## BackGround Information (For Teachers & Others)

 Background Students Standards Other Links Rubric

### Background

.......Info about density and buoyancy here.....

.The concept of buoyancy is particularly difficult. However the links to each of these may help with some understanding. For this particular lesson it is of some importance to recognize that the buoyancy of water changes as the salinity increases. For fresh water the buoyant force is approximately 62.4 lbs per cubic foot , whereas in salt water the values are 64 lbs per cubic foot . What this means is that if an object is between 62.4 and 64 lbs/cuft it will sink in fresh water but float in salt water. In this way the density of an object tells whether the object will float or sink in the way that it compares to the density of water. The density of water is approximately 1 gm/cc, but this changes also with salinity. See the examples under the buoyancy link.

### Classroom Use With Students

Most of the material explored in this activity will be new to students at Eighth Grade. They may have some knowledge of matter and mass and volume, but it is likely they will have little or no prior knowledge of the concepts covered in this activity. In particular, the concept of density is very difficult for most students of this age group. And Buoyancy is of difficulty for most adults! Reviewing the units of mass (grams, kilograms and lbs) and volume (milliters, liters, gallons, etc) is essential. Reviewing the concept of force, at least to understand that it is a "pushing" upon an object, is also essential.

Therefore, before exposing the students to this activity, some classroom preparation should be done. One pre-learning activity that might be done, is to have students create aluminum foil boats of a specific size, and then add paper clips or washers to load them until sinking, asking the question: does shape make a difference? Other questions which can be raised are: what is holding up the foil boat, since aluminum is "heavier" than water (have them fold the foil tightly into a small square or use aluminum pellets and drop into water). Why does a boat in general stay afloat even though it may be made of material which sink in water? These questions are also a part of the activity of course, but doing this as a lead-in activity maight be useful as well.

At the end of this lesson, doing the aluminum boat and comparing salt with fresh water might be a very good experiment for closure, with an important element being consistency---two small identical plastic boats might be better for this!

Dropping a cork into water might also be a good discussion-starter for this unit, as well as the whole "message in a bottle" concept.

Students with learning disabilities and English languages learners will need additional assistance with the activities in the unit. Peer coaches, parent volunteers, or paraprofessionals could be used to make the experience more successful for these learners.

### California Science Standards

California Science Standards: 08phys8a, 08phys8b, 08phys8c (having to do with Density and Buoyancy)

08Phys: Density and Buoyancy: All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept, students know:
• 08phys8a: that density is mass per unit of volume and how to calculate the density of substances (regular and irregular solids and liquids) from the measurements of mass and volume.
• 08phys8b: that the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid it has displaced, and know how to apply this principle to predict whether an object will float or sink.
• 08phys8c: how to apply their knowledge of buoyant forces to calculate the densities of solids and liquids from appropriate measurement.

### Other links for teachers: Density and Buoyancy

(Density & Buoyancy Activity)

(Density & Buoyancy Activity) (Go to "For Kids" in the menu ribboln at the page bottom, then to the density or buoyancy experiment)