Physics homework

need question     8, 22, 25 and 31 answer and must show work … need it in 4 hour no extension 

1 A cyclist moves at a constant speed of 15 mi/h. Ex-
press the speed in units of ft/s.

2 How long is required to travel 300 km along a freeway
at a constant speed of (a) 50.0 km/h; (b) 60.0 km/h?

3 Light travels at a constant speed of 3.00 3 108 m/s,
whereas sound travels through the air at a constant
speed of 340 m/s.
(a) How long does it take for light to travel from a

lightning stroke to an observer 1.00 km away?
(b) How long after the flash is seen is the thunder pro-

duced by the lightning heard?
4 The driver of a car is initially moving at a constant

speed of 72.0 km/h when a traffic light turns red. If
0.500 s reaction time is required before the brakes can
be applied, what is the distance in meters traveled by
the car before it begins to slow down?

5 A person on earth communicating with an astronaut
on the moon asks a question. How long must the per-
son on earth wait for a response if the astronaut an-
swers 1.00 s after the message is received? The moon
is 3.84 3 105 km from the earth, and the speed of ra-
dio waves is 3.00 3 108 m/s.

6 External stimuli are communicated to the brain by
means of electrical signals propagating along nerve
cells at a speed of approximately 30 m/s. Similarly,
electrical messages are sent at the same speed from
the brain along nerve cells to the muscles. Reflex ac-
tions are controlled by a relatively simple nerve circuit
from a muscle to the spine and back to the muscle. Es-
timate the reflex time for a stimulus at the knee.

7 The world record in the 100 yard dash is 9.1 s. Esti-
mate the world record in the 100 m dash.

Average speed
8 Compute the average speed in m/s of a runner who

completes a mile in 4.00 min.
9 Blood circulating from the heart to the hands and back

to the heart travels a distance of about 2.0 m in 40 s.
Find the average speed of the blood.

10 You drive from Los Angeles to Portland, a distance of
1.6 3 103 km, in 30 h, including stops. What steady
speed would have allowed you to arrive at the same
time if you had driven at this speed nonstop?

11 During 5 successive 1.00 min intervals, a runner
moves at the following constant speeds: 0.400
km/min, 0.240 km/min, 0.160 km/min, 0.160
km/min, and 0.320 km/min. Compute the total dis-
tance traveled and the average speed.

12 A tortoise and a hare race over a course 1.00 km long.
The tortoise moves at a constant speed of 2.00 m/s.
The hare moves at a speed of 10.0 m/s for 60.0 s, rests
for 10.0 min, and continues at 10.0 m/s for 40.0 s.
(a) Sketch s versus t for both the tortoise and the hare

on the same graph.
(b) Who wins the race?
(c) What is the hare’s average speed?

Instantaneous speed
13 A plane accelerates down a runway. Markers beside

the runway are 10.0 m apart. The plane moves be-
tween two adjacent markers in 0.200 s. Estimate the
instantaneous speed of the plane as it passes the first
marker.

14 By analyzing a multiflash photograph of a golfer hit-
ting a golf ball, one finds that the head of the club trav-
els 50.0 cm in 0.0100 s, just before it strikes the ball.
Find the approximate instantaneous speed of the club
head at the instant of contact.

15 Let s 5 ct3, where c 5 1.0 m/s3. Compute the average
speed over the time intervals Dt1 5 1.0 3 10]1 s, Dt2 5
1.0 3 10]2 s, and Dt3 5 1.0 3 10]3 s, with all time in-
tervals starting at the time t 5 1.0000 s. What is the in-
stantaneous speed at t 5 1.0000 s?

16 Paradoxes relating to the nature of motion were for-
mulated by the ancient Greeks. The paradoxes were
not resolved because the Greeks had no clear under-
standing of concepts like instantaneous speed. One of
Zeno’s paradoxes can be stated as follows: a runner
wishing to run 100 meters must first cover half that
distance. But before he can cover the 50 meters, he
must first travel 25 meters, and so on. It is clear that
before the runner can travel 100 meters, or any finite
distance, he must first move through an infinite num-
ber of shorter distances, and this he can never do in a
finite time. Resolve this paradox.

Problems 37

Problems (listed by section)

Speed

Uniform motion

1-2

★

★

★★

★

Displacement

17 A traveler flies from Spokane to Atlanta by way of
Los Angeles. Los Angeles is approximately 1600 km
due south of Spokane, and Atlanta is approximately
3200 km due east of Los Angeles (Fig. 1-32). Find the
traveler’s total displacement.

18 A traveler first drives 20.0 km east, then 30.0 km
southeast, and finally 10.0 km south. Find the trav-
eler’s total displacement.

Vector Algebra

Graphical vector addition

19 Four forces each have a magnitude of 50 lb. Their re-
spective directions are north, south, east, and west.
Find the resultant force.

20 A 40 lb force acts to the right, while a 100 lb force acts
at an angle of 458 above the first. Construct the vector
sum of the two forces graphically, using a convenient
scale.

21 Electric fields are vector quantities whose magnitudes
are measured in units of volts/meter (V/m). Find the
resultant electric field when there are two fields, E1
and E2, where E1 is directed vertically upward and has
magnitude 100 V/m and E2 is directed 458 to the left of
E1 and has magnitude 150 V/m.

22 Find the resultant of the vectors shown in Fig. 1-33.
23 Find the resultant of the vectors shown in Fig. 1-34.

Graphical vector subtraction
24 A force of magnitude 20.0 lb directed toward the right

is exerted on an object. What other force must be ap-
plied to the object so that the resultant force is zero?

25 The forces shown in Fig. 1-35 are applied to an object.
Find the third force that will produce a zero resultant
force.

26 A force F1 of magnitude 100 lb is directed toward the
right. Find a second force F2 that will produce a resul-
tant force of 200 lb to the left.

27 A force F1 of magnitude 100 lb is directed vertically
upward. Find a second force F2 that will produce a re-
sultant force of 141 lb, 45.08 to the right of F1.

28 A certain vector A, of magnitude 5.00 units, points in
a direction 30.08 to the left of the negative y-axis. Find
Ax and Ay.

29 Find the x and y components of the force vector shown
in Fig. 1-36.

30 Find the x and y components of the force shown in
Fig. 1-37.

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1-4

CHAPTER 1 Description of Motion38

Fig. 1-32

Fig. 1-33 Fig. 1-34

Fig. 1-35

Components of Vectors

Finding components

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