6
Chap. 8 HW2
6.
In Figure 825, a block slides along a track
that descends through distance
h
.
The track is
frictionless except for the lower section. There
the block slides to a stop in a certain distance
D
because of friction.
(a) If we decrease
h
, will the block now slide to
a stop in a distance that is greater than, less
than, or equal to
D
?
Decreasing
h
will decrease the KE of the block just before it hits the friction section..
Therefore it will take less work to stop it.
So the block will slide a smaller distance than
D
(since
W = Fd
)
.
(b) If, instead, we increase the mass of the block, will the stopping distance now be greater
than, less than, or equal to
D
?
Increasing the mass of the block will increase the KE after it slides by a certain factor
(KE
∝
mass).
The friction force (and therefore work done) is increased by the same
factor since
F
N
KE
∝
mass.
Therefore the block will slide the same distance.
Problems
28.
Figure 840 shows an 8.00 kg stone at rest on a spring.
The
spring is compressed 10.0 cm by the stone.
(a) What is the spring
constant?
(b) The stone is pushed down an additional 30.0 cm and released.
What is the elastic potential energy of the compressed spring just
before that release?
(c) What is the change in the gravitational
potential energy of the stone–Earth system when the stone moves from the release point to
its maximum height?
(d) What is that maximum height, measured from the release point?
a) Know that
kx
F
−
=
.
The spring force
F
supporting the stone is
mg
= 78.4 N and
the compression x is 0.10 m.
Therefore
k = 78.4/0.10 = 784 N/m.
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 '08
 oshea
 Energy, Force, Friction, Potential Energy, favg

Click to edit the document details