Gravitational Potential Energy Stored energy of height § 6.3

Potential Energy The energy of relative position of two objects gravity springs electric charges chemical bonds

Potential Energy Energy is stored doing work against a potential Potential energy increases when “the potential” does negative (< 0 ) work lifting a weight stretching a spring

Gravitational Potential Energy the work to raise an object to a height Ug = mgh

Gravity Doing Negative Work Source: Young and Freedman, Figure 7.2b.

Work from Potential Energy When a potential does > 0 work on a body: The body’s potential energy decreases The body’s kinetic energy increases

Poll Question When a cute furry animal moves upward in free-fall: Its gravitational potential energy increases. Its kinetic energy increases. Both A and B. Neither A nor B.

Gravity Doing Positive Work Source: Young and Freedman, Figure 7.2a.

Poll Question When a cute furry animal moves downward in free-fall: Its gravitational potential energy increases. Its kinetic energy increases. Both A and B. Neither A nor B.

Board Work Problem A 50-g egg released from rest from the roof of a 30-m tall building falls to the ground. Its fall is observed by a student on the roof of the building, who uses coordinates with origin at the roof, and by a student on the ground, who uses coordinates with origin at the ground. What values do the two students find for: Initial gravitational potential energy Ug0? Final gravitational potential energy Ugf? Change in gravitational potential energy DUg? Kinetic energy just before impact Kf?

Mechanical Energy The energy available to do work Kinetic + potential = K + U

Conservation of Mechanical Energy If the only force doing work is gravity, mechanical energy does not change. E1 = E2 K1 + Ug1 = K2 + Ug2 1/2 mv12 + mgy1 = 1/2 mv22 + mgy2

Board Work Problem A 50-g egg released from rest from the roof of a 30-m tall building falls to the ground. Its fall is observed by a student on the roof of the building, who uses coordinates with origin at the roof, and by a student on the ground, who uses coordinates with origin at the ground. What values do the two students find for: Initial total energy E0? Final total energy Ef? Change in total energy DE?