### Chapter 4 Forces in One Dimension

```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)

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 he 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 rest or uniform
velocity as long as no net force acts on it.”
Inertia is measured in terms of a ratio, the amount of force
need 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


Weight is measured in Newtons, not pounds
 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
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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=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
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
