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(Due Date: 02/09/2012)
Homework # 2, PHYS 121
Spring 2012
Mastering Physics: P2.12, P2.15, P2.29, P2.65, P2.69, P2.59, P2.70, 2.16
P2.12.
Prepare:
Assume that Richard only speeds on the 125 mi stretch of the interstate. We then need to compute the
times that correspond to two different speeds for that given distance. Rearrange Equation 1.1 to produce
distance
time
speed
Solve:
At the speed limit:
1
125 mi
60 min
time
115.4 min
65 mi/h
1h
At the faster speed:
2
125 mi
60 min
time
107.1min
70 mi/h
By subtracting we see that Richard saves 8.3 min.
Assess:
Breaking the law to save 8.3 min is normally not worth it; Richard’s parents can wait 8 min.
Notice how the hours (as well as the miles) cancel in the equations.
P2.15.
Prepare
:
Assume
x
v
is constant so the ratio
x
t
is also constant.
Solve:
(a)
30 m
30 m
=
=1.5 s
=15 m
3.0 s
1.5 s
3.0 s
x
x
(b)
30 m
30 m
=
= 9.0 s
= 90 m
3.0 s
9.0 s
3.0 s
x
x
Assess:
Setting up the ratio allows us to easily solve for the distance traveled in any given time.
P2.16.
Prepare:
Assume
x
v
is constant so the ratio
x
t
is also constant.
Solve:
(a)
100 m
400 m
400 m
=
=18 s
= 72 s
18 s
100 m
t
t
(b)
100 m
1.0 mi
1.0 mi
1609 m
=
= 290 s = 4.8 min
18 s
100 m
1.0 mi
t
t
Assess:
This pace does give about the right answer for the time required to run a mile for a good marathoner.
P2.29.
Prepare:
We’ll do this in parts, first computing the acceleration after the congestion.
Solve:
12.0 m/s
5.0 m/s
7.0 m/s
=
=
=
8.0 s
8.0 s
v
a
t
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(Due Date: 02/09/2012)
Homework # 2, PHYS 121
Spring 2012
Now use the same acceleration to find the new velocity.
2
fi
7.0
=
=12.0 m/s
m/s
(16 s) = 26 m/s
8.0
v
v
a t
Assess:
The answer is a reasonable 58 mph.
P2.59.
Prepare:
Fleas are amazing jumpers; they can jump several times their body height—something we cannot do.
We assume constant acceleration so we can use the equations in Table 2.4. The last of the three relates the three variables
we are concerned with in part
(a)
: speed, distance (which we know), and acceleration (which we want).
22
(
) = (
)
2
y
y
y
v
v
a
y
In part
(b)
we use the first equation in Table 2.4 because it relates the initial and final velocities and the acceleration
(which we know) with the time interval (which we want).
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