Today, let's talk about momentum. The University of
Houston's College of Engineering presents this
series about the machines that make our
civilization run, and the people whose ingenuity
In another episode I talk
about a museum docent who didn't understand the
role of momentum in a demonstration she ran for
school children. When my wife saw that, she said,
"Look, a whole lot of people like me never learned
about momentum. If it's important, why don't you
explain it in a program?"
That's a tough challenge because we all have at
least a gut sense of the concept. But what would
you do if you had to define momentum? The
dictionary calls it the impetus of a moving body --
not much help there! Your gut sense is probably
more accurate. When you say something gains
momentum, you mean it's increasingly hard to stop.
And that's absolutely correct.
In a physics class, we're told that you calculate
the momentum of, say, an automobile by multiplying
its velocity by its mass. So, to understand
momentum we have to understand mass. Suppose you
have a six-pound rock. That rock would only weigh
one pound on the moon but its mass would be the
same as it is on Earth.
Years ago, when I first studied physics, I came
home terribly frustrated trying to understand mass
and momentum. I complained to my father that I
couldn't see how mass differed from weight.
So he picked up a paper weight and tossed it to me.
"What happens when you catch it," he asked. "It
pushes my hand back," I said. "Okay," he went on,
"How much would that weigh on the moon?" "Only a
sixth as much," I answered, wondering where he was
headed. "All right then, what would you feel if I
tossed it to you on the moon?" And, just like that,
the scales fell from my eyes.
Of course! Matter has a property independent of its
weight. Catching a paper weight would feel just the
same on the moon as it would in my living room. A
six-pound weight is just the reaction of a certain
mass to Earth's gravity. That object would have
different weights on Jupiter or Mars. But it'd
still have the same mass -- even in outer space
where it weighs nothing at all.
Now, back to momentum. Put an object in motion, and
you give that mass a momentum which is hard to stop
or deflect. The momentum of a spinning skater can
change form. With her arms outstretched, she spins
slowly, but the outstretched tips of her fingers
are really moving quite fast. With arms drawn
inward, she spins faster, but the tips of her
indrawn elbows move fairly slowly. What stays about
the same is her net momentum.
Years ago, a construction worker told of a friend
working on the side of a building. A wrecking ball
swung toward him -- gently and slowly. Instead of
getting out of its way, he reached out to stop it.
He didn't realize that, with its enormous mass, it
also had enormous momentum even if it was moving
slowly. The result? It slowly crushed him -- left
him lame. If he'd only had a proper gut sense of
basic physics -- he might still be walking today.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds