Re: [Phys-L] real-world physics

["LaMontagne, Bob" <RLAMONT@providence.edu>]

If that episode of Mythbusters showed them performing that calculation and then declaring that it was obviously theoretically impossible - followed by moving on to the next myth - how long would they be on the air?

Even more important, If the episode consisted of them showing the calculation only and declaring impossibility - anyone watching the episode would forget about it completely (probably before the end of the show). By actually testing the myth, they have created an image that is not easily forgotten. Even a group of physics teachers has been talking about it for days.

This might be a lesson for us as teachers to bring into our work. An artful derivation or proof might contain everything one would ever need to know about a certain topic, but it will probably be forgotten by students before the next class. An absurd visual image (and hopefully the physics related to it) will be remembered and talked about long after.

Bob at PC

________________________________________
From: phys-l-bounces@mail.phys-l.org [phys-l-bounces@mail.phys-l.org] on behalf of Paul Nord [Paul.Nord@valpo.edu]
Sent: Thursday, June 07, 2012 10:23 AM
To: Phys-L@Phys-L.org; betwys1@sbcglobal.net
Subject: Re: [Phys-L] real-world physics

If the bubble wrap acts like an ideal spring, completely reversing the motion does imply that a lower limit on the maximum acceleration of 2g.

In this case, it's simpler to consider the duration of the collision.  Starting at a speed of 120 mph (50 m/s) and changing that to zero implies an average speed during collision of 25 m/s.  Traveling a distance of 4 inches (0.1 m) at this speed means that the collision takes 0.004 seconds.  Knowing our delta-v and delta-t we can say that the average acceleration during that time is 12,500 m/s/s (1250 g).

That is the calculation that the Mythbusters should have done before attempting to build this.

Paul


On Jun 6, 2012, at 5:46 PM, brian whatcott wrote:

> Hmmm..  with a compressing spring, an increasing force, providing an increasing retardation.
> So certainly not constant acceleration.   I think I was responding to the idea of doubling the acceleration by the reversed direction, which seems also mistaken
>
> Brian W
>
>
> On 6/6/2012 11:54 AM, brian whatcott wrote:
> > Comparable speed at reciprocal heading, with constant acceleration possibly??
> >
> > Brian W
> >
> > On 6/6/2012 10:57 AM, Bernard Cleyet wrote:
> > > On 2012, Jun 06, , at 08:19, John Clement wrote:
> > >
> > > > I wonder if the boxes were filled with bubble wrap.
> > > A possible worsening, as if not popped, bouncing, doubling the acceleration.
> > >
> > >
> > >
> > >
> > > >  Since these are
> > > > professionals I suspect they first tried a dummy and that they know how to
> > > > calculate forces.
> > > >
> > > > So testing an idea is not all that farfetched.
> > > __
>
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