CHAPTER 4
FORCES IN ONE DIMENSION
Honors Physics
Glencoe, 1999
4.1 Force & Motion

Force- any agent that can cause
an object to change its state of
motion
 Contact
forces- must touch to
interact (walking, throwing,
crashing)
 Field forces- no touching necessary
(gravity, magnetism, electrical)

Force (free-body) diagramshows vectors of all forces
involved in motion
Net Force (Fnet)

Sum of all forces
Newton’s Second Law

“The acceleration of an object
is equal to the sum of the forces
acting on the object, divided by
the mass of the object.”
ā
= SF/m (S is the “sum of” F)
Practice Problem 4.1.8


Three confused sleigh dogs are trying to pull a sled across the
Alaskan snow. Alutia pulls east with a force of 35N, Seward
also pulls east but with a force of 42N, and big Kodiac pulls
west with a force of 53N. What is the net force on the sled?
Known & Unknown
F1=35N E
 F3=53N W


Choose appropriate equation


F2=42N E
Fnet=?
Fnet=F1+F2+F3
Plug & Chug
Fnet=35N+42N+(-53N)
 Fnet=24N E

Newton’s First Law- Inertia


“Every object continues in its state of motion as long as no
net force acts on it.”
Inertia is measured in terms of a ratio, the amount of force
needed to cause one unit of acceleration
 Ratio
is called mass and is measured in kilogram (kg)
A
150 kg object requires 150 units of force to cause an acceleration
of 1 m/s2

The unit of force is called a Newton (N)
 Force
needed to accelerate a mass of 1 kg by 1 m/s2
Types of Forces
4.2 Using Newton’s Laws

Apparent weight- force on the object
as exerted by a gravitational field
 On
earth, g = Fg/m
 On the earth’s moon, g = 1.63m/s2
 1.63m/s2
= F/m
Your New (SI) Weight


Weight is measured in Newtons, not pounds
To determine your mass
 Your

To determine your weight in Newtons
 Your

weight (in pounds) ÷ 2.2
mass (in kg) x 9.80m/s2
Mr. Latham=185lbs
 (185lbs)/(2.2lbs/kg)=84.1kg
 (84.1kg)(9.80m/s2)=824.1N
Weightlessness

Apparent Weight of 0N
 Your
acceleration has the
same magnitude and
direction as the gravity field
 If you are not in a
measurable gravity field
 http://www.youtube.com/w
atch?v=2V9h42yspbo
Drag Force (Fd)

The force exerted by a fluid
on the object moving
through the fluid
 Dependant
on size and
shape of object, properties
of fluid, velocity of object

Terminal velocity- reached
when drag force equals the
force of gravity
Practice Problems 4.2.16


Kamaria is learning how to ice-skate. She wants her mother to
pull her along so that she has an acceleration of 0.80m/s2. If
Kamaria’s mass is 27.2kg, with what force does her mother
need to pull her?
Known & Unknown
m=27.2kg
 a= 0.80m/s2


Choose appropriate equation


F=?
F=ma
Plug & Chug
F=(27.2kg)(0.80m/s2)
 F=22kg m/s2 = 22N

Practice Problem 4.2.17


Taru & Reiko simultaneously grab a 0.75kg piece of rope and
begin tugging on it in opposite directions. If Taru pulls with a force
of 16.0N and the rope accelerates away from her at 1.25m/s2,
with what force is Reiko pulling?
Known & Unknown



FTaru=16.0N
FReiko=?
Fnet=?
Choose appropriate equation(s)



m=0.75kg
a=-1.25m/s2
Fnet=ma
FTaru+FReiko=ma
Fnet=FTaru+FReiko
Plug & Chug



16.0N+FReiko=(0.75kg)(-1.25m/s2)
FReiko=(0.75kg)(-1.25m/s2)-16.0N
FReiko=-17N
Practice Problem 4.2.19


On Earth, a scale shows that you weigh 585N. A) What is your
mass? B) What would the scale read on the Moon
(g=1.60m/s2)?
Known & Unknown
Fg=585N
 gE=9.80m/s2
 m=?


Choose appropriate equation(s) & rearrange


gM=1.60m/s2
FgM=?
Fg=mg
m=Fg/g
Plug & Chug
A) m=Fg/g
 B) Fg=mg

m=(585N)/(9.80m/s2)=59.7kg
Fg=(59.7kg)(1.60m/s2)=95.5N
4.3 Interaction Forces
Newton’s Third Law

“Whenever one object exerts a
force on a second object, the
second exerts an equal force in the
opposite direction on the first.”
 FA on B
= -FB on A
Physlets


E.4.5
E.4.7
Practice Problem 4.3.31

A suitcase sits on a stationary airport luggage cart. Draw
a free-body diagram for each object and specifically
indicate any interaction pairs between the two.
Tension (FT)

A force exerted on a string
 Equal
to the net weight of
the objects that it supports
Normal Force (FN)


The perpendicular contact force
exerted by a surface on another
object
If the surface is tilted, the FN is
tilted at the same angle
Practice Problem 4.3.32




You are helping to repair a roof by loading equipment into a
bucket that workers hoist to the rooftop. If the rope is
guaranteed not to break as long as the tension does not
exceed 450N and you fill the bucket until it has a mass of
42kg, what is the greatest acceleration that the workers can
give the bucket as they pull it to the roof?
Known & Unknown
FTmax=450N
amax=?
Fg=mg=(42kg)(9.80m/s2)
Formula
Fnet=ma
Plug & Chug
FTmax-Fg=ma
(450N-412N)=(42kg)(amax)
amax=0.91m/s2