You have asked a great question, one that
touches on one of the most common interactions
between things in our world. It's the same
question many great thinkers have asked for
If you ask any scientist on the street, "What
makes a ball stop when it is rolling?" they'll
give you at least one word: friction. Let me ask
you, what do you think of when you hear the word
friction? Maybe you think of two people arguing,
or something sliding, slowing, and heating up, or
just a general feeling of difficulty.
Friction in our world is a force. What is a force?
A force causes acceleration, or a change in
velocity. This change in velocity could be from
rest to 60 miles per hour, or from 10 miles to per
to 3 miles per hour. Say you push your friend. You
pushing your friend is a force because you cause
your friend to accelerate.
Friction is a force that resists movement. It
always opposes the motion of an object. Friction
does not happen in and of itself, it needs at
least two players that are touching one another.
These players may be a ball and the ground, a book
and a table, a skydiver and the air, or any number
of objects that are made from that fundamental
building block of nature--atoms.
When two objects are in contact and moving
relative to one another, friction happens. When I
say moving relative to one another, I mean one is
moving faster or slower or in the opposite
direction compared to the other. A book slides on
a table, and friction exerts a force in the
opposite direction the book is moving until the
book stops. A skydiver falls through the air and
speeds up until there are so many air molecules
hitting him per second that they keep gravity from
accelerating him anymore.
What is friction really? We said friction is a
force. Well the specifics of that force cannot be
found in our world, but the world of atoms. The
truth is that friction is fundamentally
electromagnetic in nature! Think about bringing
two positive ends of a bar magnet together. They
repel each other, right? The same thing is
happening in a place smaller than one billionth of
a yard. Everything in our world is pushing against
When you set your keyboard on your desk, it
doesn't fall through the desk because the
electrons in the atoms on the surface of your desk
are pushing against the electrons in the atoms on
the bottom of your keyboard. When you roll a ball
on the ground, the electrons in the atoms on the
surface of the ground push against the electrons
in the atoms on the surface of your ball that is
touching the ground.
A rolling ball stops because the surface on which
it rolls resists its motion. A rolling ball stops
because of friction.