Chapter 7 solved problems
24.
Picture the Problem
: The pine cone falls straight down under
the influence of gravity.
Strategy:
The work done by gravity equals the change in kinetic
energy according to equation 77.
The work done by gravity is
always
W = mgh
as indicated in Example 72 and Conceptual
Checkpoint 71.
Solution:
1. (a)
The
work
done by
gravity on
the pine
cone
equals the
increase
in its
kinetic
energy.
Set the
energies
equal and
solve for
v
:
(
29
(
29
2
1
2
2
2
2 9.81 m/s
16 m
18 m/s
W
K
mgh
mv
v
gh
= ∆
=
=
=
=
=
2. (b)
Air resistance did negative work because the speed and
therefore the kinetic energy of the pine cone when it landed was
reduced.
Air resistance removed energy from the pine cone.
Insight:
Kinetic friction always does negative work because the
force is always opposite to the direction of motion.
33.
Picture the Problem
: The compressed spring pushes
the block from rest horizontally on a frictionless
surface.
The block slides to the left as indicated in
the figure.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview. Sign up
to
access the rest of the document.
 Spring '11
 BEAN
 pH

Click to edit the document details