Chapter 4 – Forces and
Newton’s Laws of Motion
Newton’s First Law
(aka Law of Inertia)
Objects maintain their state of
motion unless acted on by a net
external force.
Inertia – resistance to change
in motion.
Mass – measure of an object’s
inertia (in kg)
Newton’s Second Law
 F  ma
net force
(Newtons)
mass
acceleration
(kg)
(m/s2)
The net force is the total (vector) force on the
object
Newton’s Second Law
in words…
The net force on object equals the mass
of the object x it’s acceleration.
The acceleration of an object is
inversely proportional to its mass.
Given the same net force, a small mass will
accelerate more than a larger mass.
Units for Mass, Acceleration, and Force
System
Mass
Accel.
Force
SI
kg
m/s2
newton (N)
CGS
g
cm/s2
dyne (dyn)
BE
slug
foot/s2
pound (lb)
Weight – force of gravity on a mass (Newtons)
F = ma
W = mg
A 1 kg object has a weight of
W = mg
W = (1 kg)(9.8 m/s2)
W= 9.8 N
Newton’s Third Law
For every action there is an equal and
opposite reaction.
Forces always occur in pairs.
Newton’s Third Law
Fhammer on nail = -Fnail on hammer
Newton’s Third Law
If one object exerts a force on another, then the
second object also exerts a force on the first; these
forces are equal in magnitude and opposite in
direction. FA on B = -FB on A
Remember that the forces act on different objects
and DO NOT cancel out!
When determining the motion of an object, only
consider the forces acting ON the object.
An unfortunate box elder bug splatters against the windshield of a
moving car. Compared to the force of the car on the bug, the
force of the bug on the car is
a.
b.
c.
d.
larger
smaller
the same
need more information to say
c. the same
Compared to the deceleration of the car, the deceleration of the
windshield-hitting bug is
a.
b.
c.
d.
larger
smaller
the same
need more information to say
a. larger
A box weighing 600 lbs is pushed along a horizontal floor at
constant velocity with a force of 250 lbs parallel to the floor.
What is the net force on the box?
The net force is zero since the velocity is constant (no accel.)
A car weighs 2193 lb.
(a) What is the weight of the car in N?
(b) What is the mass of the car in kg ?
(a) 9754 N
(b) 995 kg
(1 lb = 4.45 N)
(W = mg, g = 9.8 m/s2)
ASSIGN: Chapter 4 #1-13 (odds), p. 121
Due Thursday
Chapter 4 Conceptual Questions #1-18
Page 118
An object must be moving for it to have inertia.
FALSE
In a tug of war, the person who pulls with the most force wins.
If the net force on an object is zero, it can’t be moving.
FALSE
FALSE
In order for you to jump off the ground, you need to exert a force on
the floor that is greater than the force the floor exerts on you.
FALSE
Constant velocity requires that the acceleration be zero.
TRUE
As a ball falls freely, the distance it falls each second is the same. FALSE
If a horse pulls on a wagon at rest, the wagon pulls back
equally as much on the horse.
TRUE
When given the same force, a heavy mass will have a larger
FALSE
acceleration than a lighter mass.
Neglecting air resistance, all objects fall at the same rate. TRUE
If a horse pulls on a wagon at rest, the wagon pulls back equally as
much on the horse. Will the wagon be set into motion?
a. No, because the forces cancel each other out.
b. Yes, because there is a net force acting on the wagon.
c. Yes, because there is time delay between action and reaction.
d. Yes, only if the horse’s pull on the wagon is larger than the
wagon’s pull on the horse.
b. Yes, because there is a net force acting on the wagon.
An object of mass 5 kg is acted upon by exactly four forces, each of
magnitude 10 N. Which of the following could NOT be the
resulting acceleration of the object?
a. 0 m/s2
b. 2 m/s2
c. 4 m/s2
d. 8 m/s2
e. 10 m/s2
e. The maximum net force would be 40 N; 40N/5kg = 8 m/s2
Clearly state Newton’s Three Laws of Motion
in your own words.
First Law – Objects maintain their state of motion until
acted upon by a net external force.
Second Law – Given the same force, a more massive
object will have a smaller acceleration than a less
massive object.
F = ma
Third Law – Every force has an equal and opposite force.
