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Motorcycle in the Air

Date: Wed, 4 Jan 1995 12:02:00 -0800 (PST)
From: larry parsons
Subject: Motorcycle

Hello, this is Andrea Tompkins. I'm a junior at Ilwaco High School. This 
is my assignment for my math teacher. Here's the question... When a 
motorcycle is in the air, why does pushing on the gas make it go up and 
braking make it go down? 

Date: 6 Jan 1995 21:39:24 -0500
From: Dr. Ken
Subject: Motorcycle

Hello there, Andrea!

Well, let's see.  Do you mean a motorcycle that has just gone off a jump or
something?  I couldn't quite tell what you meant by "in the air."  But if
this is really what you mean, then you're right to question:  I don't think
that hitting the gas or the brakes would really have much effect in such a

See, the only way a flying motorcycle can alter its course (the natural curve
that any thrown object takes) is by interacting with the air around it.
That is, unless the motorcycle smashes into something or blows up or
something like that, the bike won't go up or down.

But stepping on the gas will produce only a very minor change in the
motorcycle's course.  What stepping on the gas does is burns more gasoline,
and then sends fumes out the tailpipe.  I suppose that if it sent the fumes
out the back at a high enough speed, that could produce kind of a
rocket-engine effect, and propel the bike forward, but we're really not
dealing with those kinds of air speeds.

As far as stepping on the brakes go, I can't really see how that would
produce any effect at all on the trajectory of the bike.  If you've got an
idea, please send it along, and I'd be happy to discuss it.

So in reaction to your question, "it doesn't."

-Ken "Dr." Math

From: Brad Leclair
Date: Sun, 29 Sep 1996 11:06:45 -0400 (EDT)

I know the answer.  A motorcycle doesn't go up or down in the air.  But you
can change the angle of the wheels.  By using more of the throttle in the
air, a rider can make the front wheel go up.  This also makes the back wheel
come down.  So the bike is pivoting on its center of gravity.  By using the
brakes a rider can drop his front wheel and simultaneously raise his back
wheel in order to make his/her wheels parallel to the landing.  I am a 10th
grader from Masconomet High School in Massachusetts and I don't know a lot
about math but I know my fair share of the habits of motorcycles and dirt
bikes in the air.

Date: 30 Sep 1996 10:39:06
From: Dr. Ken
Subject: Motorcycle

Of course!  I guess this is why I'm a mathematician and not a physicist.
What's happening here is that you're tapping into the angular momentum of
the wheels.  The wheels are spinning clockwise (as viewed from the right)
while you're in the air, and just as Brad said, you can use this fact to
alter the angle of the cycle.  The simplest to think about is braking.  If
you slam on the brakes in midair, you will make the bike (including the
wheels) one rigid hunk of mean metal.  But where did that angular momentum
of the wheels go?  It's transferred into the bike, and the bike will tilt
clockwise, bringing the front wheel down and the back wheel up.

Applying the gas will have the opposite effect - it will make the wheels
spin clockwise faster than they were.  But that force has to push against
something, and that something is the bike.  So the bike starts tilting
counterclockwise, bringing the front wheel up and the back wheel down.

But also, keep this in mind - as Brad said, neither of these methods will
really make the bike go _up_ or _down_.  The height of the center of gravity
of the bike will be unaffected.  The only way to affect that is still (as I
said in my above message) to push on the air around you, either by shoving
some exhaust out the back of your cycle, or blowing really hard.
Associated Topics:
High School Physics/Chemistry

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