Physics Ch8 Flashcards
What is Potential Energy?
(U) is energy that can be associated with the configuration of a system of objects that exert forces on one another
Key Characteristics of Potential Energy-Work on Objects
1) System consists of two or more objects
2) A force acts between a particle and the rest of the system
3) When the configuration changes, the force does work, changing the kinetic energy to another form
4) When the configuration change is reversed, the force reverses the energy transfer, doing work
Conservative Force
Forces for which Work 1 = -(Work 2) is always true
ex: gravitational force, spring force
Nonconservative force
Forces for which Work 1 != -(Work 2)
ex: kinetic friction force and drag force. Thermal energy
Gravitational PE, relative to a reference configuration with reference point yi = 0
U(y) = mgy
Mechanical Energy
The mechanical energy of a system is the sum of its potential energy U and kinetic energy K:
Emec = K + U
Work done by Mechanical Energy
Work done by conservative forces increases K and decreases U by that amount, so:
change in K = -(change in U)
with subscripts:
K2 + U2 = K1 + U2
principle of the conservation of mechanical energy:
change in Emec = change in K + change in U = 0
why is the conservation of mechanical energy important?
When the mechanical energy of a system is conserved, we can relate the sum of kinetic energy and potential energy at one instant to that at another instant without considering the intermediate motion and without finding the work one by the forces involved.
Ex: Choose the lowest point in the system as U = 0
Then at the highest point U = max, and K = min
Force from potential energy plot
F(x) = -dU(x)/dx
Key Points of Potential Energy Curves
To find K(x) at any place, take the total mechanical energy (constant) and subtract U(x)
Places where K = 0 are turning points,
- places where the particle changes direction
At equilibrium points, the slope of U(x) is 0
A particle in neutral equilibrium is stationary, with potential energy only, and net force = 0
Unstable vs. Stable Equilibrium
A particle in unstable equilibrium is stationary, with potential energy only, and net force = 0
A particle in stable equilibrium is stationary, with potential energy only, and net force = 0
Work vs. Kinetic and Potential Energies
For a frictionless system:
W = change in K + change in U
W = change in Emec
Friction and Thermal Energy
For a system with friction:
Change in thermal energy = fk*d
W = change in Emec + change in Eth
Conservation of Energy Statement
-Energy transferred between systems can always be accounted for
-The law of conservation of energy concerns:
The total energy E of a system
includes mechanical, thermal, and other internal energy
