Mechanical Energy
Conservative Forces
Conservation of Mechanical Energy
Conserved Quantities
• When a quantity is conserved, it remains
constant
• The amount of a conserved quantity is
the same at the beginning of a situation
as it is at the end of the situation
Conservation of Energy
• The TOTAL amount of energy in a closed
system will remain constant
CLOSED SYSTEM: Nothing in or out –
no interaction with the surroundings
TOTAL ENERGY: all kinds of energy
(kinetic, PEE, PEG, chemical, heat, etc.)
Mechanical Energy
• Energy associated with the motion and
position of everyday objects
• Mechanical Energy is the sum of kinetic
and potential energy
• ME = KE + PE
ME = KE + PEG + PEE
Conservation of Mechanical Energy
• When there are only conservative forces
acting on an object (gravity, springs),
MECHANICAL ENERGY IS CONSERVED
Conservative Forces
• Conservative forces DO NOT change the
amount of mechanical energy of a system
• Gravity and Elastic Forces
• Conservative forces will convert one form of
mechanical energy into another, but the total
amount of mechanical energy remains the
same
Conservation of Mechanical Energy
• When there are only conservative forces
acting on an object (gravity, springs),
MECHANICAL ENERGY IS CONSERVED
• MEi = MEf
• KEi + PEGi + PEEi = KEf + PEGf + PEEf
Conservative Forces
Conservation of Mechanical Energy
PEG decreases & KE increases, but total ME
remains constant (in the absence of air friction)!
Conservation of ME & Falling Objects
10000 J
5000 J
Time (s)
Conservation of ME & Pendulums
PEG = mgh
KE = 0
KE = ½ mv2
PEG = 0
Is Mechanical Energy Conserved Here?
Nonconservative Forces
• When nonconservative forces do work on a
system, mechanical energy IS NOT conserved
WNC = ΔME
This is the Work-Energy Theorem
Friction
• Friction does negative work on the system, so the
mechanical energy is decreased
• While ME is not conserved when nonconservative
forces (like friction) act on an object, TOTAL
ENERGY is always conserved
– Total energy would include heat energy
• Friction forces turn ME into non-ME (usually
heat), but the total amount of energy stays the
same
Friction & Pendulums