Using Physics to Understand Your World

Studying motion is fine, but it’s just the very beginning of the beginning. When
you take a look around, you see that the motion of objects changes all the
time. You see a motorcycle coming to a halt at the stop sign. You see a leaf
falling and then stopping when it hits the ground, only to be picked up again
by the wind. You see a pool ball hitting other balls in just the wrong way so
that they all move without going where they should.
Motion changes all the time as the result of force, which is what Part II is all
about. You may know the basics of force, but sometimes it takes an expert to
really know what’s going on in a measurable way. In other words, sometimes
it takes a physicist like you.


Absorbing the Energy Around You

You don’t have to look far to find your next piece of physics. You never do. As
you exit your house in the morning, for example, you may hear a crash up the
street. Two cars have collided at a high speed, and, locked together, they’re
sliding your way.

Thanks to physics (and, more specifically, Part III of this book), you can make
the necessary measurements and predictions to know exactly how far you
have to move to get out of the way. You know that it’s going to take a lot to
stop the cars. But a lot of what?

It helps to have the ideas of energy and momentum mastered at such a time.
You use these ideas to describe the motion of objects with mass. The energy
of motion is called kinetic energy, and when you accelerate a car from 0 to
60 miles per hour in 10 seconds, the car ends up with plenty of kinetic energy.
Where does the kinetic energy come from? Not from nowhere — if it did, you
wouldn’t have to worry about the price of gas. Using gas, the engine does
work on the car to get it up to speed.

say, for example, that you don’t have the luxury of an engine when you’re
moving a piano up the stairs of your new place. But there’s always time for a
little physics, so you whip out your calculator to calculate how much work
you have to do to carry it up the six floors to your new apartment.
After you move up the stairs, your piano will have what’s called potential
energy, simply because you put in a lot of work against gravity to get the
piano up those six floors.

Unfortunately, your roommate hates pianos and drops yours out the window.
What happens next? The potential energy of the piano due to its height in a
gravitational field is converted into kinetic energy, the energy of motion. It’s
an interesting process to watch, and you decide to calculate the final speed
of the piano as it hits the street.