The invisible push
The light turns green and the car starts to move quickly. It feels as if
a big, invisible hand pushes you back in the car seat and makes you stay
there. But it feels different when the car stops quickly. The same
\”hand” seems to give you an invisible push forward. It feels as if the
\”hand” wants you to keep going. If you didn’t have a seat belt on, you
might hit the front window.
What is this invisible \”something” that pushes you when a car starts
and stops? It’s called inertia (ihn UR shuh).
Inertia is a name for the way things behave when they are stopped and
when they are moving. When anything is stopped, it stays stopped—it
can’t start moving by itself. It
only starts to move when a force—a push or a pull—makes it move. And
when anything is moving, it tries to keep moving. It won’t stop until
a force stops it.
When a car starts to move, your body tries to stay stopped. So you feel
yourself pressing back as the car seat moves forward. And when the car
stops, your body tries to keep moving. Inertia \”pushes” you forward.
Your seat belt is there to hold you back.
Here’s a little trick you can do with inertia. Lay a playing card over a
drinking glass. Put a penny on the card. Very quickly, pull the card off
the glass, straight toward you.
You might think the penny would \”ride” along on the card. But it won’t.
Inertia keeps it from moving. It will stay where it is—and drop into
the glass with a clink.
