Re: [Phys-L] Physics, Errors and Differenet Teaching Styles

["John Clement" <clement@hal-pc.org>]

One does not have to talk about the normal or buoyant force.  A simple
analysis suffices.
1.  When the rock is in the boat the amount of water displaced has the same
mass as the mass of the rock+boat
2.  Wnen the rock is submerged the amount of water displaced is equal to the
volume of the rock + an amount whose mass is the same as the boat.

The change in the amount of water displaced is that it goes from an amount
whose mass=mass of rock, to a volume equal to the volume of the rock.  If
the rock is more dense than water the amount=mass of rock is greater than
the the amount=volume of rock.  Hence when the rock is in the water and not
attached to the boat the amount of water displaced is less.  It doesn't
matter if the rock is at the bottom, just if it is no longer attached to the
boat.  So the level of the lake rises a little bit when the rock is
submerged.

The normal force is red herring, as the normal force exerted by the lake
bottom stays the same to be equal to the weight of the rock, boat, & water.

Incidentally pre age 8 students think that the weight of the rock determines
the level of water when it is dropped into the lake.  They think that the
weight pushes the water up.  I think the force analysis has a bit of this
type of thinking.  Just because we use advanced thinking, we often fall back
to simpler thinking at times, especially when it is outside our expertise.

John M. Clement
Houston, TX

> >
> In more colloquial terms than I used before:  With the rock 
> in the boat, the buoyant force is great enough to hold them 
> both up, but with the rock on the bottom the buoyant force 
> holds the boat up by itself but gets help from the normal 
> force in holding the rock up so the total buoyant force is 
> smaller meaning less water is displaced meaning the lake 
> level is lower.