Topic

Topic: Buoyancy

Teacher Notes (Refer to student sheet for lab worksheet)

Objectives:

Define Buoyancy
Measure Buoyant force

Concept:

It is common knowledge that objects are lighter in water than in air. When an object is fully or partially immersed in a fluid it experiences an apparent loss in weight. In this experiment you will investigate buoyancy.

Historical background:

The Greek Mathematician and inventor Archimedes is the father of the Principle of Buoyancy. History has it that he discovered the principle of buoyancy while he took his bath one day in his bathtub. He noticed that as he lowered himself into the bath the water from the bath overflowed the sides and found that there was a relationship between his weight in the tub and the volume of water displaced. At this discovery he ran out of his bathtub into the streets yelling “heurEka” meaning, “I found it”. This principle has since been applied in the building of ships and air balloons. He is credited with proving that the King’s crown smith had been ripping off the king in doping his gold crown with silver.

National Science Standard addressed:

Physical Science - A- Properties and structure of matter, Students will observe balanced and unbalanced forces and the resulting motion of an object.

Science as Inquiry - Students will collect observational data and think critically about cause and effect relationships.

Missouri Grade Level Expectation by the lesson: Science Strand 1.1a Properties and states of matter, Strand 2.2a,b,c Forces as contact forces - pull, push, buoyancy.

Materials

Rocks (8)
Measuring Cylinders (8)
String
Spring balances (8)
Clear container (8)
Water
Aluminum bowls (8)

Instructions

Students will work in groups of two. Each group will be provided with the above listed materials.

Reasoning Strategies:

1. Induction

2. Investigation

Lab 1

Step1:

Use the spring balance provided to measure the weight of the object provided in air and record the weight. Weight in Air = ________________ N

Step2:

Pour some water in the measuring cylinder provided, about 2/3 way full, and record its volume. Volume of water = ______________ CC

Immerse the object provided in the water.

What do you notice about its weight? _________________________________

Now read off its weight and record it. Also record the new water level.

Weight of Object in water = __________________ N

New volume of water = ____________________ CC

Difference in Weight: _____________________ N

Lab 2 - Factors affecting Buoyancy

In this lab students will place the object provided, stone in our case, in a bowl of water and observe what happens. It will sink. They then put this stone in an aluminum bowl and place the bowl in water. Now it floats. This behavior should be explained using the idea of density of the system of the stone and the aluminum bowl. the increase in surface area of the system increases the volume of the system and reduces its density, hence it floats.

_____________________________________

Concept Development:

Questions: Through teacher led questions and answers groups discuss their findings.

Teacher summary:

The activity shows that the object weighed more in air than in water. Objects in any fluid experience an upward push or force known as Buoyant Force. Buoyant force is a result of unbalanced forces acting on the object in the fluid from all directions. This buoyant force is equal to the apparent loss in weight of the object. The buoyant force experienced by a body can be affected by its size (surface area) and weight. We found in lab 2 that the object sank when placed in water by itself, but floats when placed in the foil bowl. This is also a good time to introduce Archimedes Principle which states that an object that is partially or fully immersed in a fluid displaces an amount of fluid equal to its weight. Students should also be informed that Archimedes Principle is also described as the law of Buoyancy.

What do you think is the reason why it did not sink while in the bowl?

Application: The concept of buoyancy and the principle of Archimedes can be applied in finding the volume of an irregular object. First the object is weighed in air using a triple beam balance, then when submerged in water. Since a submerged object displaces its volume of the fluid in which it is submerged, the apparent loss in mass in grams is equal to the volume of the irregular object in cubic centimeters since water has a density of 1g/cc, which means that 1gram of water has a volume of 1 cubic centimeter.

References:

Douglas Llewellyn. (2002) Inquire Within.
Paul G. Hewitt. (2002) Conceptual Physics: a High School Physics Program. Ma: Prentice Hall.

Historical background

http://www.bible-history.com/resource/ff_float.htm

Application

http://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy2.html#arc