7.34.
Model:
Blocks 1 and 2 make up the system of interest and will be treated as particles. Assume a
massless rope and frictionless pulley.
Visualize:
Solve:
The blocks accelerate with the same magnitude but in opposite directions. Thus the acceleration
constraint is
2
1
,
a
a
a
=
= −
where
a
will have a positive value. There are two real action/reaction pairs. The two
tension forces will act as if they are action/reaction pairs because we are assuming a massless rope and a
frictionless pulley. Make sure you understand why the friction forces point in the directions shown in the free
body diagrams, especially force
1
f
′
G
exerted on block 2 by block 1. We have quite a few pieces of information to
include. First, Newton’s second law for blocks 1 and 2:
(
)
net on 1
1
1
k
1
1
1
1
1
x
F
f
T
n
T
m a
m a
μ
=
−
=
−
=
= −
G
(
)
net on1
1
1
1
1
0 N
y
F
n
m g
n
m g
=
−
=
⇒
=
(
)
net on 2
pull
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 Fall '09
 geller
 Acceleration, Force, Friction, Mass, frictionless pulley, massless rope, 1.77 m/s

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