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4/25/08 10:33 AM
Ch9 HW3 S2008
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http://www.webassign.net/v4cgikseiloff3@gatech/student.pl?y=20080425142654kseiloff3@gatech1286165155
Ch9 HW3 S2008 (Homework)
KRISTIN SEILOFF
Instructor: Jennifer Curtis
Description
Momentum and energy in collisions: Unequal masses; 2D collisions
Instructions
Reading: CH9
Web
Assign
Current Score:
51 out of 55
Due:
Friday, March 28, 200808:35 AM EDT
1. 9/9 points
In outer space rock 1, with mass
6
kg and velocity <
3900
,
-3200
,
2800
> m/s, struck rock 2, which was at rest. After the collision, rock 1's
velocity is <
3500
,
-2500
,
3300
> m/s. What is the final momentum of rock 2?
2
f
= < 2400
2400
, -4200
-4200
, -3000
-3000
> kg · m/s
Before the collision, what was the kinetic energy of rock 1?
K
1
i
= 99870000
9.99e+07
J
Before the collision, what was the kinetic energy of rock 2?
K
2
i
= 0
0
J
After the collision, what is the kinetic energy of rock 1?
K
1
f
= 88170000
8.82e+07
J
Suppose the collision was elastic (that is, no change in kinetic energy and therefore no change in thermal or other internal energy of the rocks).
In that case, after the collision, what is the kinetic energy of rock 2?
K
2
f
= 11700000
1.17e+07
J
On the other hand, suppose that in the collision some of the kinetic energy is converted into thermal energy of the two rocks, where
E
thermal,1
+
E
thermal,2
=
7.02
10
6
J. What is the final kinetic energy of rock 2?

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Ch9 HW3 S2008
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K
2
f
= 4680000
4.68e+06
J
In this case (some of the kinetic energy being converted to thermal energy), what was the transfer of energy
Q
(microscopic work) from the
surroundings into the two-rock system during the collision? (Remember that
Q
represents energy transfer due to a temperature difference
between a system and its surroundings.)
Q
= 0
0
J
Solution or Explanation
sys
=
net
t
= 0, so the momentum of the system doesn't change.
Therefore,
1f
+
2f
=
sys,i
If the collision was elastic then
K
1f
+
K
2f
=
K
sys,i
If some kinetic energy is converted into the thermal energy of the two rocks,
then
K
1f
+
K
2f
+
E
thermal, sys
=
K
sys,i
Although the system's temperature rises (associated with the increase in its internal or thermal energy), in the rapid collision there isn't time for
any significant thermal energy transfer
Q
to the cooler surroundings. Eventually the system will cool down as a result of thermal energy transfer
to the surroundings, but this will take a while.
2. 22/22 points

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