Re: [Phys-L] real-world application of the laws of motion

[John Denker <jsd@av8n.com>]

On 4/29/22 10:20 AM, Carl Mungan via Phys-l wrote:

> And I suppose even if he had used enough straps, the whole trailer
> could have jack-knifed vertically up into the air?

Let's talk about that.

1) By design, most boats have a rather low center of mass.
  The last non-catamaran sailboat I was on had thousands of
  pounds of lead in the keel, to make it not heel too much
  when on a reach.

  So, even if you add in the height of the trailer, I would
  expect the height of the CM to be less than half the length
  of the boat. That implies to flip the thing up in the air
  would require more than 1 gee of deceleration. I doubt that
  the truck could achieve that.

2) The same physics leads to a different conclusion in other
  situations. A person on a bicycle has a comparatively high
  CM relative to length (compared to a boat). If you have good
  brakes, you might be able to generate enough force to flip
  yourself over the front tire.

  In a panic-stop situation with a high CM, you should start
  by scrunching down to lower your CM as much as possible, then
  apply the front brake to the point where most of the down
  force is shifted to the front, i.e. just shy of the point
  where the rear wheel comes off the ground (which would result
  in loss of directional control). The point is, assuming a
  constant coefficient of friction and no skidding, this is
  the most braking force you can achieve.

  Suppose you use the back brake only. This is self-limiting,
  because more braking means less down-force on the rear (and
  more down-force on the front) so at some point the rear tire
  starts skidding. This is suboptimal. It is far less than the
  maximum possible braking.

  If you have a lower CM and/or a longer bike and/or a
  more rearward seat position, you don't need to worry about
  flipping over the front tire. Max braking will be achieved
  with "most" of the weight on the front wheel but there will
  still be "some" weight on the back, so there will still be
  some role for the rear brake.

3) The same physics applies to taildragger-type airplanes. A
  nose-over accident is definitely possible if you apply too
  much braking. Normally you can prevent this by applying
  back stick (i.e. nose-up tail-down aerodynamic force) but
  if (a) you are a klutz and/or (b) there is a tailwind that
  robs you of aerodynamic control authority, expensive things
  can happen.
     https://www.youtube.com/watch?v=o4oOhumKco4