[Phys-L] Re: A Third law question

[hhaskell at MINDSPRING.COM (Hugh Haskell)]

At 13:18  -0500 10/11/05, Rick Tarara wrote:

> In the end though, she was still
> having trouble.  She could 'understand' how she increased the force of her
> hand on the disk but couldn't really accept the inertia arguments about how
> the disk increased its force back on the hand.  'Where does the disk get the
> extra force when I push up with more force than its weight?' was her
> repeated question.
>
> Any suggestions here?  How would you try to deal with this question?

How about helping her to understand how the table exerts a force on
the book that is resting on it? If she can see that, as she
accelerates an object by pushing on it, the object gets distorted by
tiny amounts and that generates forces within the object that can end
up exerting forces back on her hand as it attempts to restore itself
to its original shape.

Another possibility would be to look at collisions, and how, if two
objects come together, and both have their velocities changed (thus
being accelerated), both have to have been accelerated, each by the
other, so therefore there is a force of each upon the other. This,
IMO, is a good justification for teaching momentum before force and
acceleration. When the law of conservation of momentum is established
empirically, then the concepts of forces and Newton's three laws
follow almost automatically.

Hugh
--

Hugh Haskell
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<mailto:hhaskell@mindspring.com>

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