Chapter 6: Forces and Equilibrium

CPO Science
Foundations of Physics
Chapter 9
Unit 2, Chapter 6
Unit 2: Motion and Force in
One Dimension
Chapter 6: Forces and Equilibrium
 6.1 Mass, Weight and Gravity
 6.2 Friction
 6.3 Equilibrium of Forces and Hooke’s
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
Chapter 6 Vocabulary Terms
 mass
 weight
 normal force
 extension
 compression
 spring constant
 weightless
 g-force
 friction
 net force
 free-body
 lubricant
 deformation
 restoring force
 coefficient of
 equilibrium
 ball bearing
 dimension
 engineering
 design cycle
 subscript
 spring
 Hooke’s law
 prototype
 coefficient of
static friction
static friction
sliding friction
rolling friction
viscous friction
 air friction
6.1 Mass, Weight, and Gravity
 Mass is a measure of
 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
Weight force (N)
Fw = mg
Gravity (9.8 m/sec2)
Mass (kg)
6.1 Free fall and weightlessness
 An elevator is accelerating downward at 9.8 m/sec2.
 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
Friction force (N)
Ff = m Fn
Normal force (N)
Coefficient of friction
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
Friction force (N)
Ff = msFn
Normal force (N)
Coefficient of
sliding friction
Table of friction coefficients
6.2 Calculate using friction
 A steel pot with a weight of 50 N sits on a steel
 How much force does it take to start the pot
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
 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
— 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
6.3 Hooke's Law
Force (N)
Deformation (m)
Spring constant N/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
6.3 Calculate using equilibrium
 The spring constant for a piece of solid wood is
1×108 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
*Students read Section 6.3 AFTER Investigation 6.3
Application: The design of structures

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