Physics 140  Discussion 05  Solutions
4.20.
I
DENTIFY
:
Weight and mass are related by
.
w
mg
=
The mass is constant but
g
and
w
depend
on location.
S
ET
U
P
:
On earth,
2
9 80 m/s
.
g
=
.
E
XECUTE
:
(a)
,
w
m
g
=
which is constant, so
E
A
E
A
.
w
w
g
g
=
E
17 5 N,
w
=
.
2
E
9 80 m/s
,
g
=
.
and
A
3 24 N.
w
=
.
2
2
A
A
E
E
3 24 N
(9 80 m/s
)
1 81 m/s
.
17 5 N
w
g
g
w
.
=
=
.
= .
.
(b)
E
2
E
17 5 N
1 79 kg.
9 80 m/s
w
m
g
.
=
=
= .
.
E
VALUATE
:
The weight at a location and the acceleration due to gravity at that location are
directly proportional.
__________________________________________________________________________________
4.29.
I
DENTIFY
:
Since the observer in the train sees the ball hang motionless, the ball must have the
same acceleration as the train car. By Newton’s second law, there must be a net force on the
ball in the same direction as its acceleration.
S
ET
U
P
:
The forces on the ball are gravity, which is
w
, downward, and the tension
T
r
in the
string, which is directed along the string.
E
XECUTE
:
(a)
The acceleration of the train is zero, so the acceleration of the ball is zero.
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 Winter '10
 Uher
 Acceleration, Force, Mass, m/s

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