Unit 2, Chapter 6
CPO Science
Foundations of Physics
Chapter 9
Unit 2: Motion and Force in
One Dimension
6.1 Mass, Weight and Gravity
6.2 Friction
6.3 Equilibrium of Forces and Hooke’s
Law
Chapter 6: Forces and Equilibrium
Chapter 6 Objectives
1.
Calculate the weight of an object using the strength
of gravity (
g
) and mass.
2.
Describe the difference between mass and weight.
3.
Describe at least three processes that cause friction.
4.
Calculate the force of friction on an object when
given the coefficient of friction and normal force.
5.
Calculate the acceleration of an object including the
effect of friction.
6.
Draw a free

body diagram and solve one

dimensional
equilibrium force problems.
7.
Calculate the force or deformation of a spring when
given the spring constant and either of the other two
variables.
Chapter 6 Vocabulary Terms
mass
weight
weightless
g

force
friction
static friction
sliding friction
rolling friction
viscous friction
air friction
normal force
extension
net force
free

body
diagram
lubricant
equilibrium
ball bearing
dimension
spring
Hooke’s law
compression
spring constant
deformation
restoring force
coefficient of
friction
engineering
design cycle
subscript
prototype
coefficient of
static friction
6.1 Mass, Weight, and Gravity
Mass
is a measure of
matter.
Mass is constant.
Weight
is a force.
Weight is
not
constant.
6.1 Mass, Weight, and Gravity
The
weight
of an object
depends on the
strength of gravity
wherever the object is.
The
mass
always stays
the same.
6.1 Weight
F
w
= mg
Gravity (9.8 m/sec
2
)
Mass (kg)
Weight force (N)
6.1 Free fall and weightlessness
An elevator is accelerating downward at 9.8 m/sec
2
.
The scale feels
no force
because it is falling away
from your feet at the same rate you are falling.
As a result, you are
weightless
.
6.1 Calculate weight
How much would a
person who weighs 490 N
(110 lbs) on Earth weigh
on Jupiter?
The value of
g
at the top
of Jupiter’s atmosphere
is 23 N/kg.
(Since Jupiter may not
actually have a surface,
“on” means at the top of
the atmosphere.)
6.1 Calculate force
A 10

kilogram ball is supported
at the end of a rope. How much
force (tension) is in the rope?
6.1 Mass, Weight, and Gravity
Key Question:
What is speed and how is it measured?
*Students read Section 6.1 BEFORE Investigation 6.1
6.2 Friction
Friction
results from relative motion
between objects.
Frictional forces are forces that resist
or oppose motion.
6.2 Types of Friction
Static friction
Sliding friction
Rolling friction
6.2 Types of Friction
Air friction
Viscous friction
6.2 Friction
F
f
=
m
F
n
Normal force (N)
Coefficient of friction
Friction force (N)
6.2 Calculate force of friction
A 10 N force pushes down on a box that weighs 100 N.
As the box is pushed horizontally, the coefficient of
sliding friction is 0.25.
Determine the force of friction resisting the motion.
6.2 Sliding Friction
F
f
=
m
s
F
n
Normal force (N)
Coefficient of
sliding friction
Friction force (N)
Table of friction coefficients
6.2 Calculate using friction
A steel pot with a weight of 50 N sits on a steel
countertop.
How much force does it take to start the pot
sliding?
6.2 Calculate using friction
The engine applies a forward force
of 1,000 newtons to a 500

kg car.
Find the acceleration of the car if the
coefficient of rolling friction is 0.07.
6.2 Friction
Key Question:
How can we describe and model friction?
*Students read Section 6.2 AFTER Investigation 6.2
6.3 Equilibrium and Hooke's Law
When the net force
acting on an object is
zero, the forces on
the object are
balanced.
We call this
condition
equilibrium
.
6.3 Equilibrium and Hooke's Law
Newton’s second law simply requires that for an object to
be in equilibrium, the net force, or the
sum of the forces
,
has to be zero.
6.3 Equilibrium and Hooke's Law
Many problems have more than one force applied to an
object in more than one place.
6.3 Calculate net force
Four people are pulling on the same 200 kg box
with the forces shown.
Calculate the acceleration of the box.
6.3 Calculate force using equilibrium
Two chains are used to lift a
small boat. One of the chains
has a force of 600 newtons.
Find the force in the other
chain if the mass of the boat is
150 kilograms.
6.3 Equilibrium and Hooke's Law
The most common type of spring is a coil of metal or
plastic that creates a
force
when it is
extended
(stretched) or
compressed
(squeezed).
6.3 Equilibrium and Hooke's Law
The force from a spring
has two important
characteristics:
—
The force
always
acts in
a direction that tries to
return the spring to its
unstretched shape.
—
The strength of the force
is
proportional
to the
amount of extension or
compression in the
spring.
6.3 Hooke's Law
F =

k x
Spring constant N/m
Force (N)
Deformation (m)
6.3 Calculate force
A spring with
k =
250 N/m is extended by
one centimeter.
How much force does the spring exert?
6.3 Equilibrium and Hooke's Law
The
restoring force
from a wall is always
exactly equal and
opposite to the force
you apply, because it
is caused by the
deformation
resulting
from the force you
apply.
6.3 Calculate using equilibrium
The spring constant for a piece of solid wood is
1
×
10
8
N/m.
Use Hooke’s law to calculate the deformation when
a force of 500 N (112 lbs) is applied.
6.3 Equilibrium of Forces and Hooke's Law
Key Question:
How do you predict
the force on a
spring?
*Students read Section 6.3 AFTER Investigation 6.3
Application: The design of structures
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