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Lecture7 (Topic6) Conservation of Energy, NG HUI YUN A20SC0187 - Coggle…
Lecture7 (Topic6)
Conservation of Energy
Conservative and Nonconservative Forces
Conservative Force
Work done by force on an object depends only on initial and final positions of the object
Wg=Fd=mgh
Nonconservative Force
Any work done depends on path
Examples: friction force, air resistance, push or pull, etc
Potential Energy
Energy associated with forces that depend on position/configuration of objects relative to surroundings
Gravitational Potential Energy
Work done in lifting objects
△U = U₂ - U₁ = W = mg(y₂-y₁)
Belongs to a system
Elastice Potential Energy
△U = ½kx²
Mechanical Energy and Its Conservation
Work-Energy Principle
△U = -Wnet
△U + △K = 0
E = K + U
E is constant (conserved)
K₁ + U₁ = K₂+ U₂
Law of Conservation of Energy
consider conservative and nonconservative forces
mechanical energy is not conserved
it is replaced by frictional force, heat, etc
TOTAL ENERGY is conserved
△K + △U + [change in all other form of energy] = 0
K₁ + U₁ = K₂+ U₂ + Ffrℓ
Gravitational Potential Energy
For points far from the Earth surface, Fg decreases inversely as r²
F = -GMm//r²
negative sign indicates force on m is opposite to r direction
W = GMm/r₂ - GMm/r₁
Since △U = -Wnet,
△U = GMm/r₁ - GMm/r₂
Escape Velocity
Objects get to escape from the Earth if its speed is high enough.
escape velocity (Earth) = 1.12 x 10^4 m/s
Power
rate at which work is done
P = W/t
work done involves transformation of energy from one type/object to another
P = E/t
SI unit
Joules/second (J/s)
Watt (W)
housepower (hp)
1J/s = 1W
1hp = 746 W
P = Fv
Efficiency
= output power/input power
< 1.0
NG HUI YUN
A20SC0187
