Problems
141
Section 5.7 Some Applications of Newton’s Laws
16.
A 3.00-kg object is moving in a plane, with its x and y coor-
dinates given by x " 5t
2
#
1 and y " 3t
3
$
2, where x and
y are in meters and t is in seconds. Find the magnitude of
the net force acting on this object at t " 2.00 s.
17.
The distance between two telephone poles is 50.0 m.
When a 1.00-kg bird lands on the telephone wire midway
between the poles, the wire sags 0.200 m. Draw a free-body
diagram of the bird. How much tension does the bird pro-
duce in the wire? Ignore the weight of the wire.
18.
A bag of cement of weight 325 N hangs from three wires as
suggested in Figure P5.18. Two of the wires make angles
!
1
"
60.0° and !
2
"
25.0° with the horizontal. If the sys-
tem is in equilibrium, find the tensions T
1
, T
2
, and T
3
in
the wires.
21. The systems shown in Figure P5.21 are in equilibrium. If
the spring scales are calibrated in newtons, what do they
read? (Neglect the masses of the pulleys and strings, and
assume the incline in part (c) is frictionless.)
22. Draw a free-body diagram of a block which slides down a
frictionless plane having an inclination of ! " 15.0°
(Fig. P5.22). The block starts from rest at the top and the
length of the incline is 2.00 m. Find (a) the acceleration of
1
θ
2
θ
T
1
T
2
T
3
Figure P5.18 Problems 18 and 19.
A bag of cement of weight F
g
hangs from three wires as
shown in Figure P5.18. Two of the wires make angles !
1
and !
2
with the horizontal. If the system is in equilibrium,
show that the tension in the left-hand wire is
T
1
"
F
g
cos !
2
/sin (!
1
$
!
2
)
20.
You are a judge in a children’s kite-flying contest, and two
children will win prizes for the kites that pull most strongly
and least strongly on their strings. To measure string ten-
sions, you borrow a weight hanger, some slotted weights,
and a protractor from your physics teacher, and use the
following protocol, illustrated in Figure P5.20: Wait for a
child to get her kite well controlled, hook the hanger onto
the kite string about 30 cm from her hand, pile on weight
until that section of string is horizontal, record the mass
required, and record the angle between the horizontal
and the string running up to the kite. (a) Explain how this
method works. As you construct your explanation, imagine
that the children’s parents ask you about your method,
that they might make false assumptions about your ability
without concrete evidence, and that your explanation is an
opportunity to give them confidence in your evaluation
19.
Figure P5.20
5.00 kg
(a)
5.00 kg
5.00 kg
5.00 kg
(b)
5.00 kg
(c)
30.0
°
Figure P5.21
technique. (b) Find the string tension if the mass is 132 g
and the angle of the kite string is 46.3°.