WHY DO WE NEED TO PUSH THINGS TO GET THEM TO MOVE?
Things don't start moving by themselves. If something is at rest, you have to push it to get it going. For a push of a given size, some objects will move more easily than others. Inertia is a measure of how much something resists being put into motion. Mass is a measure of inertia. The more massive an object is, the harder you have
to push to get it moving.
WHAT ABOUT MAKING SOMETHING ROTATE?
To get something to rotate, a twisting force has to be
used. But it is more than just the mass that determines how hard it is to rotate
something. It also matters how far that mass is from the point around which you are trying to rotate. If most of the mass is close to the center of rotation, it is easier to make it rotate. For example, a disk will roll
more easily than a ring with the same mass and size.
BASEBALL PLAYERS ARE SCIENTISTS?
"I'm sure that of all of the
home runs I hit, at least half
of them came when I was
choking up slightly."
Ted Williams
When the pitcher is bringing
the heat, Megan gets
a jump on the ball by
choking up on her bat.
Well, some might be. But almost all of them know that it is harder to get good “bat speed” with a big,
heavy bat. When a pitcher is throwing the ball at 100 miles per hour, the batter has less then a second
to hit the ball, so getting the bat around fast matters! If a batter wants to be able to swing a bat more
quickly, they choke up on the bat. This puts the heaviest part of the bat a little closer to where
they are twisting the bat with their wrists.
The next time you play baseball, do an experiment. Take your favorite bat and try choking up on
the grip a little. You’ll find that you can swing a little faster, and that’s a good thing! Tell your coach
"IT'S ALL ABOUT ROTATIONAL INERTIA!"
TRY IT!
ROTATIONAL INERTIA (Example: Twirling a baseball bat.) Hold a bat horizontally in your hand, gripping it
in the middle. Twist it back and forth to rotate it,
like in the figure above. Now grip that same bat
all the way at the end and try to rotate it again. It
is much harder. That’s because the mass is farther
away from the rotation axis.
