FRICTIONAL FORCES
FRICTIO
Free body diagram:
A book lying on a level table
Free body diagram:
A person floating in still water
Free body diagram:
A wrecking ball hanging vertically from a cable
Free body diagram:
A helicopter hovering in place
3 Free Body Diagrams:
Move R, accel R
Move R, no accel
Move R, accel L
Free Body Diagrams Notes
Key Terms
newton, - N, unit of force in the metric system
- force needed to push a 1kg object at a speed of
1m/s for a second.
mass, in physics, the quantity of matter in a body
regardless of its volume or of any forces acting on it.
weight, measure of the force of gravity on a body
What does gravity have to do with the weight of an
object?
Weight (W) varies depending upon the location of the
body in the earth's gravitational field (or the gravitational
field of some other astronomical body).
The acceleration of gravity on earth is approximately:
9.8 m/s² in SI units and
32 ft/s² in US Customary units.
To calculate the weight of an object you have to multiply
it’s mass times the acceleration of gravity.
W = m * g
The Statue of Liberty has a mass of 225,000
kg. How much does she weigh?
To calculate the weight of an object you have to
multiply it’s mass times the acceleration of
gravity.
Write the formula:
W = m *
g
Substitute known values:
W = (225,000 kg) * 9.8 m/s²
Present solution with units:
W = 2,200,000 N
What the heck is 2,200,000 N?
The Statue of Liberty weighs 2,207,250 Newtons, which is
495,000 lbs pounds!
A free-body diagram illustrates the
relative magnitude and direction of all
forces acting upon an object. The object
must be isolated and “free” of its
surroundings.
This is a free-body diagram of the Statue of Liberty. She is
represented by a simple box. The forces acting on her are
labeled with a magnitude and the arrow shows direction.
Notice the surrounding objects are stripped away and the
forces acting on the object are shown.
496210 lb
496210 lb
“FW” here represents the force of the weight of the statue.
“FN” is the normal force, which represents the force Liberty
FW = 495,000 lb
Island is pushing back up on the statue.
Normal: means perpendicular to, (ex. The walls to the
floor)
The force of the pedestal to the statue is normal
to the surface of the ground.
The island has a great resistance to compression.
The ground is exerting a force upward on the
statue perpendicular, or normal, to the surface.
FN = 495,000 lb
Think of the diagram on an XY
plane.
If “up” is assumed to be the
positive direction, then N is
positive and W is negative.
•When forces acting on the object
cancel each other out it is in a state
of static equilibrium.
(Positive y-direction)
+y
FW = -495,000 lb
FN = 495,000 lb
+x
(Positive x-direction)
Create a free body diagram (FBD) of the gorilla:
FN
Gorilla
FW
Free Body Diagram of the Sitting Gorilla
(The box represents the gorilla, W =
weight of the gorilla, N = Normal force)
Sitting Gorilla
Draw a FBD of the wooden swing:
Where are the forces
on the swing?
FT1
FT2
Swing
FW
Free Body Diagram of the wooden swing (The box
represents the wooden swing, W = weight of the
swing and the parrot, T represents the ropes that
are in tension supporting the weight)
Parrot on wooden
swing hung by ropes
Draw a FBD of bucket the bungee jumper
leaped from:
Where are the forces
on the bucket?
FT
bucket
FW
Free Body Diagram of the bucket (T represents the
tensile force of the cable the bucket is suspended from,
and W is the weight of the diver and the bucket)
Bungee jumping from
crane
Draw a FBD of the ring at point C:
Where are the forces on the ring?
A
B
FTCA
FTCB
C
D
FTCD
Traffic Light supported
by cables
Free Body Diagram of the ring at point C
(T represents the force of the cables that
are in tension acting on the ring)
Draw a FBD of the traffic light:
A
B
C
Where are the
forces on the
light?
FTCD
Light
D
FW
Traffic Light supported
by cables
Free Body Diagram of the traffic light (FTCD
represents the force of the cables acting on
the light and FW is the weight acting on the
light)
Draw a FBD of the pin at point A:
Where are the forces on
point A?
FTAB
FTAC
B
A
FTAD
FTAE
Free Body Diagram of pin A
C
E
D
Pin-Connected Pratt Through Truss Bridge
(If you consider the third dimension, then
there is an additional force acting on point A
into the paper: The force of the beam that
connects the front of the bridge to the back of
the bridge.)
Sitting Gorilla
Draw a FBD of the wooden swing:
Where are the forces on the swing?
Parrot on wooden
swing hung by ropes
Draw a FBD of bucket the bungee jumper leaped
from:
Where are the forces on the bucket?
Bungee jumping from crane
Draw a FBD of the ring at point C:
Where are the forces on the ring?
A
B
C
D
Traffic Light supported
by cables
Draw a FBD of the traffic light:
Where are the forces on the light?
A
B
C
D
Traffic Light supported
by cables
Draw a FBD of the pin at point A:
Where are the forces on point A?
B
A
C
E
D
Pin-Connected Pratt Through Truss Bridge