Physics Lesson 8
Static Equilibrium
Eleanor Roosevelt High School
Chin-Sung Lin
Net Force
Net Force
Net Force – Sum of all Forces
 The combination of all forces acting on an object is
called the net force
Net Force
Calculate Net Force – Vector Addition
 Vector addition can be used to calculate the net force
Net Force
Calculate Net Force – Vector Resolution/Addition
 Since vector addition by graphical method is not
precise, it is preferred to resolve all the force vectors
into horizontal and vertical components, and then add
them respectively
y
F
Fy
Fx = F cos θ
Fy = F sin θ
Fx
x
Net Force
Calculate Net Force
B
A
Net Force
Calculate Net Force
B
A
A+B
Net Force
Calculate Net Force
B
A
Net Force
Calculate Net Force
B
A
A+B
Net Force
Calculate Net Force
B
A
Net Force
Calculate Net Force
B
A
A–B
Net Force
Calculate Net Force
B
A
Net Force
Calculate Net Force
B
A
A–B
Net Force
Calculate Net Force
A
A
Net Force
Calculate Net Force
A
A
0
Net Force
Calculate Net Force
A
A
Net Force
Calculate Net Force
A
A
0
Gravitational Force (Fg)
The gravitational force (Fg) is the attraction force exerted
by Earth on the block
Fg
Normal Force (FN)
The normal force (FN) is the reaction force exerted by
ground on the block
FN
Fg
Normal Force (FN)
Normal force is always perpendicular to the surface
FN
FN = Fg
Fg
Friction Force (Ff)
The friction force (Ff) is the reaction force exerted by
ground on the block
Friction force is always against the motion
Ff
F
Friction Force (Ff)
When the net force is equal to zero, friction force (Ff) is
equal to the the force (F) exerted on the block
Ff = FF
Ff
F
Tension Force (FT)
The tension force (FT) is the force exerted by the rope on
the block
FT
Fg
Tension Force (FT)
When the net force is equal to zero, tension force (FT) is
equal to the the gravitational force (Fg) exerted on the
block
FT
FT = Fg
Fg
Equilibrium
Statics
Statics is the branch of mechanics concerned with the
analysis of force on physical systems in static
equilibrium, that is, in a state where the relative
positions of subsystems are at rest, or where
components and structures are at a constant
velocity
Equilibrium
When an object is at rest, or at constant velocity, with
the net force on it being zero, the object is in a state
of equilibrium
Free-Body Diagram
A diagram showing all the forces acting on an object is
called a free-body diagram
Drawing a free-body diagram should always the first
step in solving physics problems involving forces
Free-Body Diagram
We isolate, or free the object of concern from everything
else, and represent that object by a dot. We then
draw all the forces vectors acting on the object with
its tail starting on the dot. We label each vector to
indicate what type of force it represents –
•
W (or Fg) for a gravitational force
•
N (or FN) for a normal force
•
f (or Ff) for a friction force, and
•
T (or FT) for a tension force
FT
Fg
Vector Addition &
Vector Resolution
Vector Addition
An object of 10 N hangs from two spring scales
When the spring scales hang at 60o from the vertical,
their readings are 10 N each
When the angle has increased to 75.5o, the readings are
20 N each
Vector Addition
As the angle between the scales increases, the tension
in the scales must increase for the resultant to remain
10 N
Vector Addition
For any pair of scales, ropes, or wires supporting a load, the
greater their angles from the vertical, the larger the tension
force in them. The resultant of the tension forces, or the
diagonal of the parallelogram they describe, must be equal
and opposite to the load being supported
Vector Resolution
2 FT cos
N
= 2 x 10 N x cos 60o
2 FT cos
= 2 x 20 N x cos 75.5o = 2 x 20 N x 1/4 = 10 N
= 2 x 10 N x 1/2 = 10
Q&A
The End