Chapter 3 Acceleration and free fall - Light and Matter

it must be zero.
128 Chapter 4 Force and motion
In a future physics course or in another textbook, you may encounter
the term “net force,” which is simply a synonym for total
force.
What happens if the total force on an object is not zero? It
accelerates. Numerical prediction of the resulting acceleration is the
topic of Newton’s second law, which we’ll discuss in the following
section.
This is the first of Newton’s three laws of motion. It is not
important to memorize which of Newton’s three laws are numbers
one, two, and three. If a future physics teacher asks you something
like, “Which of Newton’s laws are you thinking of?,” a perfectly
acceptable answer is “The one about constant velocity when there’s
zero total force.” The concepts are more important than any specific
formulation of them. Newton wrote in Latin, and I am not
aware of any modern textbook that uses a verbatim translation of
his statement of the laws of motion. Clear writing was not in vogue
in Newton’s day, and he formulated his three laws in terms of a concept
now called momentum, only later relating it to the concept of
force. Nearly all modern texts, including this one, start with force
and do momentum later.
An elevator example 1
⊲ An elevator has a weight of 5000 N. Compare the forces that the
cable must exert to raise it at constant velocity, lower it at constant
velocity, and just keep it hanging.
⊲ In all three cases the cable must pull up with a force of exactly
5000 N. Most people think you’d need at least a little more than
5000 N to make it go up, and a little less than 5000 N to let it down,
but that’s incorrect. Extra force from the cable is only necessary
for speeding the car up when it starts going up or slowing it down
when it finishes going down. Decreased force is needed to speed
the car up when it gets going down and to slow it down when it
finishes going up. But when the elevator is cruising at constant
velocity, Newton’s first law says that you just need to cancel the
force of the earth’s gravity.
To many students, the statement in the example that the cable’s
upward force “cancels” the earth’s downward gravitational force implies
that there has been a contest, and the cable’s force has won,
vanquishing the earth’s gravitational force and making it disappear.
That is incorrect. Both forces continue to exist, but because they
add up numerically to zero, the elevator has no center-of-mass acceleration.
We know that both forces continue to exist because they
both have side-effects other than their effects on the car’s center-ofmass
motion. The force acting between the cable and the car continues
to produce tension in the cable and keep the cable taut. The