Physics 211 Test 2 spring 2008
Some answers are given with
three significant figures for clarity.
1.
Body
A
weighs 211 N, and body
B
weighs 75 N. The coefficients of friction between
A
and the incline
are µ
s
= 0.56 and µ
k
= 0.25. Angle
θ
is 40°.
Find the acceleration of
A
if
A
is moving down the incline.
Assume the acceleration, a, is in the positive direction in both freebody diagrams.
2.
In the figure, blocks
A
and
B
have weights of 48 N and 37 N, respectively.
(b) Block
C
suddenly is lifted off
A
. What is the acceleration of block
A
, if µ
k
between
A
and the table is
0.10?
(a) Determine the minimum
weight of block
C
to keep
A
from
sliding, if µ
s
between
A
and the
table is 0.30.
F
N
 (48 N + W
C
) = 0
F
T
 f
SMax
= 0
F
T
– 37 N = 0
f
SMax
=
S
F
N
W
C
= 75
F
N
F
T
F
T
(48 N
f
SMax
37 N
a = 0
a = 0
F
N
F
T
F
T
48 N
f
k
37 N
a
a
F
N
 48 N
= 0
F
T
– f
k
= M
A
a
37 N
 F
T
= M
B
a
f
k
=
k
F
N
a = 3.72 m/
M
A
= 48N/g
M
B
= 37N/g
θ
+y
+x
F
N
F
T
f
k
W
A
F
T
W
B
+y
W
A
Sin
θ
 f
k
– F
T
= (W
A
/g)a
and F
N
 W
A
Cos
θ
= 0
and
f
k
= .25F
N
for
A
F
T
– W
B
= (W
B
/g)a
for B
The result of these equations is a = 0.69 m/s down the incline for A
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The figure below shows an initially stationary block of mass
m
on a floor. A force of magnitude
0.570
mg
is then applied at upward angle
θ
= 21°.
What is the magnitude of the acceleration of the block across the floor if
s
= 0.600 and
k
= 0.400?
FSin
θ
+ F
N
–
mg
= 0
FCos
θ
 f =
m
a
Assuming f = .
s
F
N,
results in a i
indicating that there is motion; thus we must use f =
k
F
N
This results in a = 2.1 m/s
2
4.
A 2.5 kg block
is pushed along a horizontal floor by a force
F
of magnitude 34 N at an angle
θ
= 42
o
with the horizontal. The coefficient of kinetic friction between the block and floor is 0.25.
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 Spring '08
 staff
 mechanics, Force, Friction, Mass, FN

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