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Name: _________________________
**04-02 Forces Reading
Directions: Read and annotate this text. In your notebook, record key details such as:
 The types of forces and their definitions
 The steps for constructing a force diagram
Your notes will be checked as an official at-bat for standard 8b – Read Critically
Standard
8b - Read
Critically
5
Advanced Proficient
4
Proficient
3
Emerging Proficient
2
Advanced Beginner
1
Beginner
Critically reads
professional scientific
literature to determine
central ideas and
supporting information;
selects and paraphrases
textual evidence in
original language that
skillfully and accurately
explains complex
information in simpler
terms
Critically reads
scientific literature
adapted for
classroom use to
determine central
ideas and
supporting
information; selects
and paraphrases
textual evidence in
own language
Critically reads
scientific literature
adapted for classroom
use to determine
central ideas and
supporting information;
selects and paraphrases
textual evidence in
basic terms that
capture the main ideas
but may lose some
supporting details
Reads scientific texts
to determine the
central idea and some
supporting
information; can
summarize the main
ideas but struggles to
use original language
when paraphrasing or
does not include
essential details
Struggles to
independently
determine the
central ideas
in scientific
texts, or
cannot
summarize
using own
words
Forces can be thought of as pushes and pulls. For example, you exert a force to push open a door. Gravity exerts a
force on you which pulls you to the surface of the earth. In every situation, forces are an interaction between
two objects - you can't touch without being touched. The door also pushes back on your hand, the earth is also
gravitationally attracted to you.
Types of Forces
There are many types of forces between objects that are differentiated by the way the two objects interact.
Here are some of the ones we will use in class:
When two surfaces touch each other, there are always two forces involved –
a normal (N) force and a friction (f) force. The normal force is a support
force that’s always perpendicular to, or straight up from, a surface (here
“normal” is a math term that means perpendicular). For example, if a car is
parked on a hill, the road exerts an upward normal force to support the car.
The friction force is always parallel to, or along, the surface and usually
opposes the motion of an object. For example, there must be a friction
force acting on the car along the surface of the road, otherwise it would
slide down the hill.
FN
Ff
Fg
Extended materials such as a string or chain, exert tension (T) forces on an object. If the material can
stretch and compress, like a spring or rubber band, it’s called an elastic or spring (s) force. When an
object moves through water or air, resistive forces are called drag (d).
It is also possible for two objects to interact without touching. For example, when a skydiver jumps out of
a plane, the Earth exerts a force of gravity (g) on the skydiver even though the Earth and the skydiver
aren’t directly touching. When this happens, the objects exert forces through a field. For example, there
is a gravitational field between the Earth and the skydiver that allows the Earth to pull the skydiver (the
same gravitational field is responsible for keeping you on the ground right now).
Representing Forces
To understand how to represent forces, we will analyze the example below.
Example: A tractor with a rope pulls a log at a constant speed
Step 1: Carefully select the object of interest that
will be the focus of our analysis. We will call that
object the system, and everything else in the
environment that might in any way affect the system
are the surroundings.
Step 2: Simplify the system by drawing it as a point.
Step 3: Draw and label a vector (arrow) for each
force acting on the system. When we label forces on
our diagram, we want to indicate the type of
interaction between the objects, what object the
force is acting on and what object the force is by. We
will use the following notation:
From the example above, we will choose the system to
be the log.
The tractor, rope, ground, and earth are part of our
surroundings that can affect the log.
Step 2
Step 3
Fkind, on victim, by agent
Step 4: Mark which forces (if any) are equal in
magnitude to other forces. To show this, we will use a
notation from geometry where we put hash mark
through vectors that are equal.
Step 4
Tip: If the object has a constant velocity, the forces
must be balanced and you should use hash marks to
show this. If the object is accelerating, at least one
force must be unbalanced and won’t have hash marks.
The log is pulled at a constant velocity, so our forces
must be balanced. The single hash mark through the up
and down forces show that they’re equal to each other.
The double hash marks through the left and right
forces show that they’re equal.