# Chapter 6: Forces and Equilibrium - CPO Science

Urban and Civil

Nov 29, 2013 (4 years and 5 months ago)

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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