Ch9 HW2 (Homework)
BRANDON JARROTT FELDKAMP
PHYS 2211, section M, Fall 2008
Instructor: Jennifer Curtis
Web
Assign
Current Score:
58 out of 59
Due:
Friday, October 31, 2008 09:00 AM EDT
Description
Momentum and energy in collisions: Unequal masses; 2D collisions
Instructions
Reading: CH9
The due date for this assignment is past.
Your work can be viewed below, but no changes can be
made.
1. 9/9 points
In outer space rock 1, with mass
9
kg and velocity <
4100
,
3200
,
2400
> m/s, struck rock 2, which
was at rest. After the collision, rock 1's velocity is <
3600
,
2500
,
2700
> m/s. What is the final
momentum of rock 2?
2
f
= <
4500
4500
,
6300
6300
,
2700
2700
> kg · m/s
Before the collision, what was the kinetic energy of rock 1?
K
1
i
=
1.476e8
1.48e+08
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
=
1.193e8
1.19e+08
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
=
2.835e7
2.84e+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
=
1.70
10
7
J. What is the final
kinetic energy of rock 2?
K
2
f
=
1.13e7
1.14e+07
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 tworock 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
Ch9 HW2
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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
Object A has mass
m
A
=
10
kg and initial momentum
A,i
= <
19
,
8
, 0 > kg · m/s, just before it
strikes object B, which has mass
m
B
=
13
kg. Just before the collision object B has initial
momentum
B,i
= <
6
,
6
, 0 > kg · m/s.
Consider a system consisting of both objects A and B. What is the total initial momentum
of this system, just before the collision?
sys,
i
= <
25
25
,
2
2
, 0 > kg · m/s
The forces that A and B exert on each other are very large but last for a very short time. If we
choose a time interval from just before to just after the collision, what is the approximate
value of the impulse applied to the twoobject system due to forces exerted on the system
by objects outside the system?
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 Physics, Current, Kinetic Energy, Momentum, Work

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