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force concepts, and textbooks, including this one, almost universally place
the force description of physics before the energy description. During the
long period after the introduction of force and before the careful definition
of energy, students are therefore vulnerable to situations in which, without
realizing it, they are imputing the properties of energy to phenomena of
force.
Incorrect statement: “How can my chair be making an upward
force on my rear end. It has no power!”
# Power is a concept related to energy, e.g. 100-watt lightbulb
uses up 100 joules per second of energy. When you sit in a chair,
no energy is used up, so forces can exist between you and the
chair without any need for a source of power.
Force is not stored or used up.
Because energy can be stored and used up, people think force also can
be stored or used up.
Incorrect statement: “If you don’t fill up your tank with gas, you’ll
run out of force.”
# Energy is what you’ll run out of, not force.
Forces need not be exerted by living things or machines.
Transforming energy from one form into another usually requires some
kind of living or mechanical mechanism. The concept is not applicable to
forces, which are an interaction between objects, not a thing to be trans-
ferred or transformed.
Incorrect statement: “How can a wooden bench be making an
upward force on my rear end. It doesn’t have any springs or
anything inside it.”
# No springs or other internal mechanisms are required. If the
bench didn’t make any force on you, you would obey Newton’s
second law and fall through it. Evidently it does make a force on
you!
A force is the direct cause of a change in motion.
I can click a remote control to make my garage door change from being
at rest to being in motion. My finger’s force on the button, however, was
not the force that acted on the door. When we speak of a force on an object
in physics, we are talking about a force that acts directly. Similarly, when
you pull a reluctant dog along by its leash, the leash and the dog are making
forces on each other, not your hand and the dog. The dog is not even
touching your hand.
Self-Check
Which of the following things can be correctly described in terms of force.
(a) A nuclear submarine is charging ahead at full steam.
(b) A nuclear submarine’s propellers spin in the water.
(c) A nuclear submarine needs to refuel its reactor periodically.
Discussion questions
A. Criticize the following incorrect statement: “If you shove a book across a
table, friction takes away more and more of its force, until finally it stops.”
B. You hit a tennis ball against a wall. Explain any and all incorrect ideas in the
following description of the physics involved: “The ball gets some force from
you when you hit it, and when it hits the wall, it loses part of that force, so it
doesn’t bounce back as fast. The muscles in your arm are the only things that
a force can come from.”
(a) This is motion, not force. (b) This is a description of how the sub is able to get the water to produce a forward
force on it. (c) The sub runs out of energy, not force.
Section 4.4What Force Is Not