fisica1-youn-e-freedman-exercicios-resolvidos

2.6: The s-waves travel slower, so they arrive 33 s after the p-waves.
d
3.5
t
s
d
d
v
s
km
s
d
= t
=
p
p
d
=
6.5
+ 33s
= vt → t =
d
v
+ 33s
km
s
= 250km
d
v
+ 33s
2.7: a) The van will travel 480 m for the first 60 s and 1200 m for the next 60 s, for a
total distance of 1680 m in 120 s and an average speed of 14 .0 m s.
b) The first stage of
240
the journey takes = 30 s and the second stage of the journey takes
m
8.0 m s
( 240 m 20 m s) = 12 s, so the time for the 480-m trip is 42 s, for an average speed of
11.4 m s. c) The first case (part (a)); the average speed will be the numerical average
only if the time intervals are the same.
2.8: From the expression for x(t), x(0) = 0, x(2.00 s) = 5.60 m and x(4.00 s) = 20.8 m. a)
5.60 m−0
20 .8 m 0
= 2.80 m s b) − = 5.2 m s c) 20.8 m−5.60
m
= 7.6 m s
2.00 s
4.00 s
2.00 s
2.9: a) At t = , x 0 , so Eq (2.2) gives
v
1
0
1
=
2
2
3
3
x2
(2.4 m s )(10.0 s) − (0.120 m s )(10.0 s)
av
= =
=
t
2
(10.0 s)
b) From Eq. (2.3), the instantaneous velocity as a function of time is
12.0 m s.
v x
= 2bt
− 3ct
2
= (4.80 m
2
2
s ) t − (0.360 m s ) t ,
3
so i) v ( 0) = 0,
x
2
2
ii) v (5.0 s) = (4.80 m s )(5.0 s) − (0.360 m s )(5.0 s) = 15.0 m s,
x
2
2
and iii) v (10.0 s) = (4.80 m s )(10.0 s) − (0.360 m s )(10.0 s) = 12.0m s.
x
2
c) The car is at rest when v = 0 . Therefore (4.80 m s ) t − (0.360 m s ) t = 0 . The
x
4.80 m s
only time after t = 0 when the car is at rest is t = = 13.3s
3
3
2
3
0.360 m s
2
3