EWTON ’ S
Physics of the Life Sciences
, DOI: 10.1007/978-0-387-77259-2_2,
© Springer Science+Business Media, LLC 2008
Living cells exchange energy and matter with their surroundings. They reproduce.
Often they move about. To understand such basic aspects of life, it is essential
to understand how motion is related to force and how force is related to energy.
Explaining these relations for an object moving in one dimension is the goal of this
and the next two chapters.
Before beginning to read and master the formal discussion of motion that follows
in this chapter, however, it is very useful to remind ourselves what it feels like to
move at constant velocity and to accelerate. Recall how it feels to ride in a car along
a straight flat highway that has recently been resurfaced. If the car’s speedometer is
fixed at a constant reading you can close your eyes and not know you are moving at
all, no matter how fast the speedometer says you are moving. Of course, roads aren’t
straight and flat for very long stretches. You feel clues that you are moving from the
little bumps and turns the car makes. Riding in an elevator is probably a better exam-
ple. Once the elevator gets going, only the flashing floor numbers give any hint that
anything is happening, no matter how fast the elevator is traveling or whether you are
going up or down. In both car and elevator examples, when you feel as if you are at
rest you are moving in a straight line at a constant rate. This kind of motion is called
. Constant velocity feels exactly like standing still.
When the car turns or goes over a bump or speeds up or slows down, or when the
elevator starts or stops, you definitely feel it. All such instances involve change in
velocity. Change in velocity is called
and acceleration can be felt. If a trin-
ket dangles by a thread from the car’s rear view mirror you can see it deflect from hang-
ing vertically at the same instant you feel acceleration. If by some bizarre chance, you
are standing on a scale as the elevator starts or stops, the scale’s reading will change
when you feel the acceleration.
Why you feel acceleration but not constant velocity, why acceleration causes the
trinket to deflect and the scale reading to change, all require an explanation. That
explanation is contained in Newton’s laws of motion, discussed in this chapter. In
order to understand the content of Newton’s laws, we have to be able to describe
motion with quantitative precision. The major goal of this chapter is to demonstrate
how a body’s interactions with its surroundings can explain changes in its motion. We
use the term force to denote a quantitative measure of interaction. The theme of this
chapter, then, is that force explains (causes) acceleration. As discussed previously, any
macroscopic body is a collection of smaller, more fundamental pieces. A complete
understanding of the changes in motion of a macroscopic body requires keeping track