Oslund (hno56) – Homework 6 – sai – (58015)
1
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beFore answering.
001
(part 1 oF 2) 10.0 points
Two blocks,
A
and
B
, are connected by a
compressed spring. The mass oF block
A
is
twice that oF block
B
.
Immediately aFter
release, compare the net Force on block
A
to the net Force on block
B
.
The table is
Frictionless and the mass oF the spring is zero.
1.
F
A,net
and
F
A,net
are both directed toward
the leFt with
F
A,net
larger than
F
A,net
, but not
twice as large. Block
A
is pulling block
B
.
2.
F
A,net
is equal in magnitude and opposite
in direction to
F
B,net
, and not zero.
correct
3.
F
A,net
will be in the opposite direction
and halF as large as
F
B,net
.
4.
There is no net Force on block
A
or block
B
.
5.
F
A,net
is equal in magnitude and direction
to
F
B,net
, and not zero.
6.
F
B,net
will be in the opposite direction
and halF as large as
F
A,net
.
Explanation:
F
A,net
is the Force on block
A
From the
spring, it’s reaction Force is the Force acting
on the spring by block
A
, which we denote
as
F
S,A
. They have the same magnitude but
they are in opposite directions.
Similarly,
F
B,net
has a reaction Force
F
S,B
. The total
Force on the spring
±
F
S,net
=
±
F
S,A
+
±
F
S,B
=

±
F
A,net

±
F
B,net
=

±
±
F
A,net
+
±
F
B,net
²
=
m
spring
·
a
=0
·
a
.
So that
±
F
A,net
and
±
F
B,net
must have the
same magnitude but in opposite directions.
Note:
This is correct only when the mass
oF the spring can be neglected as mentioned
in the problem.
002
(part 2 oF 2) 10.0 points
Which oF the Following best describes the sit
uation aFter the blocks are released?
1.
The total momentum oF the system oF
blocks is conserved because there is no net
external Force.
correct
2.
The total momentum oF the system oF
blocks is not conserved because there is an
external velocity acting on the system.
3.
The total momentum oF the system is not
conserved because there are external Forces;
e.g.
, gravity.
4.
Momentum For any body is always con
served.
5.
The momentum oF block
A
is conserved
and the momentum oF block
B
is conserved.
However, the total momentum oF the system
oF blocks is not conserved.
6.
The momentum oF block
A
is conserved
and the momentum oF block
B
is conserved.
It Follows that the total momentum oF the
system oF blocks is also conserved.
Explanation:
Neither block
A
nor
B
conserves its mo
mentum since there is a Force on each oF them.
However, once we consider the two blocks and
the spring as a whole system, its momentum
is conserved since there is no external Force.
The contribution oF momentum From the
spring is zero since its mass is zero.
003
10.0 points
A 62.8 kg astronaut is on a space walk when
the tether line to the shuttle breaks.
The
astronaut is able to throw a 11.0 kg oxygen
tank in a direction away From the shuttle with
a speed oF 13.7 m/s, propelling the astronaut
back to the shuttle.
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View Full DocumentOslund (hno56) – Homework 6 – sai – (58015)
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Assuming that the astronaut starts from
rest, Fnd the Fnal speed of the astronaut after
throwing the tank.
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 Spring '08
 Kaplunovsky
 Physics, Kinetic Energy, Momentum, Work, m/s

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