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Lesson 17 Homework Solutions
1.
The figure shows two masses 27 kg
and 32 kg connected by a spring that
passes over a frictionless pulley of
mass 5 kg and radius 0.4 m. The 27
kg is resting on the table top and the
32 kg is 1.2 m above the table top. If
the system is released, use the
conservation of energy to find the
speed of the 32 kg mass just before
touching the table.
1.2 m
32 kg
27 kg
0.4 m
5 kg
We assume that the potential energy is zero at the table top.
Initial potential energy of the 27kg mass = 0
Initial potential energy of the 32kg mass = 32
×
9.8
×
1.2 = 376.32 J
Since the potential energy of the pulley does not change, we will
ignore it.
Since the initial kinetic energy of the system is zero, the total initial
energy = 376.32 J
When the 32kg mass falls through 1.2 m, the 27kg mass rises
through 1.2 m. Just before the 32kg reaches the table top,
The potential energy of the 27kg = 27
×
9.8
×
1.2 =
317.52 J
Potential energy of the 32kg = 0
Kinetic energy of the 27kg = (1/2)27 v
2
= 13.5 v
2
Kinetic energy of the 32kg = 16v
2
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 Fall '10
 George
 Physics, Friction, Mass, Work

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