PHYS1401 PHYS1401 College Physics Instructor Solutions Manual Chapter 4 2026

College Physics Instructor Solutions Manual Chapter 4


CHAPTER 4: DYNAMICS:
FORCE AND NEWTON’S LAWS
OF MOTION 4.3 NEWTON’S SECOND LAW OF
MOTION: CONCEPT OF A SYSTEM

1.
Solution 2.
Solution
A 63.0Gkg sprinter starts a race with an acceleration of 4.20 m/s 2 .
What is the net external force on him?
net F = ma = (63.0 kg)(4.20 m/s2) = 265 N
If the sprinter from the previous problem accelerates at that rate
for 20 m, and then maintains that velocity for the remainder of
the 100Gm dash, what will be his time for the race?

Running from rest, runner attains a velocity of v2 =v2
+2ax=0+2(4.20m/s2)(20m)

0or v 12.96 m/s at end of acceleration.

3.
Solution

,Total time 3.09 s 6.17 s 9.26 s (a record!) A cleaner pushes a
4.50Gkg laundry cart in such a way that the net external force on
it
is 60.0 N. Calculate its acceleration.
2
a=netF/m=60.0N/4.50kg=13.3m/s of the cart. 103


2
Find the time for this first part using x = 20 m 0 0.5at , or t 11

3.09s. For the second part, with no acceleration, x2 vt2, or t2
x2/v80m/12.96m/s6.17s.

Copyright 2011-2013, Rice University. All Rights Reserved.

College Physics Instructor Solutions Manual Chapter 4

4. Since astronauts in orbit are apparently weightless, a clever
method of measuring their masses is needed to monitor their
mass gains or losses to adjust diets. One way to do this is to exert
a known force on an astronaut and measure the acceleration
produced. Suppose a net external force of 50.0 N is exerted and
the astronaut’s acceleration is measured to be 0.893 m/s2 . (a)
Calculate her mass. (b) By exerting a force on the astronaut, the
vehicle in which they orbit experiences an equal and opposite
force. Discuss how this would affect the measurement of the
astronaut’s acceleration. Propose a method in which recoil of the
vehicle is avoided.

Solution netF 50.0N
(a) m= = 56.0kg

0.893 m/s2

,a (b) a a
mastroaastro mship
 .

a If the force could be exerted on the astronaut by another
source (other than the
5. In Figure 4.7, the net external force on the 24Gkg mower is
stated to be 51 N. If the force of friction opposing the motion is 24
N, what force F (in newtons) is the person exerting on the mower?
Suppose the mower is moving at 1.5 m/s when the force F is
removed. How far will the mower go before stopping?
,where:a

astro


spaceship), then the spaceship would not experience a recoil.
meas

ship

ship


f
Solution netFFf FnetFf 51N24N75N Fma
a
24N
 1.0m/s2 sousingv2 v2 2ax





2
0 01.5m/s 21.0m/s2 x
x1.125m1.1m

, m 24kg 

6. The same rocket sled drawn in Figure 4.31 is decelerated at a
rate of 196 m/s2 . What force is necessary to produce this
deceleration? Assume that the rockets are off. The mass of the
system is 2100 kg.
104



Copyright 2011-2013, Rice University. All Rights Reserved.

College Physics Instructor Solutions Manual Chapter 4 Solution net F ma
2 5
(2100 kg)(196 m/s ) 4.12 10 N

7. (a) If the rocket sled shown in Figure 4.32 starts with only one
rocket burning, what is its acceleration? Assume that the mass of
the system is 2100 kg, and the force of friction opposing the
motion is known to be 650 N. (b) Why is the acceleration not
oneGfourth of what it is with all rockets burning?
Solution (a) Use the thrust given for the rocket sled in Figure 4.8,
4
T 2.5910 N. With only one rocket burning, net F T f or

netF Tf 2.59104 N650N 2 m m 2100 kg
a   12m/s

(b) The acceleration is not one[fourth of what it was with all
rockets burning because the frictional force is still as large as it
was with all rockets burning.
8. What is the deceleration of the rocket sled if it comes to rest in
1.1 s from a speed of 1000 km/h? (Such deceleration caused one
test subject to black out and have temporary blindness.)