An elevator cab of mass
m=500kg Is descending with speed vi = 4.0 m/s when
Its supporting cable
begins to slip, allowing it to fall with constant acceleration
a = g/5
a) During the fall through a distance d=12 m, what is the work
Wg done on the cab by the gravitational force Fg?
b) During the 12 m fall, what is the work WT done on the cab by
upward pull T of
the elevator cable?
c) What is the net work
W done on the cab during the fall?
d) What is the cab
kinetic energy at the end of the 12m fall?
Sample Problem 7-6
a) Wg = mgd = 59 kJ
b) mg - T = ma = mg/5
T = 4/5 mg
WT = - Td = - 4/5
mgd = -47 kJ
c) Wnet = Wg + WT = 12 kJ
D) Kf = Ki + Wnet = ½ mvi2 + Wnet =
16 kJ