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Solution for Homework 2, October 3, 2011
Problem 1.
(5 points)
An egg which is initially at rest is dropped on a hard surface and breaks. We
wait that any temperature change equilibrates. Take the egg as system.
(a) What is the sign of
W
?
(b) What is the sign of Δ
E
pot
?
(c) What is Δ
E
kin
(just before and just after impact)?
(d) What is Δ
U
tot
?
(e) What is the sign of
Q
?
Solution
(a)
W
= 0 as overall no work is performed. (Gravity performs ﬁrst acceleration
work on the system
W >
0 and then the system performs deformation work on
the ground
W <
0.)
(b) Δ
E
pot
=
mg
Δ
z <
0 as the egg looses height
(c) Before Impact: Δ
E
kin
=

mg
Δ
z
After Impact Δ
E
kin
= 0
(d) Δ
U
tot
= 0 as the overall temperature and pressure do not change (state
functions).
(e)
Q <
0 as the egg gives up its potential energy at impact
Problem 2.
(4 points) Complete the table for a four step cycle of a closed
system.
Step
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 Fall '09
 Faller

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