Free Body Diagrams and Vector Diagrams
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Recap from ICT Package
Free Body Diagrams
Types of Component Forces
Vector Diagrams
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A force is a push or a pull
A force is a vector
Force has both magnitude and direction
Unit for force is Newtons (N)
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When a more than one force acts on a body
at the same time, each individual force is
called a component force
The total sum of all the component forces
acting on an object is the resultant force
For any one object, there can be several
component forces, but only be one resultant
force.
Sometimes, resultant force is also called “net
force”
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When there is no resultant force (or resultant
force = 0), the object is said to be balanced
When there is a resultant force, the object is
said to be unbalanced.
We will discuss more about balanced and
unbalanced forces in this chapter, and later in
chapter 5.
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A free body diagram is a diagram used to
describe the forces acting on an object
It only describes the forces acting on ONE
single object, and not other objects
connected to it
Forces are represented by arrows.
 Direction of arrow represents direction of force
 Length of arrow represents magnitude of force
 Each arrow must be labelled
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Objects are typically represented either using
a silhouette of the object itself, or a
simplification thereof.
E.g. free-body diagram of a cow in free fall
COW
Gravitational Force (Weight)
Gravitational Force (Weight)
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Forces can be classified into 2 broad
categories: contact forces & long range
forces.
Contact forces require an object to physically
touch something else in order for the force to
take effect
Non-contact do not require physical contact
for the force to take effect
Contact Forces
Normal Contact Force
Friction
Tension
Non-Contact Forces
Gravitational Force
(Weight)
Electric Force
Magnetic Force
Electromagnetic Force
Breaking The Magician’s Code - Levitation Tricks
1) Levitating Girl 1
(https://www.youtube.com/watch?v=OAHsY_w4lc
M)
 2) Levitating Girl 2
(https://www.youtube.com/watch?v=F64L4CtGshw)
 3) Flying
(https://www.youtube.com/watch?v=O0MOVgzSU_
E)
 4) Walking on Water
(https://www.youtube.com/watch?v=z0ukAescNC0)
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Weight is simply the gravitational force
exerted by the Earth on an object
Formula: Gravitational Force = mg,
 m = mass of the object (which the force is exerting
on)
 g = gravitational field strength. This is also
identical to the acceleration due to free fall, which
is 10 ms-2.
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It is advisable to use “mg” instead of “W” or
any other symbol to represent weight or
gravitational force.
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To draw gravitational force of a free body
diagram, locate the center of gravity of the
object (typically the center of the object)
Draw an arrow downwards (towards the
center of the Earth)
Label the arrow “mg”
mg
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Sometimes called “normal reaction force”
The normal contact force is the force objects
exert on each other when they press each
other
Normal contact force is always a push force
Usually represented by symbol “N” or “n”
“Normal” here refers to “perpendicular”
The direction of normal contact force is
always perpendicular to the surface of
contact
To draw normal contact force on a free body
diagram, locate the surface of contact
 Draw a force originating from the middle of the
contact surface, going perpendicular and away from
the surface
 Label the arrow “N” or “n”
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N
N
Object on the ground
Object on an inclined
surface
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Tension is the force exerted by a taut string,
rope, spring, etc. pulling on an object
Usually represented by symbol “T”
Tension is always a pull force
To draw tension in a free body diagram,
locate the point where the string (for e.g.) is
pulling the object
Draw an arrow originating from that point, in
the same direction as the string
Label the arrow “T”
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Consider this scenario:
 Ball suspended from the
ceiling with a string
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Free Body Diagram of the
ball:
T
mg
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Consider this scenario:
 Object being pulled along
the ground by a string
N
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Free Body Diagram:
T
mg
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Friction is the contact force that opposes of
tends to oppose motion between surfaces in
contact
There is no symbol for friction. Either spell
the word out in full, or use “Ffriction“
To draw friction in a free body diagram,
identify the contact surfaces, and draw an
arrow parallel to the surfaces, in the direction
which would oppose motion
moving to the right
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If an object is moving in
one direction, friction
acts in the opposite
direction
If friction is holding up
an object which would
otherwise slide away,
friction acts in the
direction opposing the
sliding
Friction
Friction
Step 1: draw the object first. Do NOT draw other
objects connected to it
 Step 2: identify the center of gravity, draw and
label the mg of the object
 Step 3: draw all other component forces. The
length of the arrows should be proportional to
the magnitude of the force.
 Note: NEVER draw the resultant force on a Free
Body Diagram!! If there is a need to indicate the
resultant force, draw it outside the FBD.
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Draw the FBD of a stationary object on a
rough slope
N
Friction
mg
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Free Body Diagrams are to display all the
component forces acting on an object
Vector Diagrams help to determine the
resultant force, given all the component
forces
Note: Vector Diagrams are drawn to scale
Vector diagrams use the “head to tail”
method
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Step 1: Draw any one component force
Step 2: Draw the next component force
starting from the “head” of the previous
component force.
Step 3: Repeat Step 2 until all component
forces are drawn
Step 4: The resultant force is draw from the
tail of the first arrow, to the head of the last
arrow.
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A car is being pulled by 2 ropes as shown
below. The 2 forces are at an angle of 15° to
the horizontal. Find the resultant force acting
on the car.
500 N
500 N
500 N
500 N
500 N
500 N
A picture is hung on the wall with 2 strings from a
nail.
(a)
(b)
Draw a free body diagram of the picture.
Sketch the vector diagram and determine the
resultant force acting on the picture.
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Force is a push or pull, unit: Newtons (N)
Force is a vector (magnitude & direction)
Free Body Diagrams
Types of Component Forces
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Gravitational Force (weight)
Normal Contact Force
Tension
Friction
Air Resistance (Drag Force)
Vector Diagrams
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Force is a push or pull, unit: Newtons (N)
Force is a vector (magnitude & direction)
Free Body Diagrams
Types of Component Forces
 Gravitational Force (weight)
 Normal Contact Force
 Tension
 Friction