Goal: Honing the concept of normal force and reasoning.
Consider the five situations appearing below. All the blocks have the
same size and density.
Which of the following statements is true regarding the normal force
that the table or incline exerts on the block in contact with it?
- ND > NC = NA = NE > NB
- ND = NA > NC > NE = NB
- NC > ND > NA > NE > NB
- ND = NA = NC > NE > NB
- NA > ND = NC > NB > NE
- ND = NA = NE > NC > NB
- More than one of the above is true.
- None of the above is true.
- Cannot be determined.
(8) All that can be said with certainty is that none of the above is
true. First, it is impossible to determine the relationship between
NB and NE. Both are less than the weight, but
which is least depends upon unknown angles and the tension in the rope.
Further, NC is largest because all of the other cases have a
buoyancy force due to air. Some students may indicate (9) for the same
reasons as stated above. However, technically, the truth or falsity of
(1) to (7) can be determined.
Students often think that the Normal force must be vertical because all
of the examples they have seen are of this type. Other students may
think that the deformed table is indicative of a greater (or lesser)
normal force. A subtle issue is that the block under the bell jar will
not have a buoyant force due to the air. While this is a small force,
its absence means that the normal force in case C is largest of all.
Have students identify the basis of falseness of each of the statements.