Physics 111.01
October 8, 2008
PS 6b (15)
Problem 1 = Walker P 821.
A 0.26 kg rock is thrown
vertically upward from the top of a cliff that is
28
m high.
When it hits the ground at the base of the cliff the rock
has a speed of
28
m/s.
(a) Assuming that air resistance can be ignored, find the
initial speed of the rock.
m/s
(b) Find the greatest height of the rock as measured from
the base of the cliff.
m
Solution.
This problem can be done using forces
and acceleration.
But it is much easier to do just
using conservation of energy.
We will evaluate the total energy at three points:
(1) is the start of the motion; (2) is the highest point; and (3) is at the base of the
cliff.
g
h
v
1
2
3
y
y=0
H
(a)
We compare start and finish.
()
13
11
33
22
2
2
2
0
2
28 m/s
2 9.8 m/s
28 m
EE
KU KU
mv
mgh
mv
vvg
h
15.34 m/s
=
+=+
+=
+
=−
=
(b)
Now compare highest and lowest points:
23
2
1
3
2
2
3
2
2
00
2
28 m/s
2 9.8 m/s
KU
mgH
mv
v
H
g
40.0 m
=
+
=
=
=
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View Full DocumentProblem 2 = Walker P 852.
A
74
kg skier encounters a dip in the snow's surface that
has a circular cross section with a radius of curvature of 12 m. If the skier's speed at point
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 Fall '08
 Staff
 Physics, Conservation Of Energy, Energy, Force, Potential Energy, m/s

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