108
Discussion questions
A. Show that the Newton can be reexpressed in terms of the three basic mks
units as the combination kg
.
m/s
2
.
B. What is wrong with the following statements.
1. “g is the force of gravity.”
2. “Mass is a measure of how much space something takes up.”
C. Criticize the following incorrect statement:
“If an object is at rest and the total force on it is zero, it stays at rest.
There can also be cases where an object is moving and keeps on moving
without having any total force on it, but that can only happen when there’s
no friction, like in outer space.”
D. The table on the left gives laser timing data for Ben Johnson’s 100 m dash
at the 1987 World Championship in Rome. (His world record was later revoked
because he tested positive for steroids.) How does the total force on him
change over the duration of the race.
4.4What Force Is Not
Violin teachers have to endure their beginning students’ screeching. A
frown appears on the woodwind teacher’s face as she watches her student
take a breath with an expansion of his ribcage but none in his belly. What
makes physics teachers cringe is their students’ verbal statements about
forces. Below I have listed several dicta about what force is not.
Force is not a property of one object.
A great many of students’ incorrect descriptions of forces could be cured
by keeping in mind that a force is an interaction of two objects, not a
property of one object.
Incorrect statement: “That magnet has a lot of force.”
# If the magnet is one millimeter away from a steel ball bearing,
they may exert a very strong attraction on each other, but if they
were a meter apart, the force would be virtually undetectable.
The magnet’s strength can be rated using certain electrical units
(ampere-meters
2
), but not in units of force.
Force is not a measure of an object’s motion.
If force is not a property of a single object, then it cannot be used as a
measure of the object’s motion.
Incorrect statement: “The freight train rumbled down the tracks
with awesome force.”
# Force is not a measure of motion. If the freight train collides
with a stalled cement truck, then some awesome forces will
occur, but if it hits a fly the force will be small.
Force is not energy.
There are two main approaches to understanding the motion of objects,
one based on force and one on a different concept, called energy. The SI
unit of energy is the Joule, but you are probably more familiar with the
calorie, used for measuring food’s energy, and the kilowatt-hour, the unit
the electric company uses for billing you. Physics students’ previous famil-
iarity with calories and kilowatt-hours is matched by their universal unfa-
miliarity with measuring forces in units of Newtons, but the precise opera-
tional definitions of the energy concepts are more complex than those of the
Discussion question D.
x (m)t (s)
101.84
202.86
303.80
404.67
505.53
606.38
707.23
808.10
908.96
1009.83
Chapter 4Force and Motion