​ ​
General Physics I
Year 1, 1st Semester​​
​
​
​
​
​
​ ​
​
​
​
​
​
​
Translational and Rotational Equilibrium
Human balance and Friction
Module 1
Conditions for Equilibrium
●​ Equilibrium refers to a state where an
object is not accelerating; it can be at rest
or moving with constant velocity.
●​ For an object to be in equilibrium, the net
force acting on it must be zero (ΣF = 0), and
the net torque (rotational force) around any
axis must also be zero (Στ = 0).
●​ Newton’s laws of motion
1.​ Law of Inertia - objects will not change
motion unless a force acts upon it
2.​ Law of Force - force of on object =
mass times acceleration (F=MA)
3.​ Law of action & reaction - when 2
bodies interact, they apply forces to
each other of magnitude and opposite
direction
-​ “Every action has an equal, opposite
reaction”
●​ Types of Equilibrium
1.​ Static equilibrium - bodies are at rest,
object is at both translational and
rotational equilibrium at the same time
2.​ Dynamic equilibrium - bodies are
moving at constant velocity
-​ In either cases, the sum of the forces
acting on them is zero, wherein the
rates are equal and unchanging
●​ Stable vs. Unstable vs. Neutral equilibrium
1.​ Stable
when
displaced
from
equilibrium, a system experiences a net
force/torque in a direction OPPOSITE to
direction of displacement
-​ E.g. water bottle on a table
-​ If displaced from equilibrium, center of
mass goes up and gravitational
potential energy increases
-​ This is only stable up until one point
2.​ Unstable - when displaced from
equilibrium, a system experiences a net
force/torque in a direction SAME to
direction of displacement
-​ E.g. marker on table
-​ If displaced from equilibrium, it will
DECREASE gravitational potential, and
therefore fall over
3.​ Neutral - center of mass goes up and
down
-​ E.g. a ball rolling on a table
●​ Translational equilibrium deals with forces
that balance each other in such a way that
there is no net force in any direction.
-​ When the net force acting on the object
= 0 = the object has no linear
acceleration
-​ Object can be moving at constant
velocity or be at rest
-​ Identify: object, direction
●​ Rotational equilibrium deals with torques
that balance each other, resulting in no net
torque and no rotational acceleration.
-​ Object can be at rest (not rotating) or
moving at a constant angular velocity
-​ Identify: object, direction, axis of
rotation
●​ Resource:
The
Physics
Classroom:
Equilibrium
and
Statics
(https://www.physicsclassroom.com/class/
newtlaws/u2l1a.cfm)
Center of Gravity
●​ The center of gravity (CoG) of an object is
the point through which the entire weight of
the object acts, making it behave as if all
its weight is concentrated at that point.
●​ CoG is essential for maintaining balance
and stability.
●​ As long as the object is in a constant
gravitational field, CoM = CoG; e.g. on earth
where g = 9.81 m/s² which is constant
●​ If you apply a force to an object at/toward
the CoM, the force will only cause linear
acceleration
(will
not
rotationally
accelerate)
Bones, Muscles, and Posture
●​ Understanding the human skeletal and
muscular
systems
is
crucial
for
comprehending how our bodies maintain
balance and move.
●​ Posture is influenced by the alignment of
bones, muscles, and the center of gravity.
Roles of Static and Kinetic Friction in
Human Activities
●​ Static friction prevents relative motion
between surfaces in contact when there is
no relative motion.
●​ Kinetic friction opposes the relative motion
between surfaces that are in contact and
moving.