>> Publications 1994-1996

BACKGROUND

Students come to classrooms with many misconceptions. The misconceptions interfere with the students' ability to understand concepts presented in the classroom. This interference occurs regardless of how clearly teachers present concepts. The misconceptions must be corrected. Instead of giving the correct answer, the teacher gives an example that contradicts the misconception. Once the student preceives the inconsistency and accepts the challenge of resolving it, a more promising learning opportunity exists.

OBJECTIVES

1. To highlight the students' wrong preconceptions and renders them more ready to
consciously absorb new information to modify their preconceptions.
2. To correct preconceptions / misconceptions about Archimedes' Principle and upthrust.

BENEFITS FOR TEACHING-LEARNING PROCESS

1. The guided inquiry approach used is more interesting to the students then mere teacher talk or demonstration.
2. Students' wrong preconceptions are highlighted and renders them more ready to consciously absorb new information.
3. Students learn to formulate activities for investigation and disprove their own misconceptions one by one under teacher's guidance.
4. Students listen, watch and participate actively in the investigations together with the teacher.

APPARATUS / MATERIALS NEEDED

Spring balance, wooden block, dry twig, rock, beaker, balloon, plasticine, merkury, and string

IMPLEMENTATION

This approach involves 4 steps :

Strategy A	Pretest to find out students' preconceptions two days before the lesson
Strategy B	Announce results of the pretest before the lesson to shake their precenceptions.
Strategy C	Teaching by comparison between right and wrong responses through the teacher demonstration and student activity.
Strategy D	An exercise on further concept development on Archimedes' Principle.

 

The test results are announced as soon as possible 2 days before the lesson starts. If results are bad, it will surprise the students so that their preconceptions will lose much of their control over their organizing and interpretive frameworks. They may become upset and refuse to listen. However, after a period of aversion ( 2 days ), they will begin to think seriously about their preconceptions and are ready to accept new information.

1. Teacher gets the students to show the law of flotation by immersing a block of wood over different base areas. It is shown that a floating object displaces a weight of liquid equal to its own weight, depending on the volume of the object and not on the base area submerged.

2. Next the students perform experiments to show that the loss in weight of the steel ball in water is equal to the loss in weight of the hollow steel ball in water. The loss in weight of the balls in water is equal to the upthrust of either balls, dependent completely upon its submerged volume and not on whether or not there was air in the ball.

3. Demonstration: Both parts of a plasticine ball will sink because both are denser than water . The smaller part has the same density.

4. The difference in the upward and downward force acting on the submerged block is the same at any depth.

5. Yes , a piece of paraffin pressed to the bottom of the beaker rises when released.

6. Yes, upthrust is dependent on the volume of the object. Demonstration: Suspend both alternately on a spring balance and note the loss in weight of each object in water.

7. Study activity : Sink a dry twig in liquid less dense than dry wood.

8. Student demonstration : Float a block of iron on a beaker of mercury.

9. Student demonstration : Float a block of iron on the mercury and then repeat in water.

Contrasting this new information organized according to scientific concepts and the students' preconceptions appeal to their reasoning, thus making it easy for them to absorb the new information and modify preconceptions. Thus self-regulation will emerge and develop in this prosess of teaching by comparison.

Archimedes' Principle

1. Consider a ballon filled with 1 liter ( 1 000 cm3 ) of water in equilibrium in a container of water as shown in the figure.

(a) What is the mass of the 1 liter of water ?
(b) What is the weight of the 1 litre of water ?
(c) What is the weight of water displaced by the balloon ?
(d) What is upthrust on the balloon ?
(e) Sketch a pair of vectors in the figure, one for the weight of the balloon and the other for the upthrust that acts on it .
(f) How do the size and directions of your vectors compare ?

2. As a thought experiment, pretend we could remove the water from the balloon but still
have it remain at the same size of 1 litrer and the inside of the balloon is a vacuum.

(a) What is the mass of the litre of nothing ?
(b) What is the weight of the litre of nothing ?
(c) What is the weight of water displaced by the massless balloon ?
(d) What is the upthrust on the massless balloon ?
(e) In which direction would the massless balloon be accelerated ?

3. Assume the balloon is replaced by a 0.5 kilogram piece of wood that has exactly the same
volume (1000 cm3 ) as shown in the figure. The wood is held in the same submerged
position beneath the surface of the water.

(a) What volume of water is displaced by the wood ?
(b) What is the mass of the water displaced by the wood ?
(c) What is the weight of the water displaced by the wood ?
(d) How much upthrust does the surrounding water exert on the wood ?
(e) When the hand is removed, what is the net force on the wood ?
(f) In which direction does the wood accelerate when released ?

4. Repeat parts (a) through (f) in the previous question for a 5 kg rock that has the same
volume (1000 cm3 ) as shown in the figure. Assume the rock is suspended by a string in
the container of water .

(a) What volume of water is displaced by the rock ?
(b) What is the mass of the water displaced by the rock ?
(c) What is the weight of the water displaced by the rock ?
(d) How much upthrust does the surrounding water exert on the rock ?
(e) When the hand is removed, what is the net force on the rock ? ( hand here refers to
the string )
(f) In which direction does the rock accelerate when released ?

SUGGESTIONS FOR MODIFICATION

1. Questions given in the Strategy A can either be extended or refined. Examination format could be used to contain these questions.

2. Before carrying the prescribed experiments to justify the explanations of questions in Strategy A, it will be a great help if a teacher could invoke in students' mind, the concept of a "fair test".

3. It will foster a better thinking skills if students are asked to design a fair test so as to equilibrate whatever cognitive dissonances that arise due to the conflict of preinstructional ideas (before views) and the scientific view.