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.
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., a 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 transferred 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.”154 Chapter 3 Conservation of Momentum