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College Physics I
2011/12 Assignment 2
Suggested solutions
1.
A body of mass
m
1
+
m
2
is split into two parts of masses
m
1
and
m
2
by an internal explosion which
generates kinetic energy
K
. If the two parts move in the same line after the explosion, show that the
speed of one part relative to the other is
2
1
2
1
)
(
2
m
m
m
m
K
+
Conservation of mechanical energy and momentum give:
2.
A 62kg bungee jumper jumps from a bridge. She is tied to a bungee cord whose unstretched length is
12 m, and falls a total of 31 m.
(a) Calculate the spring stiffness constant k of the bungee cord, assuming Hooke’s law applies.
(b) Calculate the maximum acceleration she experiences.
(a)
The mechanical energy of the jumper is conserved.
Use
y
for the distance from the 0 of gravitational
PE and
x
for the amount of bungee cord “stretch” from its unstretched length.
Subscript 1 represents
the jumper at the start of the fall, and subscript 2 represents the jumper at the lowest point of the fall.
The bottom of the fall is the zero location for gravitational PE
(
29
0
y
=
, and the location where the
bungee cord just starts to be stretched is the zero location for elastic PE
(
29
0
x
=
.
We have
1
0
v
=
,
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31 m
y
=
,
1
0
x
=
,
2
0
v
=
,
2
0
y
=
, and
2
19 m
x
=
.
Apply conservation of energy.
(
29
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This note was uploaded on 02/22/2012 for the course CHEM yscn0027 taught by Professor Drtong during the Fall '10 term at HKU.
 Fall '10
 drtong
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