PHYS 17200 Lecture Notes - Lecture 11: Net Force

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PHYS 17200 - Modern Mechanics - Lecture 11_Potential Energy
Energy Principle
delta_E(system) = W_surroundings = F_net * delta_r
W_surroundings = Work done on object by objects in surroundings
F_net = Net force on object
delta_r = Change in displacement
Identification of the system (choosing the system can affect the work done on an object)
ie. Lifting a book
System = Book
delta_E(system) = W_surroundings
delta_E(book) = delta_K + delta_m*c^2 = 0 + 0 = 0
W_surroundings = W_Earth + W_hand = F_Earth * delta_r + F_hand * delta_r =
0
System = Earth
delta_E(system) = W_surroundings
delta_E(Earth) = delta_K + delta_m*c^2 = 0 + 0 = 0
W_surroundings = F_earth * delta_r = 0
System = Book + Earth
delta_E(book) + delta_E(Earth) -F_Earthonbook * delta_r = F_hand * delta_r
-F_Earthonbook * delta_r = delta_U
delta_E(book) + delta_E(Earth) + delta_U = F_hand * delta_r
In this case the large mass of the Earth meant that there was no
significant contribution to ΔU from work done by the book on the Earth. In
general, both objects in two-body systems system do significant work on
each other.
Force and Gravitational Potential Energy
Force of Attraction between two objects
F = -G*m_1*m_2/(R^2) ; G = 6.673×10^−11
Potential Energy between two objects
U = -G*m_1*m_2/(R) ; G = 6.673×10^−11
If F_g = -m*g, then U=mgy
Bound and Unbound States
Bound State: K + U < 0
Unbound State: K + U >= 0
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