6
Chap.6
HW2
Questions
9.
In Figure 617, a horizontal force of 100 N is to be
applied to a 10 kg slab that is initially stationary on a
frictionless floor, to accelerate the slab.
A 10 kg block lies
on top of the slab; the coefficient of friction
μ
between the
block and the slab is not known, and the block might slip.
(a) Considering that possibility, what is the possible range of values for the magnitude of
the slab's acceleration
a
slab
?
(
Hint
: You don't need written calculations; just consider
extreme values for
.)
If
= 0,
a
slab
= F/m
slab
= 100/10 = 10 m/s
2
.
If
= max value, slab+ block stick together and,
a
slab
= F/(m
slab
+ m
block
) = 100/(10+10) =
5 m/s
2
.
Range of acceleration is 5 – 10 m/s
2
.
(b) What is the possible range for the magnitude
a
block
of the block's acceleration?
If
= 0,
a
block
= 0 since there is no force on the block.
.
If
= max value, slab+block stick together and so
a
block
is 5 m/s
2
.
Range of acceleration is 0 – 5 m/s
2
.
Hint: If the coefficient of static friction between the block and slab is large the block
accelerates with the slab and you can consider the two to be a single object. If the
coefficient of static friction is zero the block remains stationary and the slab accelerates
from under it.
10.
The path of a park ride that travels at constant
speed through five circular arcs of radii
R
0
, 2
R
0
, and
3
R
0
is shown.
Rank the arcs according to the
magnitude of the centripetal force on a rider
traveling in the arcs, greatest first.
Centripetal force =
mv
2
/
r
.
Since
v
is constant, we just need to focus on r.
The curve
with the smallest radius will have the largest centripetal force.
So the ranking is:
4,
3, and then 5, 1, 2 all tie for smallest.
Problems
28.
In Figure 635, blocks
A
and
B
have weights of 44
N and 22 N, respectively. (a) Determine the minimum
weight of block
C
to keep
A
from sliding if
s
between
A
and the table is 0.20. (b) Block
C
suddenly is lifted off
A
. What is the acceleration of block
A
if
k
between
A
and the table is 0.15?
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 Spring '08
 oshea
 Force, Friction

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