Section 4.7 – The Gravitational Force
Newton’s Law of Universal Gravitation
All masses attract each other with a force
determined by
force of
attraction
(in N)
G  6.67259 x 10
masses (in kg)
distance b/w mass
centers (m)
-11 N  m 2
kg
2
Universal Gravitational Constant (G)
G  6.67259 x 10
-11 N  m 2
kg
2
The force of gravity between masses depends on
The value of each mass
The distance between their center of masses
Inverse Square Law – The force of gravity
becomes 4x less when r doubles.
True or False –
1. You exert the same amount of force on
the earth as it exerts on you. TRUE!
2. If you threw something fast enough it
would never land.
TRUE!
3. The sun exerts more of a force on you
than the moon.
TRUE!
How many more times?
Weightlessness – feeling experienced
during free-fall due to lack of support
force.
Apparent Weight – Weight read by a scale;
reaction force applied to the person.
True Weight – Actual weight due to
gravitational force.
Zero g or micro g – Zero acceleration of
gravity experienced by astronaut; Not
zero gravity, gravity holds them in orbit!
True or False –
1. There is barely any gravity in an orbiting
space shuttle.
FALSE!
2. The earth exerts the same gravitational
force on a falling penny as it does on a ton
FALSE!
of bricks.
3. Neglecting air resistance, a falling ton of
bricks will fall faster than a falling penny.
FALSE!
Weight of objects on the surface of the earth
W  mg
g = 9.80 m/s2
ASSIGNMENT:
Ch. 4 #18, 20, 24, 30
Due Friday
Section 4.8 – The Normal Force
The normal force (FN) is a support force
that a surface exerts on an object.
FN = W since net force on box is zero.
Section 4.8 – The Normal Force
FN > W since box is being pressed down.
Section 4.8 – The Normal Force
FN < W since box is being lifted up.
Section 4.8 – The Normal Force
FN is always perpendicular to the surface.
Section 4.8 – The Normal Force
Apparent Weight – weight read by the scale.
True Weight – weight due to gravity.
Section 4.8 – The Normal Force
 F  ma
 F y  FN  m g
FN  m g  m a
FN  m g  m a
Apparent
Weight
True
Weight
Section 4.9 – Friction
Friction – force that opposes motion.
Depends on
1. Types of surfaces in contact
2. Amount of force pressing surfaces
together (the normal force)
Does NOT depend on contact surface
area (according to the books)
Static Friction (fs) – friction holding an
object in place.
Kinetic Friction (fk) – friction while
object is moving.
fs
M AX
  s FN
 s  coefficient of
static friction
f k   k FN
 k  co efficien t o f
k in etic frictio n
ASSIGNMENT:
Read 4.8 – 4.9
Chapter 4 #34 – 39 (p. 123)
Help with solving problems…
1. Draw a picture of the object; include all
forces drawn with the correct direction.
2. Write an expression for the net force
(ΣF). Have a separate expression for the
horizontal (x) and vertical (y) direction.
3. Set the net force expression equal to ma.
4. Try substituting to eliminate the
number of variables. Keep trying!!
Tension (T) – the reaction force of a pulling on an object.
Pull w/ 100 N, box pulls back / 100 N.
Tension in rope is 100 N, not 200 N
Spring
scale
reads
100 N
What does
spring
scale
read
now?
100 N
Applying Newton’s 2nd Law
Equilibrium (a = 0)
ΣFx = 0
ΣFy = 0
Non-Equilibrium (a ≠ 0)
ΣFx = max
ΣFy = may
Free-Body Diagrams – Show relative
magnitudes and directions of all forces
acting on an object.
Free-Body Diagrams help to visualize
all the forces acting to determine the
net force.
Don’t forget that ΣF = ma
Free-Body Diagram Examples…
A book is at rest on a table top.
A girl is suspended motionless from the ceiling
by two ropes.
Free-Body Diagram Examples…
A rightward force is applied to a book in order to move it
across a desk with a rightward acceleration. Consider
frictional forces. Neglect air resistance.
A rightward force is applied to a book in order to move it
across a desk at constant velocity. Consider frictional
forces. Neglect air resistance.
Free-Body Diagram Examples…
A car is coasting to the right and slowing down.
An leave is free-falling from a tree. Neglect air
resistance.
Free-Body Diagram Examples…
A block on an inclined plane.
ASSIGNMENT:
Ch. 4 #46,47,50,51,53,58,63,67
due Tuesday