8.31.
IDENTIFY:
Treat the comet and probe as an isolated system for which momentum is conserved.
SET UP:
In part (a) let object
A
be the probe and object
B
be the comet. Let
x
be the direction the probe is
traveling just before the collision. After the collision the combined object moves with speed
2
v
. The change in
velocity is
21
x
B x
v v
v
. In part (a) the impact speed of 37,000 km/h is the speed of the probe relative to the
comet just before impact:
11
37,000 km/h
A x
B x
vv
. In part (b) let object
A
be the comet and object
B
be the
earth. Let
x
be the direction the comet is traveling just before the collision. The impact speed is 40,000 km/h,
so
40,000 km/h
A x
B x
.
EXECUTE:
(a)
12
xx
PP
.
2
A A x
B B x
x
AB
m v
m v
v
mm
.
2
1
1
1
1
1
A
B
A
B
A
x
B x
A x
B x
A x
B x
A
B
A
B
A
B
m
m
m
m
m
v v
v
v
v
v
v
m
m
m
m
m
m
.
6
14
372 kg
( 37,000 km/h)
1.4 10 km/h
372 kg
0.10 10 kg
v
.
The speed of the comet decreased by
6
. This change is not noticeable.
(b)
14
8
14
24
( 40,000 km/h)
6.7 10 km/h
5.97 10 kg
v
. The speed of the earth would
change by
8
. This change is not noticeable.
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 Fall '07
 Evrard
 Kinetic Energy, Momentum, Potential Energy, Collision, kg, ma mb

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