Download Draw It!! - BEHS Science

Just as PICTORIAL
models and motion
diagrams are useful in
solving problems about
MOTION,
similar representations
will help you analyze
how FORCES affect
motion.
Take the example of a ball
tied to a string and sketch
the situation to represent
the forces acting on the
ball.
CIRCLE the system and
identify every place the
system touches the
EXTERNAL world.
It is at these places the
contact forces are
EXERTED.
Identify the
CONTACT
forces.
Force from
string
Then identify any FIELD
forces on the system. This
gives you the pictorial model.
Gravity?
Magnetic?
Force of
Electric?
gravity
To make a pictorial
representation of the forces
acting on the ball, apply the
particle model and represent
the object with a DOT.
Represent each force with
an arrow (VECTOR) that
points in the direction
that the force is applied.
Try to make the LENGTH of
each arrow proportional to
the SIZE of the force.
Always draw the force
arrow pointing away from
the object, even if it is a
PUSH.
Make sure that you
label each force.
Use an uppercase F
with a SUBSCRIPT
describing the force.
Fstring
Fgravity
This type of physical
model, which represents
the forces acting on a
system, is called a FREEbody diagram.
Draw free-body diagrams for the
following objects.
The only RULE for
drawing free-body
diagrams
is to depict all the
forces which exist for
that object in the given
situation.
Thus, to CONSTRUCT free-
body diagrams, it is
extremely important to know
the various types of forces.
If necessary, refer to the list
of forces and their
description in order to
understand the various force
types and their appropriate
SYMBOLS.
Applied Force
Fapp
Contact Force
An applied force is a force which is
APPLIED to an object by a person
or another object.
Gravity force
 Fgrav
 Field
 The force of GRAVITY is the force with which the
earth, moon, or other MASSIVELY large object attracts
another object towards itself. By definition, this is the
WEIGHT of the object. The force of gravity on earth is
always equal to the weight of the object as found by
the equation:
 Fgrav = mg, where g = 9.8 m/s2 (on Earth) and m = mass
(in kg).
Normal Force
 Fnorm
 Contact
 The normal force is the SUPPORT force
exerted upon an object which is in contact
with another stable object. For example, if a
book is resting upon a SURFACE, then the
surface is exerting an upward force upon the
book in order to support the weight of the
book.
Friction Force
 Ffrict
 Contact
 The friction FORCE is the force exerted by a
surface as an object moves across it or makes an
effort to move across it. There are at least two types
of friction force - SLIDING and static friction.
Thought it is not always the cast, the friction force
often opposes the motion of an object.
Tension Force
 Ftens
 Contact
 The tension force is the force which is
TRANSMITTED through a string, rope,
cable or WIRE when it is pulled tight
by forces acting from opposite ends.
Spring Force
Fspring
Contact
The spring force is the force exerted
by a compressed or STRETCHED
spring upon any object which is
attached to it.
Electrical
 Felec
 Field
 The electrical force is the force exerted
by CHARGED particles on each other.
Depending on the type of charged
particles, Felec could be ATTRACTIVE
or repulsive.
Magnetic
Fmag
Field
The magnetic force exerted
between magnetic POLES,
producing magnetization.