CHAPTER 4 FORCES AND NEWTON'S LAWS
ANSWERS TO FOCUS ON CONCEPTS QUESTIONS
1. (b) If only one force acts on the object, it is the net force; thus, the net force must be nonzero.
Consequently, the velocity would change, according to Newtons first
The City College of New York
Department of Physics
Physics 20300 - Sections ST, ST2, ST3
Prof. V. Petricevic:
office: MR423B; 212-650-5550;
Physics, (10th ed.) by Cutnell & Johnson (Wiley)
Town A lies 20 km north of town B. Town C lies 13 km west of town A. A small plane
flies directly from town B to town C. What is the displacement of the plane?
A) 33 km, 33 north of west
B) 19 km, 33 north of west
C) 24 km, 57 north of west
D) 31 km
Prof. J. Gersten
Lab Report #1: Measurements
The main idea behind the first lab was to help us better understand how to obtain
accurate measurements to be used in calculations. The measurement
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Homework #1 Solutions
1. (If you didnt do this last week because of the impending test,
try it now.) The figure shows an oddly shaped pipe, which is 2 cm
thick everywhere. Water flows in from the left, moving at 10 m/s.
4. If we make the units explicit, the function is
2.0 rad 4.0 rad/s 2 t 2 2.0 rad/s3 t 3
but in some places we will proceed as indicated in the problemby letting these units be
(a) We evaluate the function at t = 0 to obtain 0 = 2.
2. Our notation is as follows: x1 = 0 and y1 = 0 are the coordinates of the m1 = 3.0 kg
particle; x2 = 2.0 m and y2 = 1.0 m are the coordinates of the m2 = 4.0 kg particle; and x3 =
1.0 m and y3 = 2.0 m are the coordinates of the m3 = 8.0 kg par
1. The potential energy stored by the spring is given by U 21 kx , where k is the spring
constant and x is the displacement of the end of the spring from its position when the
spring is in equilibrium. Thus
b g 8.9 10 N m .
2 25 J
1. (a) Since the gas is ideal, its pressure p is given in terms of the number of moles n, the
volume V, and the temperature T by p = nRT/V. The work done by the gas during the
isothermal expansion is
W p dV n RT
n RT ln 2 .
2. The initial speed of the car is
v 80 km/h (1000 m/km)(1 h/3600 s) 22.2 m/s .
The tire radius is R = 0.750/2 = 0.375 m.
(a) The initial speed of the car is the initial speed of the center of mass of the tire, so Eq.
11-2 leads to
3. In solving the ideal-gas law equation pV = nRT for n, we first convert the temperature
to the Kelvin scale: Ti (40.0 273.15) K 313.15 K , and the volume to SI units:
Vi 1000 cm3 103 m 3 .
(a) The number of moles of oxygen present is