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Phys 298
Brown
Solutions to graded problems from week 2 homework.
Ch. 2 #78
This is a free fall problem.
Given/Known:
"
y
=
1.30 m
"
t
=
0.22 s
a
y
=
g
,
t
0
=
0,
y
0
=
0
•
I choose to call down my positive ydirection.
You don’t have to make the same
choice, but you have to be consistent with your choice once you make it.
•
The way I want to approach this is to find the speed/velocity of the balloon as it
enters from the top of the window.
Then I can figure out how far the balloon has
fallen from rest above the window to reach this speed.
I use:
y
=
y
0
+
v
0
y
t
+
1
2
a
y
t
2
"
y
#
y
0
#
1
2
gt
2
=
v
0
y
t
"
v
0
y
=
$
y
#
1
2
gt
2
t
=
1.3m
#
1
2
(9.8m/s
2
)(0.22s)
2
0.22s
=
4.83 m/s
Knowing that the balloon is moving at 4.83 m/s when it reaches the top of the window,
use the equations of kinematics to figure out how far it has fallen from rest.
It is like a
separate problem where now 4.83 m/s is the FINAL velocity and 0 is the initial velocity.
Given/Known:
v
y
=
4.83 m/s,
v
0
y
=
0
a
y
=
g
y
0
=
0
Find
y
.
Start from the equation of kinematics that doesn’t involve time:
v
y
2
=
v
0
y
2
+
2
a
y
(
y
"
y
0
)
v
y
2
=
0
+
2
g
(
y
"
0)
=
2
gy
#
y
=
v
y
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This note was uploaded on 05/15/2008 for the course PHYS 298 taught by Professor Morrison during the Spring '08 term at University of Louisville.
 Spring '08
 Morrison
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