Solutions05 - Physics 140 Discussion 05 Solutions 4.20...

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.