force of 2.0x104N acting against
the motion. What will be the acceleration of the cars
if the engine uses a force of 3.0x104N?
Journal#

Two train engines are accelerating while pulling two
box cars of equal mass of 1.0x104-kg. If EACH engine
uses a force of 3.0x104N and the acceleration of the
cars is only 2.0m/s2, what is the opposing frictional
force acting against the motion?