Teaching Physics with the Physics Suite Edward F. Redish Problems Sorted by Type | Problems Sorted by Subject | Problems Sorted by Chapter in UP

Pushing iron

For each of the following partial sentences, indicate whether they are correctly completed by the symbol corresponding to the phrase greater than (>), less than (<), or the same as (=).

1. A chunk of iron is sitting on a table. It is then moved from the table into a bucket of water sitting on the table. The iron now rests on the bottom of the bucket. The force the bucket exerts on the block when the block is sitting on the bottom of the bucket is __________the force that the table exerted on the block when the block was sitting on the table.

2. A chunk of iron is sitting on a table. It is then moved from the table into a bucket of water sitting on the table. The iron now rests on the bottom of the bucket. The total force on the block when it is sitting on the bottom of the bucket __________ it was on the table.

3. A chunk of iron is sitting on a table. It is then covered by a bell jar which has a nozzle connected to a vacuum pump. The air is extracted from the bell jar The force the table exerts on the block when the block is sitting in a vacuum is __________the force that the table exerted on the block when the block was sitting in the air.

4. A chunk of iron is sitting on a scale. The iron and the scale are then both immersed in a large vat of water. After being immersed in the water, the scale reading will be __________the scale reading when they were simply sitting in the air. (Assume the scale would read zero if nothing were sitting on it, even when it is under water.)

Note to the instructor: This is an unexpectedly challenging problem for students because it mixes fluid statics with standard Newton's law dynamics. Students have to separate the idea of individual and net forces and understand that being immersed in a fluid produces an upward force, despite the "weight of the fluid pressing down on you."