Equilibrium
Force
Force is
Force is not

A push or pull on an object.

Energy.

A vector with magnitude and
direction.

Power.

Momentum.

Velocity.
Fundamental Forces




Gravity is a fundamental force.
It acts upon objects from a distance away from the
source (such as the Earth).
There are two other fundamental forces.
Electroweak force is
common in everyday life.
•
•
•
•
Electricity
Magnetism
Light
Radioactive decay

Nuclear force is uncommon
in everyday life.
• Nuclear fission (nuclear
power plants)
• Nuclear fusion (stars)
Contact Forces

Many forces are due to contact between objects.
•
•
•
•

Kick a ball
Push with a bulldozer
Tug from a rope
Friction due to the ground
The actual force is electricity, but the atoms are so
small we can treat the forces as coming from contact
by larger objects.
Newton’s Laws

Ancient scientists looked to the
natural properties of objects.
• Motion was a result of the object’s
properties.

Newton defined motion based on
forces acting from outside an object.
• Motion was the result of external
forces.

Three laws were used to define the
behavior of forces on objects.
First Law: Law of Inertia
1
An object continues at rest, or in uniform motion in a
straight line, unless a force is imposed on it.

This describes constant velocity, including zero.
No change means no force, and vice versa.

no force
rocket
constant
velocity
Zero Net Force

An object at rest with no net force is in static
equilibrium.

The net force is due to the sum of forces acting on
the object.
• The forces are vectors


Fnet   F  0
Static Forces

An advertising sign weighs
210 N. It is supported from a
post with a horizontal beam,
and by a chain making an
angle of 35 from the
horizontal. What is the force
in the chain?
q = 35º
Newton
Legal
W = 210 N
Vector Forces

With no motion, forces must
sum to zero.

Identify forces on the sign.
• C is the force on the chain
• B is the force on the beam
• W is the weight

Vector sum is zero.

B

C

W

W

C

B
Force Components

To find the values, use
components

Find the vertical components
for the force on the chain.
•
•
•
•
q = 35º
Cx
Cy
Bx
Wy = -210 N

Cy = C sinq
Wy = -210 N
0 = Cy + Wy = C sinq + Wy
C = -Wy / sinq = 370 N
Use horizontal components
for the force on the beam.
• 0 = Bx + Cx = Bx + (-C cosq)
• Bx = C cosq = 300 N
Constant Velocity

Constant velocity means no
change in motion.
v0

Dynamic equilibrium applies
in states of constant, nonzero velocity.

Zero net force used here:
• FN + Fg + Fy = 0
• Fx + Ffr = 0
FN
Ffr
Fx
Fg
Fy
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