# Unit IV Reading -Force Diagrams - PDF

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```					        UNIT III READING 1: INERTIA & EQUILIBRIUM
SYSTEM SCHEMA
It is important when studying dynamics problems in physics to be able to identify all
objects in a system and also to identify how those objects interact with each other and
with their environment. It is often difficult for students not only to identify which objects
interact but also to identify the type of interaction (F g, FT, Ff, FN) occurring between any
given pair of interacting objects. System schema are a first step in helping to rectify
these issues.

In this representational tool, objects are represented as circles. The circles are
identified with letters that correspond to the objects being represented. The circles are
also spatially arranged similarly to their actual arrangement in space. Consider the
example below.

A block is pulled horizontally by a string along the surface of a table. There are
essentially five objects of interest in this system: the
String
block, the string, the table, the ground, and the
Block
earth. To start the system schema, we will use five
circles labeled with B (for block), S (for string), T (for
table), G (for ground) and E (for earth). These                  Table
circles will be arranged
S     spatially so that the entire
B
earth, ground, table, and
block are in a vertical
arrangement with the block
T                     at the top, and the string will                Ground
be placed off to the right side
(as shown to the left). Notice that the ground and earth are
G                     enclosed in an oval. This represents that they are both the
same object (the ground is a local part of the earth), but when
identifying interactions, sometimes the ground is the key object,
and sometimes the entire earth is the key object (illustrated in
the complete diagram below).
E
Our system schema so far shows only the objects involved in
the interactions, but does not yet show which pairs of objects interact and the types of
interactions involved. To show interactions,
use a line to connect each object to every                                   tension
B                  S
other object with which it interacts. A solid
friction
line will represent a contact force and a dotted
line will represent a long-range force.                      normal
Alongside each line, identify the type of                         T
interaction. The system schema now appears
as shown to the right.                                          normal           gravitational

Study of this system schema illustrates the          gravitational
following: a) the block and the string interact                      G
with each other via the tension force, b) the
block and the table interact with each other
via two different interactions—the normal
E
Unit III Reading 1: Inertia & Equilibrium                                                             2

force and the friction force, c) the table interacts with the block, the ground (locally) and
the also the entire earth but not with the string, and d) the earth interacts with all of the
other objects in the system via the long-range gravitational force. Technically, all of the
objects interact with all other objects via the long-range gravitational force, but when
neither of the interacting objects is extremely massive, like a planet or star, we will
ignore these interactions as they are, for all intents and purposes, approximately equal
to zero.

Later in the unit when we study specific objects in a system, it will be necessary to a)
identify how many different interactions the given object experiences and b) identify the
magnitude and direction of each of these
tension           forces. System schema allow you to do
B                 S        the first step.
friction
normal                         For example, let’s identify the interactions
in which the block is involved. On the
T                          system schema, draw a closed box
around the block as shown to the left.
normal          gravitational Note that four different lines protrude into
the box. This represents that the block is
gravitational                                involved in four different interactions, two
G
from the table (the normal and friction
forces), one from the string (tension

tension
B                     S
E
friction
force), and one from the earth (long-range                       normal
gravitational force).                                               T

Let’s now use the system schema to study                                              gravitational
a different object in the system—the table.                       normal
Draw a closed box around the table as
gravitational
shown to the right. Note that four different                        G
lines protrude into this box as well. This
represents that the table is involved in four
different interactions, two from the block
(the normal and friction forces), one from
the ground (normal force), and one from                             E
the earth (long-range gravitational force).

In summary, system schema allow us to identify how all of the objects in a system
interact with other objects in the system. Since each interaction is bound by two
objects, this illustrates that interactions always come in pairs; we will never find an
interaction line ending in free space, but rather at another object.

```
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