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Ph 111.01
Fall, 2008
Solutions, Problem set 3c (8)
Problem 1.
In Denver, children bring their old jackolanterns to the top of a tower and
compete for accuracy in hitting a target on the ground (figure 418).
Suppose that the
tower height is
h
= [9.60] m, and that the bullseye's horizontal distance is d = [3.7] m
from the launch point.
If the pumpkin is thrown horizontally, what is the launch speed
needed to hit the bullseye? (Neglect air resistance.)
Figure 418
Solution.
The time to fall a distance
h
under gravity, with no initial velocity in the
vertical direction, is given by
2
1
00
2
2
1
2
y
yyv
t g
t
hg
t
=+ −
=+−
=
giving
()
2
1
2
2
29
.60
m
2
1.400 s
9.8 m/s
gt
h
h
t
g
=
==
=
Then the horizontal velocity, which is constant, is given by
( )
0
3.7 m
2.643 m/s
1.400 s
xx
x
vv
t
∆
== =
=
∆
Problem 2 (Walker Ch. 5 P. 2).
In a grocery store, you push a [15.5] kg shopping cart
with a force of [10.5] N.
If the cart starts at rest, how far does it move in 3.00 s?
F
m
x
m
F
G
""
:
x
F
ma
x
Fm
a
=
=
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View Full DocumentThe figure above illustrates the threefold approach to a problem:
drawing, free
body diagram, and Newton'slaw equations.
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 Fall '08
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