Re: How does gravity control the motion of vehicles and why?

Hi Kathy,

I'm glad to hear that you are having fun in the fifth grade. My wife taught
fifth grade and we had lots of fun with their science projects too.

Let's assume you are thinking of ground vehicles like cars, bikes, trucks,
and racing vehicles � even skates and skis. They work a little different
than planes and rockets. Notice that all of these forms of transportation
rely on being on the ground or some other surface of the earth. That will
be an important key idea.

The importance of gravity in the motion of ground or land vehicles is not
too much different than gravity's importance in your being able to walk. I
think if you can see how they are similar, you will be able to understand
other comparisons when they are presented to you.

First of all, think of yourself standing still, just before you take a
step. Maybe without even thinking about it, the first thing you do is push
back with one foot as you lift the other. It's a rule in nature that if you
push back on one thing, it will push forward on you. That is Newton's Third
Law and one of the most difficult to understand; but it always works. The
floor, pushing forward on you, is what makes you go forward as you begin to
walk. The force, between you and the ground, is called "friction"; but it
wouldn't happen at all if you weren't in contact with the ground. But you
normally are, and that's because of GRAVITY. 

Now, the same is true with vehicles. As the wheels begin to turn, the
bottoms (touching the ground) are turning backwards. This tries to push the
road backwards, and sometimes we even see some loose parts of the road fly
backwards too. But, in general, the road follows the rule and pushes the
wheel forward, just as intended. Think for a minute how this might also be
true for skates and rollerblades. Everything moves because it is in contact
with the ground and the strength of that contact is normally due to gravity. 

Braking works much the same way. While the engine can make the wheels turn
and the brakes make the wheels stop turning, it is the ground (friction)
that makes the vehicle go or stop or even change direction. The real danger
of going too fast over bumps in the road is that the ground may no longer
control the motion of the vehicle: no braking and no turning. In fact, the
stronger the force making contact with the ground, the stronger the
friction to control motion. Racing cars are built to use wind resistance to
push the car down harder on the road than gravity alone would do. This
gives the racer more control in curves and in changing speed. 

This makes me ask another question. What do you think moving around would
be like if there were less gravity than we are used to?

Have you ever had to move about on a waxed floor in your socks? I have and
it is hard to stop or turn quickly. This is true because of a low-friction
coating on the floor. But what if it was caused by just low gravity, like
if we were on the moon?

Many people have wondered what it would be like to play sports on the moon.
Do you think you could run as fast? Could you turn corners as well? The
moon has only about one-sixth as much gravity as we  are used to on the
earth. So that would mean only about one-sixth as much friction to help us
move about.

I'll leave it to you to consider what types of games we could play on the
moon. Then you might want to check this NASA site:
http://science.nasa.gov/headlines/y2006/04aug_xmoon.htm

Keep thinking about these things and you'll become a very good scientist in
time.

Gene Ewald